Gang Chen

Gang Chen

Professor in the Department of Atmospheric and Oceanic Sciences (AOS) at University of California-Los Angles (UCLA)

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portfolio

publications

The impacts of changing transport and precipitation on pollutant distributions in a future climate

Published in Journal of Geophysical Research, 2011

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Recommended citation: Fang, Yuanyuan, Arlene M. Fiore, Larry W. Horowitz, Anand Gnanadesikan, Isaac M. Held, Gang Chen, Gabriel Vecchi and Hiram Levy, 2011: The impacts of changing transport and precipitation on pollutant distributions in a future climate, Journal of Geophysical Research, 116, D18303, doi:10.1029/2011JD015642.

Delineating the Eddy–Zonal Flow Interaction in the Atmospheric Circulation Response to Climate Forcing: Uniform SST Warming in an Idealized Aquaplanet Model

Published in Journal of the Atmospheric Sciences, 2013

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Recommended citation: Chen, Gang, Jian Lu and Lantao Sun, 2013: Delineating the Eddy--Zonal Flow Interaction in the Atmospheric Circulation Response to Climate Forcing: Uniform SST Warming in an Idealized Aquaplanet Model, Journal of the Atmospheric Sciences, 70, 2214--2233, doi:10.1175/JAS-D-12-0248.1.

The robust dynamical contribution to precipitation extremes in idealized warming simulations across model resolutions

Published in Geophysical Research Letters, 2014

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Recommended citation: Lu, Jian, L. Ruby Leung, Qing Yang, Gang Chen, William D. Collins, Fuyu Li, Z. Jason Hou and Xuelei Feng, 2014: The robust dynamical contribution to precipitation extremes in idealized warming simulations across model resolutions, Geophysical Research Letters, 41, 2971--2978, doi:10.1002/2014GL059532.

The Role of Stratospheric Polar Vortex Breakdown in Southern Hemisphere Climate Trends

Published in Journal of the Atmospheric Sciences, 2014

ABSTRACT: This paper investigates the connection between the delay in the final breakdown of the stratospheric polar vortex, the stratospheric final warming (SFW), and Southern Hemisphere climate trends. We first analyze ERA-Interim reanalysis and three climate model outputs with different climate forcings. Climate trends appear when there is a delay in the timing of SFWs. When regressed onto the SFW dates (which reflects the anomaly when the SFW is delayed for one standard deviation of its onset dates), the anomaly pattern bears a resemblance to the observed climate trends, for all the model outputs, even without any trends. This suggests that the stratospheric and tropospheric circulations are organized by the timing of SFWs, in both the interannual time scale and climate trends due to external forcings.We further explore the role of the SFW using a simplified dynamical model, in which the ozone depletion is mimicked by a springtime polar stratospheric cooling. The responses of zonal-mean atmospheric circ…

Recommended citation: Sun, Lantao, Gang Chen and Walter A. Robinson, 2014: The Role of Stratospheric Polar Vortex Breakdown in Southern Hemisphere Climate Trends, Journal of the Atmospheric Sciences, 71, 2335--2353, doi:10.1175/JAS-D-13-0290.1.

Sensitivities of the Lower-Stratospheric Transport and Mixing to Tropical SST Heating

Published in Journal of the Atmospheric Sciences, 2014

ABSTRACT: The sensitivities of the Brewer-Dobson Circulation (BDC) to different distributions of tropical SST heating are investigated in an idealized aqua-planet model. It is found that an increase in tropical SSTs generally leads to an acceleration of tropical upwelling and an associated reduction in the Age of Air (AOA) in the polar stratosphere, and that the AOA near the subtropical tropopause is correlated with local isentropic mixing of tropospheric air with stratospheric air.The zonal distribution of SST perturbations has a major impact on the vertical and meridional structure of the BDC as compared with other SST characteristics. Zonally localized SST heatings tend to generate a shallow acceleration of the stratospheric residual circulation, enhanced isentropic mixing associated with a weakened stratospheric jet, and a reduction in AOA mostly within the polar vortex. Contrarily, SST heatings with a zonally symmetric structure tend to produce a deep strengthening of the stratospheric residual circula…

Recommended citation: Yang, Huang, Gang Chen and Daniela I.V. V. Domeisen, 2014: Sensitivities of the Lower-Stratospheric Transport and Mixing to Tropical SST Heating, Journal of the Atmospheric Sciences, 71, 2674--2694, doi:10.1175/JAS-D-13-0276.1.

Variations in Titan's dune orientations as a result of orbital forcing

Published in Icarus, 2016

Wind-blown dunes are a record of the climatic history in Titan's equatorial region. Through modeling of the climatic conditions associated with Titan's historical orbital configurations (arising from apsidal precessions of Saturn's orbit), we present evidence that the orientations of the dunes are influenced by orbital forcing. Analysis of 3 Titan general circulation models (GCMs) in conjunction with a sediment transport model provides the first direct intercomparison of results from different Titan GCMs. We report variability in the dune orientations predicted for different orbital epochs of up to 70°. Although the response of the GCMs to orbital forcing varies, the orbital influence on the dune orientations is found to be significant across all models. Furthermore, there is near agreement among the two models run with surface topography, with 3 out of the 5 dune fields matching observation for the most recent orbital cycle. Through comparison with observations by Cassini, we find situations in which the observed dune orientations are in best agreement with those modeled for previous orbital configurations or combinations thereof, representing a larger portion of the cycle. We conclude that orbital forcing could be an important factor in governing the present-day dune orientations observed on Titan and should be considered when modeling dune evolution.

Recommended citation: McDonald, George D., Alexander G. Hayes, Ryan C. Ewing, Juan M. Lora, Claire E. Newman, Tetsuya Tokano, Antoine Lucas, Alejandro Soto and Gang Chen, 2016: Variations in Titan's dune orientations as a result of orbital forcing, Icarus, 270, 197--210, doi:10.1016/j.icarus.2015.11.036.

Delineating the Barotropic and Baroclinic Mechanisms in the Midlatitude Eddy-Driven Jet Response to Lower-Tropospheric Thermal Forcing

Published in Journal of the Atmospheric Sciences, 2016

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Recommended citation: Nie, Yu, Yang Zhang, Gang Chen and Xiu-Qun Yang, 2016: Delineating the Barotropic and Baroclinic Mechanisms in the Midlatitude Eddy-Driven Jet Response to Lower-Tropospheric Thermal Forcing, Journal of the Atmospheric Sciences, 73, 429--448, doi:10.1175/JAS-D-15-0090.1.

Barotropic and Baroclinic Eddy Feedbacks in the Midlatitude Jet Variability and Responses to Climate Change–Like Thermal Forcings

Published in Journal of the Atmospheric Sciences, 2017

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Recommended citation: Burrows, D Alex, Gang Chen and Lantao Sun, 2017: Barotropic and Baroclinic Eddy Feedbacks in the Midlatitude Jet Variability and Responses to Climate Change–Like Thermal Forcings, Journal of the Atmospheric Sciences, 74, 111--132, doi:10.1175/JAS-D-16-0047.1.

Examining the Hydrological Variations in an Aquaplanet World Using Wave Activity Transformation

Published in Journal of Climate, 2017

ABSTRACT: Building on the recent advent of the concept of finite-amplitude wave activity, a contour-following diagnostics for column water vapor (CWV) is developed and applied to a pair of aquaplanet model simulations to understand and quantify the higher moments in the global hydrological cycle. The Lagrangian nature of the diagnostics leads to a more tractable formalism for the transient, zonally asymmetric component of the hydrological cycle, with a strong linear relation emerging between the wave activity and the wave component of precipitation minus evaporation (). The dry-versus-wet disparity in the transient hydrological cycle is measured by , and it is found to increase at a super-Clausius–Clapeyron rate at the poleward side of the mean storm track in response to a uniform sea surface temperature (SST) warming and the meridional structure of the increase can be largely attributed to the change of the meridional stirring scale of the midlatitude Rossby waves. Further scaling for indicates that the ra…

Recommended citation: Lu, Jian, Koichi Sakaguchi, Qing Yang, L. Ruby Leung, Gang Chen, Chun Zhao, Erik Swenson and Zhangshuan J. Hou, 2017: Examining the Hydrological Variations in an Aquaplanet World Using Wave Activity Transformation, Journal of Climate, 30, 2559--2576, doi:10.1175/JCLI-D-16-0561.1.

Wave Events: Climatology, Trends, and Relationship to Northern Hemisphere Winter Blocking and Weather Extremes

Published in Journal of Climate, 2017

ABSTRACT: Diagnostics of finite-amplitude local wave activity (LWA) are applied to the 500-hPa geopotential height field to diagnose persistent synoptic weather events of anomalously large wave activity in the Northern Hemisphere. By considering the cyclonic and anticyclonic components of LWA separately, persistent weather systems associated with large-amplitude troughs and ridges are detected.While anticyclonic wave events are predominantly found over Europe and Alaska, cyclonic wave events usually occur over East Asia and northeastern Canada. Those preferred regions correspond to the location of planetary-scale ridges and troughs, which contribute, together with transient anomalies, to the formation of wave events. Although wave events are not blocking events per definition, they are typically associated with increased blocking in their vicinity. Their spatial relationship to blocking, however, varies depending on their cyclonic or anticyclonic nature and the type of wave-breaking signatures. Wave events are also shown to be accompanied by warm or cold temperature extremes, whose spatial pattern depends on the type of events, cyclonic or anticyclonic, and the sector affected. Trends in the frequency of wave events indicate that cyclonic wave events and the associated cold extremes affecting East Asia have become more frequent in recent decades and could be linked to recent trends toward La Niña–like conditions in the Pacific and trends toward the negative phase of Arctic Oscillation.

Recommended citation: Martineau, Patrick, Gang Chen and D. Alex Burrows, 2017: Wave Events: Climatology, Trends, and Relationship to Northern Hemisphere Winter Blocking and Weather Extremes, Journal of Climate, 30, 5675--5697, doi:10.1175/JCLI-D-16-0692.1.

On the Linkage Between the Asian Summer Monsoon and Tropopause Folds

Published in Journal of Geophysical Research: Atmospheres, 2018

ABSTRACT: This study uses a set of idealized aquaplanet model experiments to investigate the linkage between the Asian summer monsoon circulation and tropopause fold activity. It is found that folds tend to occur on the northwestern side of the upper-level anticyclone associated with the monsoon circulation and are generated due to intensified monsoon circulation and resulting intensified subsidence. In addition, the impact of tropopause folds on extreme rainfall events is also examined. It is found that while the likelihood of extreme rainfall is largely decreased at and upstream of the occurrence of folds, the likelihood is significantly increased downstream. This pattern of suppression of extreme rainfall upstream and promotion downstream also persists for about 1–2 days, which likely suggests a positive feedback between extreme rainfall downstream and tropopause folds. Finally, changes in tropopause folds with monsoon intensity are also discussed.

Recommended citation: Wu, Yutian, Gang Chen, Lindsey Taylor and Pengfei Zhang, 2018: On the Linkage Between the Asian Summer Monsoon and Tropopause Folds, Journal of Geophysical Research: Atmospheres, 123, 2037--2049, doi:10.1002/2017JD027870.

Lower-Stratospheric Control of the Frequency of Sudden Stratospheric Warming Events

Published in Journal of Geophysical Research: Atmospheres, 2018

ABSTRACT: The sensitivity of stratospheric polar vortex variability to the basic-state stratospheric temperature profile is investigated by performing a parameter sweep experiment with a dry dynamical core general circulation model where the equilibrium temperature profiles in the polar lower and upper stratosphere are systematically varied. It is found that stratospheric variability is more sensitive to the temperature distribution in the lower stratosphere than in the upper stratosphere. In particular, a cold lower stratosphere favors a strong time-mean polar vortex with a large daily variability, promoting frequent sudden stratospheric warming events in the model runs forced with both wavenumber-1 and wavenumber-2 topographies. This sensitivity is explained by the control exerted by the lower-stratospheric basic state onto fluxes of planetary-scale wave activity from the troposphere to the stratosphere, confirming that the lower stratosphere can act like a valve for the upward propagation of wave activity. It is further shown that with optimal model parameters, stratospheric polar vortex climatology and variability mimicking Southern and Northern Hemisphere conditions are obtained with both wavenumber-1 and wavenumber-2 topographies.

Recommended citation: Martineau, Patrick, Gang Chen, Seok-Woo Son and Joowan Kim, 2018: Lower-Stratospheric Control of the Frequency of Sudden Stratospheric Warming Events, Journal of Geophysical Research: Atmospheres, 123, 3051--3070, doi:10.1002/2017JD027648.

Enhanced hydrological extremes in the western United States under global warming through the lens of water vapor wave activity

Published in npj Climate and Atmospheric Science, 2018

ABSTRACT: A novel diagnostic framework based on the wave activity of column integrated water vapor (CWV) is used to probe into the higher moments of the hydrological cycle with bearings on the extremes. Applying the CWV wave activity analysis to the historical and RCP8.5 scenario simulations by the CMIP5 models reveals a super Clausius–Clapeyron rate of increase in the wet vs. dry disparity of daily net precipitation due to the enhanced stirring length of wave activity at the poleward flank of the storm track, despite a decrease in the hydrological cycling rate (HCR) measured by the reciprocal of wave activity residence time. The local variant of CWV wave activity unravels the unique characteristics of atmospheric rivers (ARs) in terms of their transport function and locally enhanced mixing efficiency. Under RCP8.5, the local moist wave activity increases by ~40% over the northeastern Pacific and western United States by the end of the 21st century, indicating lengthening and more frequent landfalling ARs with a local HCR. These results imply that the unusually wet winter the west coast just experienced in 2016/17 might be a harbinger of consequence of a ~20% increase in the related hydrological extremes in the west coast, despite a robust weakening of the more frequent wet extremes in a warmer climate.

Recommended citation: Lu, Jian, Daokai Xue, Yang Gao, Gang Chen, L. Ruby Leung and Paul Staten, 2018: Enhanced hydrological extremes in the western United States under global warming through the lens of water vapor wave activity, npj Climate and Atmospheric Science, 1, 7, doi:10.1038/s41612-018-0017-9.

Large-scale Atmospheric Control on Non-Gaussian Tails of Midlatitude Temperature Distributions

Published in Geophysical Research Letters, 2018

ABSTRACT: Observed surface temperature distributions are non-Gaussian, which has important implications for the likelihood of extreme events in a changing climate. We use a two-dimensional advection-diffusion model of temperature stirred by stochastically generated Rossby waves with a sustained background temperature gradient to explore non-Gaussian temperature distributions. We examine how these distributions change with changes to thermal relaxation and eddy stirring. Weakening the background temperature gradient leads to decreased variance but no changes in other moments, while the eddy properties affect both the variance and skewness. A poleward movement of eddy stirring latitude leads to reduced skewness for most latitudes, implying a shift toward longer negative tails in temperature distributions, all else being equal. In contrast, the dependence of temperature skewness on eddy speed is a nuanced, nonlinear relationship.

Recommended citation: Linz, Marianna, Gang Chen and Zeyuan Hu, 2018: Large-scale Atmospheric Control on Non-Gaussian Tails of Midlatitude Temperature Distributions, Geophysical Research Letters, 1--9, doi:10.1029/2018GL079324.

Thermodynamic and Dynamic Mechanisms for Hydrological Cycle Intensification over the Full Probability Distribution of Precipitation Events

Published in Journal of the Atmospheric Sciences, 2018

ABSTRACT: Precipitation changes in a warming climate have been examined with a focus on either mean precipitation or precipitation extremes, but changes in the full probability distribution of precipitation have not been well studied. This paper develops a methodology for the quantile-conditional column moisture budget of the atmosphere for the full probability distribution of precipitation. Analysis is performed on idealized aqua- planet model simulations under 3-K uniform SST warming across different horizontal resolutions. Because the covariance of specific humidity and horizontal mass convergence is much reduced when conditioned onto a given precipitation percentile range, their conditional averages yield a clear separation between the moisture (thermodynamic) and circulation (dynamic) effects of vertical moisture transport on precipitation. The thermodynamic response to idealized climate warming can be understood as a generalized ‘‘wet get wetter’’ mechanism, in which the heaviest precipitation of the probability distribution is enhanced most from increased gross moisture stratification, at a rate controlled by the change in lower-tropospheric moisture rather than column moisture. The dynamic effect, in contrast, can be interpreted by shifts in large-scale atmospheric circulations such as the Hadley cell circulation or midlatitude storm tracks. Furthermore, hor- izontal moisture advection, albeit of secondary role, is important for regional precipitation change. Although similar mechanisms are at play for changes in both mean precipitation and precipitation extremes, the thermodynamic contributions of moisture transport to increases in high percentiles of precipitation tend to be more widespread across a wide range of latitudes than increases in the mean, especially in the subtropics.

Recommended citation: Chen, Gang, Jesse Norris, J. David Neelin, Jian Lu, L. Ruby Leung and Koichi Sakaguchi, 2018: Thermodynamic and Dynamic Mechanisms for Hydrological Cycle Intensification over the Full Probability Distribution of Precipitation Events, Journal of the Atmospheric Sciences, JAS--D--18--0067.1, doi:10.1175/JAS-D-18-0067.1.

Thermodynamic versus dynamic controls on extreme precipitation in a warming climate from the Community Earth System Model Large Ensemble

Published in Journal of Climate, 2018

ABSTRACT: The moisture budget is evaluated as a function of the probability distribution of precipitation for the end of the twentieth century and projected end of the twenty-first century in the Community Earth System Model Large Ensemble. For a given precipitation percentile, a conditional moisture budget equation relates pre- cipitation minus evaporation (P 2 E) to vertical moisture transport, horizontal moisture advection, and moisture storage. At high percentiles, moisture advection and moisture storage cancel and evaporation is negligible, so that precipitation is approximately equal to vertical moisture transport, and likewise for pro- jected changes. Therefore, projected changes to extreme precipitation are approximately equal to the sum of thermodynamic and dynamic tendencies, representing changes to the vertical profiles of moisture content and mass convergence, respectively. The thermodynamic tendency is uniform across percentiles and regions as an intensification of the hydrological cycle, but the dynamic tendency is more complex. For extreme events, per degree of warming, in the mid-to-high latitudes the dynamic tendency is small, so that pre- cipitation approximately scales by the Clausius–Clapeyron 7%K21 increase. In the subtropics, a drying tendency originating from dynamics offsets the thermodynamic wetting tendency, with the net effect on precipitation varying among regions. The effect of this dynamic drying decreases with increasing per- centile. In the deep tropics, a positive dynamic tendency occurs with magnitude similar to or greater than the positive thermodynamic tendency, resulting in generally a 10%–15% K-1 precipitation increase, and with a >25%K-1 increase over the tropical east Pacific. This reinforcing dynamical tendency increases rapidly for high percentiles.

Recommended citation: Norris, Jesse, Gang Chen and J. David Neelin, 2018: Thermodynamic versus dynamic controls on extreme precipitation in a warming climate from the Community Earth System Model Large Ensemble, Journal of Climate, JCLI--D--18--0302.1, doi:10.1175/JCLI-D-18-0302.1.

Larger increases in more extreme local precipitation events as climate warms

Published in Geophysical Research Letters, 2019

ABSTRACT: Climate models project that extreme precipitation events will intensify in proportion to their intensity during the 21st century at large spatial scales. The identification of the causes of this phenomenon nevertheless remains tenuous. Using a large ensemble of North American regional climate simulations, we show that the more rapid intensification of more extreme events also appears as a robust feature at finer regional scales. The larger increases in more extreme events than in less extreme events are found to be primarily due to atmospheric circulation changes. Thermodynamically induced changes have relatively uniform effects across extreme events and regions. In contrast, circulation changes weaken moderate events over western interior regions of North America and enhance them elsewhere. The weakening effect decreases and even reverses for more extreme events, whereas there is further intensification over other parts of North America, creating an “intense gets intenser” pattern over most of the continent.

Recommended citation: Li, Chao, Francis Zwiers, Xuebin Zhang, Gang Chen, Jian Lu, Guilong Li, Jesse Norris, Yaheng Tan, Ying Sun and Min Liu, 2019: Larger increases in more extreme local precipitation events as climate warms, Geophysical Research Letters, 2019GL082908, doi:10.1029/2019GL082908.

Changes in frequency of large precipitation accumulations over land in a warming climate from the CESM Large Ensemble: the roles of moisture, circulation and duration

Published in Journal of Climate, 2019

ABSTRACT: Projected changes in the frequency of major precipitation accumulations (hundreds of millimeters), integrated over rainfall events, over land in the late twenty-first century are analyzed in the Community Earth System Model (CESM) Large Ensemble, based on the RCP8.5 scenario. Accumulation sizes are sorted by the local average recurrence interval (ARI), ranging from 0.1 to 100 years, for the current and projected late- twenty-first-century climates separately. For all ARIs, the frequency of exceedance of the given accumulation size increases in the future climate almost everywhere, especially for the largest accumulations, with the 100-yr accumulation becoming about 3 times more frequent, averaged over the global land area. The moisture budget allows the impacts of individual factors—moisture, circulation, and event duration—to be isolated. In the tropics, both moisture and circulation cause large future increases, enhancing the 100-yr accumulation by 23% and 13% (average over tropical land), and are individually responsible for making the current-climate 100-yr accumulation 2.7 times and 1.8 times more frequent, but effects of shorter durations slightly offset these effects. In the midlatitudes, large accumulations become about 5% longer in duration, but are predominantly controlled by enhanced moisture, with the 100-yr accumulation (land average) becoming 2.4 times more frequent, and 2.2 times more frequent due to moisture increases alone. In some monsoon-affected regions, the 100-yr accumulation becomes more than 5 times as frequent, where circulation changes are the most impactful factor. These projections indicate that changing duration of events is a relatively minor effect on changing accumulations, their future enhancement being dominated by enhanced intensity (the combination of moisture and circulation).

Recommended citation: Norris, Jesse, Gang Chen and J. David Neelin, 2019: Changes in frequency of large precipitation accumulations over land in a warming climate from the CESM Large Ensemble: the roles of moisture, circulation and duration, Journal of Climate, JCLI--D--18--0600.1, doi:10.1175/JCLI-D-18-0600.1.

Large‐scale meteorological control on the spatial pattern of wintertime PM 2.5 pollution over China

Published in Atmospheric Science Letters, 2019

ABSTRACT: The frequent episodes of severe air pollution over China during recent years have posed serious health threats to densely populated eastern China. Although several studies investigated the linkage between enhanced severity and frequency of air pol- lution and the large-scale weather patterns over China, the day-to-day covariability between them, as well as its local and remote mechanisms, has not been systemati- cally documented. The wintertime synoptic covariability between PM2.5 and large- scale meteorological fields is studied using surface observations of PM2.5 in 2013/2014–2016/2017 and ERA-Interim meteorological fields through maximum covariance analysis (MCA). The first MCA mode (MCA1) suggests a consistent accumulation of ambient PM2.5 as a result of weakened winds that block the pollut- ant removal passage in heavily polluted areas of eastern China, as well as moist air from southeast coast favoring haze formation. A northeast–southwest belt that extends into northeastern China and central China on each end is more sensitive to MCA1. The second MCA mode (MCA2) shows a north–south dipole in PM2.5 linked to the contrast of boundary layer height and surface wind speed between northern and southern regions of China. Spatial patterns of both modes are supported by the GEOS-Chem chemistry transport model with realistic emission inventory. The spatial patterns of the two modes are robust on the interannual time scales. Based on that, we investigate the variability of the first two modes of the identified modes on the multidecadal scale by projecting GPM_500 pattern to 1981–2010. Correlation analysis of the projected time series and climate indices over 30 years indicates the possible linkage of Arctic oscillation, ENSO indices, Pacific decadal oscillation and east Atlantic/western Russia to regional air pollution patterns over China.

Recommended citation: Wang, Ziwei, Gang Chen, Yu Gu, Bin Zhao, Qiao Ma, Shuxiao Wang and Kuo‐Nan Liou, 2019: Large‐scale meteorological control on the spatial pattern of wintertime PM 2.5 pollution over China, Atmospheric Science Letters, 1--9, doi:10.1002/asl.938.

How waviness in the circulation changes surface ozone: a viewpoint using local finite-amplitude wave activity

Published in Atmospheric Chemistry and Physics, 2019

ABSTRACT: Local finite-amplitude wave activity (LWA) mea- sures the waviness of the local flow. In this work we relate the anticyclonic part of LWA, AWA (anticyclonic wave activ- ity), to surface ozone in summertime over the US on interan- nual to decadal timescales. Interannual covariance between AWA diagnosed from the European Centre for Medium- Range Weather Forecast Era-Interim reanalysis and ozone measured at EPA Clean Air Status and Trends Network (CASTNET) stations is analyzed using maximum covariance analysis (MCA). The first two modes in the MCA analysis explain 84% of the covariance between the AWA and MDA8 (maximum daily 8 h average ozone), explaining 29% and 14% of the MDA8 ozone variance, respectively. Over most of the US we find a significant relationship between ozone at most locations and AWA over the analysis domain (24– 53◦ N and 130–65◦ W) using a linear regression model. This relationship is diagnosed (i) using reanalysis meteorology and measured ozone from CASTNET, or (ii) using meteo- rology and ozone simulated by the Community Atmospheric Model version 4 with chemistry (CAM4-chem) within the Community Earth System Model (CESM1). Using the linear regression model we find that meteorological biases in AWA in CAM4-chem, as compared to the reanalysis meteorology, induce ozone changes between −4 and +8 ppb in CAM4- chem. Future changes (ca. 2100) in AWA are diagnosed in different climate change simulations in CAM4-chem, simu- lations which differ in their initial conditions and in one case differ in their reactive species emissions. All future simula- tions have enhanced AWA over the US, with the maximum enhancement in the southwest. As diagnosed using the linear regression model, the future change in AWA is predicted to cause a corresponding change in ozone ranging between −6 and 6 ppb. The location of this change depends on subtle features of the change in AWA. In a number of locations this change is consistent with the magnitude and the sign of the overall simulated future ozone change.

Recommended citation: Sun, Wenxiu, Peter Hess, Gang Chen and Simone Tilmes, 2019: How waviness in the circulation changes surface ozone: a viewpoint using local finite-amplitude wave activity, Atmospheric Chemistry and Physics, 19, 12917--12933, doi:10.5194/acp-19-12917-2019.

Projected Changes to Extreme Precipitation Along North American West Coast From the CESM Large Ensemble

Published in Geophysical Research Letters, 2020

ABSTRACT: Precipitation events along the North American (NA) west coast are strongly modulated by atmospheric rivers, yet the mechanisms of their influences on the probability distributions of precipitation events are not well studied. Simulations from the Community Earth System Model (CESM) large ensemble under global warming are investigated using a moisture budget conditioned onto precipitation events for the recurrence intervals ranging from 0.1 to 50 years. In the midlatitudes, the increases in precipitation intensity and accumulation for all events over the NA west coast are predominantly controlled by moisture increases. In contrast, changes in the subtropical precipitation distributions in southwestern NA are associated with moisture increases and duration decreases for all events, with additional dynamical amplification for the heaviest precipitation events. These interpretations from the conditional moisture budget are more consistent with future projection of atmospheric rivers than the conventional mean and transient decomposition of the moisture budget.

Recommended citation: Ma, Weiming, Jesse Norris and Gang Chen, 2020: Projected Changes to Extreme Precipitation Along North American West Coast From the CESM Large Ensemble, Geophysical Research Letters, 47, 1--10, doi:10.1029/2019GL086038.

Sensitivity of the latitude of the westerly jet stream to climate forcing

Published in Geophysical Research Letters, 2020

ABSTRACT: The latitude of the westerly jet stream is influenced by a variety of climate forcings, but their effects on the jet latitude often manifest as a tug ofwar between tropical forcing (e.g., tropical upper-tropospheric warming) and polar forcing (e.g., Antarctic stratospheric cooling or Arctic amplification). Here we present a unified forcing-feedback framework relating different climate forcings to their forced jet changes, in which the interactions between the westerly jet and synoptic eddies are synthesized by a zonal advection feedback, analogous to the feedback framework for assessing climate sensitivity. This framework is supported by a prototype feedback analysis in the atmospheric dynamical core of a climate model with diverse thermal and mechanical forcings. Our analysis indicates that the latitude of a westerly jet is most sensitive to the climate change-induced jet speed changes near the tropopause. The equatorward jet shift also displays a larger deviation from linearity than the poleward counterpart.

Recommended citation: Chen, Gang, Pengfei Zhang and Jian Lu, 2020: Sensitivity of the latitude of the westerly jet stream to climate forcing, Geophysical Research Letters, e2019GL086563.

A framework for understanding how dynamics shape temperature distributions

Published in Geophysical Research Letters, 2020

ABSTRACT: Understanding what physically sets the shape of temperature distributions will enable more robust predictions of local temperature with global warming. We derive the relationship between the temperature distribution shape and the advection of temperature conditionally averaged at each temperature percentile. This enables quantification of the shift of each percentile that is due to changes in the mean temperature, in horizontal temperature advection, and other processes (e.g., radiation and convection). We use this relationship to examine global model simulations in an idealized aquaplanet model with increasing carbon dioxide. Changes in the distribution with doubling and quadrupling of carbon dioxide are significant, and they are caused by different processes. We find that midlatitude temperature distributions can be explained mostly by the horizontal advection, except in the upper and lower 10% of the distribution.

Recommended citation: Linz, Marianna, Gang Chen, Boer Zhang and Pengfei Zhang, 2020: A framework for understanding how dynamics shape temperature distributions, Geophysical Research Letters, e2019GL085684, doi:10.1029/2019GL085684.

Tropical widening: From global variations to regional impacts

Published in Bulletin of the American Meteorological Society, 2020

ABSTRACT: Over the past 15 years, numerous studies have suggested that the sinking branches of Earth’s Hadley circulation and the associated subtropical dry zones have shifted poleward over the late twentieth century and early twenty-first century. Early estimates of this tropical widening from satellite observations and reanalyses varied from 0.25° to 3° latitude per decade, while estimates from global climate models show widening at the lower end of the observed range. In 2016, two working groups, the U.S. Climate Variability and Predictability (CLIVAR) working group on the Changing Width of the Tropical Belt and the International Space Science Institute (ISSI) Tropical Width Diagnostics Intercomparison Project, were formed to synthesize current understanding of the magnitude, causes, and impacts of the recent tropical widening evident in observations. These working groups concluded that the large rates of observed tropical widening noted by earlier studies resulted from their use of metrics that poorly capture changes in the Hadley circulation, or from the use of reanalyses that contained spurious trends. Accounting for these issues reduces the range of observed expansion rates to 0.25°–0.5° latitude decade–1 —within the range from model simulations. Models indicate that most of the recent Northern Hemisphere tropical widening is consistent with natural variability, whereas increasing greenhouse gases and decreasing stratospheric ozone likely played an important role in Southern Hemisphere widening. Whatever the cause or rate of expansion, understanding the regional impacts of tropical widening requires additional work, as different forcings can produce different regional patterns of widening.

Recommended citation: Staten, Paul W., Kevin M. Grise, Sean M. Davis, Kristopher B. Karnauskas, Darryn W. Waugh, Amanda Maycock, Qiang Fu, Kerry Cook, Ori Adam, Isla R. Simpson, Robert J Allen, Karen Rosenlof, Gang Chen, Caroline C. Ummenhofer, Xiao-Wei Quan, James P. Kossin, Nicholas A. Davis and Seok-Woo Son, 2020: Tropical widening: From global variations to regional impacts, Bulletin of the American Meteorological Society, preprint, doi:10.1175/BAMS-D-19-0047.1.

Reduced European aerosol emissions suppress winter extremes over northern Eurasia

Published in Nature Climate Change, 2020

ABSTRACT: Winter extreme weather events receive major public attention due to their serious impacts, but the dominant factors regulating their interdecadal trends have not been clearly established. Here, we show that the radiative forcing due to geospatially redistributed anthropogenic aerosols mainly determined the spatial variations of winter extreme weather in the Northern Hemisphere during 1970–2005, a unique transition period for global aerosol forcing. Over this period, the local Rossby wave activity and extreme events (top 10% in wave amplitude) exhibited marked declining trends at high latitudes, mainly in northern Eurasia. The combination of long-term observational data and a state-of-the-art climate model revealed the unambiguous signature of anthropogenic aerosols on the wintertime jet stream, planetary wave activity and surface temperature variability on interdecadal timescales. In particular, warming due to aerosol reductions in Europe enhanced the meridional temperature gradient on the jet’s poleward flank and strengthened the zonal wind, resulting in significant suppression in extreme events over north- ern Eurasia. These results exemplify how aerosol forcing can impact large-scale extratropical atmospheric dynamics, and illustrate the importance of anthropogenic aerosols and their spatiotemporal variability in assessing the drivers of extreme weather

Recommended citation: Wang, Yuan, Tianhao Le, Gang Chen, Yuk L Yung, Hui Su, John H Seinfeld and Jonathan H Jiang, 2020: Reduced European aerosol emissions suppress winter extremes over northern Eurasia, Nature Climate Change, doi:10.1038/s41558-020-0693-4.

Role of atmospheric variability in driving the "Warm-Arctic, Cold-continent" pattern over the North America sector and sea ice variability over the Chukchi‐Bering Sea

Published in Geophysical Research Letters, 2020

ABSTRACT: While the observed decline of sea ice over the Chukchi-Bering Sea (CBS) has coincided with the ‘‘warm-Arctic, cold-continent” (WACC) pattern over the North America (NA) sector, there is a debate on the causes of the WACC pattern. Here we present a very similar WACC pattern over the NA sector on both interannual and subseasonal time scales. Lead-lag regression analyses on the shorter time scale indicate that an anomalous anticyclonic circulation over Alaska/Yukon in conjunction with the downward surface turbulent heat flux and long-wave radiation anomalies over CBS leads the formation of the WACC pattern by about 1-2 days, while the latter further leads CBS sea ice reduction by about 3 days. These results indicate that atmospheric variability may play an active role in driving both the WACC pattern over NA and CBS sea ice variability.

Recommended citation: Guan, Weina, Xianan Jiang, Xuejuan Ren, Gang Chen and Qinghua Ding, 2020: Role of atmospheric variability in driving the "Warm-Arctic, Cold-continent" pattern over the North America sector and sea ice variability over the Chukchi‐Bering Sea, Geophysical Research Letters, doi:10.1029/2020GL088599.

Wintertime particulate matter decrease buffered by unfavorable chemical processes despite emissions reductions in China

Published in Geophysical Research Letters, 2020

ABSTRACT:

Recommended citation: Leung, Danny M., Hongrong Shi, Bin Zhao, Jing Wang, Elizabeth M. Ding, Yu Gu, Haotian Zheng, Gang Chen, Kuo‐Nan Liou, Shuxiao Wang, Jerome D. Fast, Guangjie Zheng, Jingkun Jiang, Xiaoxiao Li and Jonathan H. Jiang, 2020: Wintertime particulate matter decrease buffered by unfavorable chemical processes despite emissions reductions in China, Geophysical Research Letters, doi:10.1029/2020GL087721.

Rapid Warming in Summer Wet Bulb Globe Temperature in China with Human-Induced Climate Change

Published in Journal of Climate, 2020

ABSTRACT: On the basis of a newly developed observational dataset and a suite of climate model simulations, we evaluate changes in summer mean wet bulb globe temperature (WBGT) in China from 1961 through 2080. We show that summer mean WBGT has increased almost everywhere across China since 1961 as a result of human-induced climate change. Consequently, hot summers as measured by summer mean WBGT are be- coming more frequent and more conducive to heat stress. Hot summers like the hottest on record during 1961–2015 in western or eastern China are now expected occur once every 3–4 years. These hot WBGT summers have become more than 140 times as likely in eastern China in the present decade (2010s) as in the 1961–90 baseline period and more than 1000 times as likely in western China. The substantially larger in- fluence in western China is associated with its stronger warming signal, which is likely due to the high Bowen ratio of sensible to latent heat fluxes of dry soils and increases in absorbed solar radiation from the decline in mountain snow cover extent. Observation-constrained projections of future summer mean WBGT under the RCP8.5 emissions scenario indicate that, by the 2040s, almost every summer in China will be at least as hot as the hottest summer in the historical record, and by the 2060s it will be common (on average, every other year) for summers to be as much as 3.08C hotter than the historical record, pointing to potentially large increases in the likelihood of human heat stress and to a massive adaption challenge.

Recommended citation: Li, Chao, Ying Sun, Francis Zwiers, Dongqian Wang, Xuebin Zhang, Gang Chen and Hui Wu, 2020: Rapid Warming in Summer Wet Bulb Globe Temperature in China with Human-Induced Climate Change, Journal of Climate, 33, 5697--5711, doi:10.1175/JCLI-D-19-0492.1.

Dynamic Amplification of Subtropical Extreme Precipitation in a Warming Climate

Published in Geophysical Research Letters, 2020

ABSTRACT: Projected precipitation changes in a warming climate vary considerably, spatially, and between intensities. The changes can be greater or less than the ∼7% K−1 Clausius‐Clapeyron (CC) prediction, owing to dynamic effects. Using two global‐climate‐model large ensembles, we quantify the dynamically induced changes to precipitation extremes from the present (1996–2005) to late‐21st‐century (2071–2080) climates, as a function of recurrence interval, focusing on the subtropics. We separate non‐CC changes into a term proportional to the present‐day vertical‐velocity spatial pattern (i.e., an amplification or damping thereof by a constant factor) and a residual. The amplitude term varies with recurrence interval, approximately canceling (doubling) CC for moderate (large) extremes, increasing precipitation variability. Contrastingly, the residual is quasi‐uniform across recurrence intervals but spatially heterogeneous, weakening extremes over dry zones. This residual may be related to Hadley cell expansion, although this explanation is insufficient to explain many features, and other possible mechanisms are discussed.

Recommended citation: Norris, Jesse, Gang Chen and Chao Li, 2020: Dynamic Amplification of Subtropical Extreme Precipitation in a Warming Climate, Geophysical Research Letters, 47, 1--15, doi:10.1029/2020GL087200.

Assessing Global and Regional Effects of Reconstructed Land-Use and Land-Cover Change on Climate since 1950 Using a Coupled Land–Atmosphere–Ocean Model

Published in Journal of Climate, 2020

Land-use and land-cover change (LULCC) is one of the most important forcings affecting climate in the past century. This study evaluates the global and regional LULCC impacts in 1950–2015 by employing an annually updated LULCC map in a coupled land–atmosphere–ocean model. The difference between LULCC and control experiments shows an overall land surface temperature (LST) increase by 0.48 K in the LULCC regions and a widespread LST decrease by 0.18 K outside the LULCC regions. A decomposed temperature metric (DTM) is applied to quantify the relative contribution of surface processes to temperature changes. Furthermore, while precipitation in the LULCC areas is reduced in agreement with declined evaporation, LULCC causes a southward displacement of the intertropical convergence zone (ITCZ) with a narrowing by 0.5°, leading to a tripole anomalous precipitation pattern over the warm pool. The DTM shows that the temperature response in LULCC regions results from the competing effect between increased albedo (cooling) and reduced evaporation (warming). The reduced evaporation indicates less atmospheric latent heat release in convective processes and thus a drier and cooler troposphere, resulting in a reduction in surface cooling outside the LULCC regions. The southward shift of the ITCZ implies a northward cross-equatorial energy transport anomaly in response to reduced latent/sensible heat of the atmosphere in the Northern Hemisphere, where LULCC is more intensive. Tropospheric cooling results in the equatorward shift of the upper-tropospheric westerly jet in both hemispheres, which, in turn, leads to an equatorward narrowing of the Hadley circulation and ITCZ.

Recommended citation: Huang, Huilin, Yongkang Xue, Nagaraju Chilukoti, Ye Liu, Gang Chen and Ismaila Diallo, 2020: Assessing Global and Regional Effects of Reconstructed Land-Use and Land-Cover Change on Climate since 1950 Using a Coupled Land–Atmosphere–Ocean Model, Journal of Climate, 33, 8997--9013, doi:10.1175/JCLI-D-20-0108.1.

Dependence of Atmospheric Transport Into the Arctic on the Meridional Extent of the Hadley Cell

Published in Geophysical Research Letters, 2020

Recent studies have shown a large spread in the transport of atmospheric tracers into the Arctic among a suite of chemistry climate models and have suggested that this is related to the spread in the meridional extent of the Hadley Cell (HC). Here we examine the HC-transport relationship using an idealized model, where we vary the mean circulation and isolate its impact on transport to the Arctic. It is shown that the poleward transport depends on the relative position between the northern edge of the HC and the tracer source, with maximum transport occurring when the HC edge lies near the middle of the source region. Such dependence highlights the critical role of near-surface transport by the Eulerian mean circulation rather than eddy mixing in the free troposphere and suggests that variations in the HC edge and the tracer source region are both important for modeling Arctic composition.

Recommended citation: Yang, Huang, Darryn W. Waugh, Clara Orbe and Gang Chen, 2020: Dependence of Atmospheric Transport Into the Arctic on the Meridional Extent of the Hadley Cell, Geophysical Research Letters, 47, 1--11, doi:10.1029/2020GL090133.

The Leading Intraseasonal Variability Mode of Wintertime Surface Air Temperature over the North American Sector

Published in Journal of Climate, 2020

In this study, detailed characteristics of the leading intraseasonal variability mode of boreal winter surface air temperature (SAT) over the North American (NA) sector are investigated. This intraseasonal SAT mode, characterized by two anomalous centers with an opposite sign—one over central NA and another over east Siberia (ES)/Alaska—bears a great resemblance to the “warm Arctic–cold continent” pattern of the interannual SAT variability over NA. This intraseasonal SAT mode and associated circulation exert a pronounced influence on regional weather extremes, including precipitation over the northwest coast of NA, sea ice concentration over the Chukchi and Bering Seas, and extreme warm and cold events over the NA continent and Arctic region. Surface warming and cooling signals of the intraseasonal SAT mode are connected to temperature anomalies in a deep-tropospheric layer up to 300 hPa with a decreasing amplitude with altitude. Particularly, a coupling between the troposphere and stratosphere is found during evolution of the intraseasonal SAT variability, although whether the stratospheric processes are essential in sustaining the leading intraseasonal SAT mode is difficult to determine based on observations alone. Two origins of wave sources are identified in contributing to vertically propagating planetary waves near Alaska: one over ES/Alaska associated with local intraseasonal variability and another from the subtropical North Pacific via Rossby wave trains induced by tropical convective activity over the western Pacific, possibly associated with the Madden–Julian oscillation.

Recommended citation: Guan, Weina, Xianan Jiang, Xuejuan Ren, Gang Chen, Pu Lin and Hai Lin, 2020: The Leading Intraseasonal Variability Mode of Wintertime Surface Air Temperature over the North American Sector, Journal of Climate, 33, 9287--9306, doi:10.1175/JCLI-D-20-0096.1.

Pacific sea surface temperature anomalies as important boundary forcing in driving the interannual Warm Arctic-Cold Continent pattern over the North American sector

Published in Journal of Climate, 2021

The leading interannual mode of winter surface air temperature over the North American (NA) sector, characterized by a “Warm Arctic, Cold Continents” (WACC) pattern, exerts pronounced influences on NA weather and climate, while its underlying mechanisms remain elusive. In this study, the relative roles of surface boundary forcing versus internal atmospheric processes for the formation of the WACC pattern are quantitatively investigated using a combined analysis of observations and large-ensemble atmospheric global climate model simulations. Internal atmospheric variability is found to play an important role in shaping the year-to-year WACC variability, contributing to about half of the total variance. An anomalous SST pattern resembling the North Pacific Mode is identified as a major surface boundary forcing pattern in driving the interannual WACC variability over the NA sector, with a minor contribution from sea ice variability over the Chukchi- Bering Seas. Findings from this study not only lead to improved understanding of underlying physics regulating the interannual WACC variability, but also provide important guidance for improved modeling and prediction of regional climate variability over NA and the Arctic region.

Recommended citation: Guan, Weina, Xianan Jiang, Xuejuan Ren, Gang Chen and Qinghua Ding, 2021: Pacific sea surface temperature anomalies as important boundary forcing in driving the interannual Warm Arctic-Cold Continent pattern over the North American sector, Journal of Climate, 1--43, doi:10.1175/JCLI-D-20-0867.1.

Quantifying the Mechanisms of Atmospheric Circulation Response to Greenhouse Gas Increases in a Forcing-Feedback Framework

Published in Journal of Climate, 2021

While there is substantial evidence for tropospheric jet shift and Hadley cell expansion in response to greenhouse gas increases, quantitative assessments of individual mechanisms and feedback for atmospheric circulation changes remain lacking. We present a new forcing-feedback analysis on circulation response to increasing CO 2 concentration in an aquaplanet atmospheric model. This forcing-feedback framework explicitly identifies a direct zonal wind response by holding the zonal mean zonal wind exerting on the zonal advection of eddies unchanged, in comparison with the additional feedback induced by the direct response in zonal mean zonal wind. It is shown that the zonal advection feedback accounts for nearly half of the changes to the eddy-driven jet shift and Hadley cell expansion, largely contributing to the subtropical precipitation decline, when the CO 2 concentration varies over a range of climates. The direct response in temperature displays the well-known tropospheric warming pattern to CO2 increases, but the feedback exhibits negative signals. The direct response in eddies is characterized by a reduction in upward wave propagation and a poleward shift of midlatitude eddy momentum flux (EMF) convergence, likely due to an increase in static stability from moist thermodynamic adjustment. In contrast, the feedback features a dipole pattern in EMF that further shifts and strengthens midlatitude EMF convergence, resulting from the upper-level zonal wind increase seen in the direct response. Interestingly, the direct response produces an increase in eddy kinetic energy (EKE), but the feedback weakens EKE. Thus, the forcing-feedback framework highlights the distinct effect of zonal mean advecting wind from direct thermodynamic effects in atmospheric response to greenhouse gas increases.

Recommended citation: Zhang, Pengfei, Gang Chen and Yi Ming, 2021: Quantifying the Mechanisms of Atmospheric Circulation Response to Greenhouse Gas Increases in a Forcing-Feedback Framework, Journal of Climate, 1--50, doi:10.1175/JCLI-D-20-0778.1.

Robust atmospheric river response to global warming in idealized and comprehensive climate models

Published in Journal of Climate, 2021

Atmospheric rivers (ARs), narrow intense moisture transport, account for much of the poleward moisture transport in midlatitudes. While studies have characterized AR features and the associated hydrological impacts in a warming climate in observations and comprehensive climate models, the fundamental dynamics for changes in AR statistics (e.g., frequency, length, width) are not well understood. Here we investigate AR response to global warming with a combination of idealized and comprehensive climate models. To that end, we developed an idealized atmospheric GCM with Earth-like global circulation and hydrological cycle, in which water vapor and clouds are modeled as passive tracers with simple cloud microphysics and precipitation processes. Despite the simplicity of model physics, it reasonably reproduces observed dynamical structures for individual ARs, statistical characteristics of ARs, and spatial distributions of AR climatology. Under climate warming, the idealized model produces robust AR changes similar to CESM large ensemble simulations under RCP8.5, including AR size expansion, intensified landfall moisture transport, and an increased AR frequency, corroborating previously reported AR changes under global warming by climate models. In addition, the latitude of AR frequency maximum shifts poleward with climate warming. Further analysis suggests the thermodynamic effect (i.e., an increase in water vapor) dominates the AR statistics and frequency changes while both the dynamic and thermodynamic effects contribute to the AR poleward shift. These results demonstrate that AR changes in a warming climate can be understood as passive water vapor and cloud tracers regulated by large-scale atmospheric circulation, whereas convection and latent heat feedback are of secondary importance.

Recommended citation: Zhang, Pengfei, Gang Chen, Weiming Ma, Yi Ming and Zheng Wu, 2021: Robust atmospheric river response to global warming in idealized and comprehensive climate models, Journal of Climate, 1--52, doi:10.1175/JCLI-D-20-1005.1.

Atmospheric River Response to Arctic Sea Ice Loss in the Polar Amplification Model Intercomparison Project

Published in Geophysical Research Letters, 2021

The atmospheric river (AR) response to Arctic sea ice loss in the Northern hemisphere winter is investigated using simulations from the Polar Amplification Model Intercomparison Project (PAMIP). Results have shown that the midlatitude responses are dominated by dynamic effects. Poleward of around 60 °N, the dynamic and thermodynamic effects cancel each other, resulting in relatively small responses. The response uncertainty can be characterized by leading uncertainty modes, with the responses over the Pacific and Atlantic projecting onto the northeastward extension and equatorward shift mode, respectively. In addition, the responses seem to be mean state dependent: under the same forcing, models with more poleward-located climatological ARs tend to show stronger equatorward shifts over the Atlantic; over the Pacific, models with more westward-located climatological AR core tend to show stronger northeastward extensions. These relationships highlight the importance of improving the AR climatology representation on reducing the response uncertainty to Arctic sea ice loss.

Recommended citation: Ma, Weiming, Gang Chen, Yannick Peings and Noah Alviz, 2021: Atmospheric River Response to Arctic Sea Ice Loss in the Polar Amplification Model Intercomparison Project, Geophysical Research Letters, 48, 1--12, doi:10.1029/2021GL094883.

Interpreting Observed Temperature Probability Distributions Using a Relationship between Temperature and Temperature Advection

Published in Journal of Climate, 2022

The nonnormality of temperature probability distributions and the physics that drive it are important due to their relationships to the frequency of extreme warm and cold events. Here we use a conditional mean framework to explore how horizontal temperature advection and other physical processes work together to control the shape of daily temperature distributions during 1979–2019 in the ERA5 dataset for both JJA and DJF. We demonstrate that the temperature distribution in the middle and high latitudes can largely be linearly explained by the conditional mean horizontal temperature advection with the simple treatment of other processes as a Newtonian relaxation with a spatially variant relaxation time scale and equilibrium temperature. We analyze the role of different transient and stationary components of the horizontal temperature advection in affecting the shape of temperature distributions. The anomalous advection of the stationary temperature gradient has a dominant effect in influencing temperature variance, while both that term and the covariance between anomalous wind and anomalous temperature have significant effects on temperature skewness. While this simple method works well over most of the ocean, the advection–temperature relationship is more complicated over land. We classify land regions with different advection–temperature relationships under our framework, and find that for both seasons the aforementioned linear relationship can explain ∼30{\%} of land area, and can explain either the lower or the upper half of temperature distributions in an additional ∼30{\%} of land area. Identifying the regions where temperature advection explains shapes of temperature distributions well will help us gain more confidence in understanding the future change of temperature distributions and extreme events.

Recommended citation: Zhang, Boer, Marianna Linz and Gang Chen, 2022: Interpreting Observed Temperature Probability Distributions Using a Relationship between Temperature and Temperature Advection, Journal of Climate, 35, 705--724, doi:10.1175/JCLI-D-20-0920.1.

Jet Stream Meandering in the Northern Hemisphere Winter: An Advection–Diffusion Perspective

Published in Journal of Climate, 2022

Large meridional excursions of a jet stream are conducive to blocking and related midlatitude weather extremes, yet the physical mechanism of jet meandering is not well understood. This paper examines the mechanisms of jet meandering in boreal winter through the lens of a potential vorticity (PV)-like tracer advected by reanalysis winds in an advection–diffusion model. As the geometric structure of the tracer displays a compact relationship with PV in observations and permits a linear mapping from tracer to PV at each latitude, jet meandering can be understood by the geometric structure of tracer field that is only a function of prescribed advecting velocities. This one-way dependence of tracer field on advecting velocities provides a new modeling framework to quantify the effects of time mean flow versus transient eddies on the spatiotemporal variability of jet meandering. It is shown that the mapped tracer wave activity resembles the observed spatial pattern and magnitude of PV wave activity for the winter climatology, interannual variability, and blocking-like wave events. The anomalous increase in tracer wave activity for the composite over interannual variability or blocking-like wave events is attributed to weakened composite mean winds, indicating that the low-frequency winds are the leading factor for the overall distributions of wave activity. It is also found that the tracer model underestimates extreme wave activity, likely due to the lack of feedback mechanisms. The implications for the mechanisms of jet meandering in a changing climate are also discussed.

Recommended citation: Chen, Gang, Yu Nie and Yang Zhang, 2022: Jet Stream Meandering in the Northern Hemisphere Winter: An Advection–Diffusion Perspective, Journal of Climate, 35, 2055--2073, doi:10.1175/JCLI-D-21-0411.1.

Distinct North American Cooling Signatures Following the Zonally Symmetric and Asymmetric Modes of Winter Stratospheric Variability

Published in Geophysical Research Letters, 2022

Download paper: journal website; Slides: here

Recommended citation: Ding, Xiuyuan, Gang Chen, Lantao Sun and Pengfei Zhang, 2022: Distinct North American Cooling Signatures Following the Zonally Symmetric and Asymmetric Modes of Winter Stratospheric Variability, Geophysical Research Letters, 49, doi:10.1029/2021GL096076.

Precipitation Extremes and Water Vapor

Published in Current Climate Change Reports, 2022

Download paper: journal website

Recommended citation: Neelin, J. David, Cristian Martinez-Villalobos, Samuel N Stechmann, Fiaz Ahmed, Gang Chen, Jesse Norris, Yi-Hung Kuo and Geert Lenderink, 2022: Precipitation Extremes and Water Vapor, Current Climate Change Reports, 8, 17--33, doi:10.1007/s40641-021-00177-z.

An Intraseasonal Mode Linking Wintertime Surface Air Temperature over Arctic and Eurasian Continent

Published in Journal of Climate, 2022

Key processes associated with the leading intraseasonal variability mode of wintertime surface air temperature (SAT) over Eurasia and the Arctic region are investigated in this study. Characterized by a dipole distribution in SAT anomalies centered over north Eurasia and the Arctic, respectively, and coherent temperature anomalies vertically extending from the surface to 300 hPa, this leading intraseasonal SAT mode and associated circulation have pronounced influences on global surface temperature anomalies including the East Asian winter monsoon region. By taking advantage of realistic simulations of the intraseasonal SAT mode in a global climate model, it is illustrated that temperature anomalies in the troposphere associated with the leading SAT mode are mainly due to dynamic processes, especially via the horizontal advection of winter mean temperature by intraseasonal circulation. While the cloud–radiative feedback is not critical in sustaining the temperature variability in the troposphere, it is found to play a crucial role in coupling temperature anomalies at the surface and in the free atmosphere through anomalous surface downward longwave radiation. The variability in clouds associated with the intraseasonal SAT mode is closely linked to moisture anomalies generated by similar advective processes as for temperature anomalies. Model experiments suggest that this leading intraseasonal SAT mode can be sustained by internal atmospheric processes in the troposphere over the mid- to high latitudes by excluding forcings from Arctic sea ice variability, tropical convective variability, and the stratospheric processes.

Recommended citation: Xiu, Junyi, Xianan Jiang, Renhe Zhang, Weina Guan and Gang Chen, 2022: An Intraseasonal Mode Linking Wintertime Surface Air Temperature over Arctic and Eurasian Continent, Journal of Climate, 35, 2675--2696, doi:10.1175/JCLI-D-21-0495.1.

The relationship between PM2.5 and anticyclonic wave activity during summer over the United States

Published in Atmospheric Chemistry and Physics, 2022

ABSTRACT: . To better understand the role of atmospheric dynamics in modulating surface concentrations of fine particulate matter (PM2.5), we relate the anticyclonic wave activity (AWA) metric and PM2.5 data from the Interagency Monitoring of Protected Visual Environment (IMPROVE) data for the period of 1988–2014 over the US. The observational results are compared with hindcast simulations over the past 2 decades using the National Center for Atmospheric Research–Community Earth System Model (NCAR CESM). We find that PM2.5 is positively correlated (up to R=0.65) with AWA changes close to the observing sites using regression analysis. The composite AWA for high-aerosol days (all daily PM2.5 above the 90th percentile) shows a similarly strong correlation between PM2.5 and AWA. The most prominent correlation occurs in the Midwestern US. Furthermore, the higher quantiles of PM2.5 levels are more sensitive to the changes in AWA. For example, we find that the averaged sensitivity of the 90th-percentile PM2.5 to changes in AWA is approximately 3 times as strong as the sensitivity of 10th-percentile PM2.5 at one site (Arendtsville, Pennsylvania; 39.92∘ N, 77.31∘ W). The higher values of the 90th percentile compared to the 50th percentile in quantile regression slopes are most prominent over the northeastern US. In addition, future changes in US PM2.5 based only on changes in climate are estimated to increase PM2.5 concentrations due to increased AWA in summer over areas where PM2.5 variations are dominated by meteorological changes, especially over the western US. Changes between current and future climates in AWA can explain up to 75 {\%} of PM2.5 variability using a linear regression model. Our analysis indicates that higher PM2.5 concentrations occur when a positive AWA anomaly is prominent, which could be critical for understanding how pollutants respond to changing atmospheric circulation as well as for developing robust pollution projections.

Recommended citation: Wang, Ye, Natalie Mahowald, Peter Hess, Wenxiu Sun and Gang Chen, 2022: The relationship between PM2.5 and anticyclonic wave activity during summer over the United States, Atmospheric Chemistry and Physics, 22, 7575--7592, doi:10.5194/acp-22-7575-2022.

Quantifying Eddy Generation and Dissipation in the Jet Response to Upper-versus Lower-level Thermal Forcing

Published in Journal of the Atmospheric Sciences, 2022

The relative roles of upper- and lower-level thermal forcing in shifting the eddy driven jet are investigated using a multi-level nonlinear quasi-geostrophic channel model. The numerical experiments show that the upper-level thermal forcing is more efficient in shifting the eddy-driven jet. The finite-amplitude wave activity diagnostics of numerical results show that the dominance of the upper-level thermal forcing over the lower-level thermal forcing can be understood from their different influence on eddy generation and dissipation that affects the jet shift. The upper-level thermal forcing shifts the jet primarily by affecting the baroclinic generation of eddies. The lower-level thermal forcing influences the jet mainly by affecting the wave breaking and dissipation. The former eddy response turns out to be more efficient for the thermal forcing to shift the eddy-driven jet.

Recommended citation: Nie, Yu, Yang Zhang, Gang Chen and Xiu-qun Yang, 2022: Quantifying Eddy Generation and Dissipation in the Jet Response to Upper-versus Lower-level Thermal Forcing, Journal of the Atmospheric Sciences, doi:10.1175/JAS-D-21-0307.1.

What Controls the Interannual Variability of the Boreal Winter Atmospheric River Activities over the Northern Hemisphere?

Published in Journal of Climate, 2022

Interannual variability of the winter AR activities over the Northern hemisphere is investigated. The leading modes of AR variability over the North Pacific and North Atlantic are first identified and characterized. Over the Pacific, the first mode is characterized by a dipole structure with enhanced AR frequency along the AR peak region at about 30° N and reduced AR frequency further north. The second mode exhibits a tri-pole structure with a narrow band of positive AR anomalies at about 30° N and sandwiched by negative anomalies. Over the Atlantic, the first mode exhibits an equatorward shift of the ARs with positive anomalies and negative anomalies located on the equatorward and poleward side of the AR peak region at about 40° N , respectively. The second mode is associated with the strengthening and eastward extension of the AR peak region which is sandwiched by negative anomalies. A large ensemble of atmospheric global climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6), which shows high skills in simulating these modes, is then used to quantify the roles of sea surface temperature (SST) forcing versus internal atmospheric variability in driving the formation of these modes. Results show that SST forcing explains about half of the variance for the Pacific leading modes, while that number drops to about a quarter for the Atlantic leading modes, suggesting higher predictability for the Pacific AR variability. Additional ensemble driven only by observed tropical SST is further utilized to demonstrate the more important role that tropical SST plays in controlling the Pacific AR variability while both tropical and extratropical SST exert comparable influences on the Atlantic AR variability.

Recommended citation: Ma, Weiming and Gang Chen, 2022: What Controls the Interannual Variability of the Boreal Winter Atmospheric River Activities over the Northern Hemisphere?, Journal of Climate, 1--39, doi:10.1175/JCLI-D-22-0089.1.

Why Seasonal Prediction of California Winter Precipitation Is Challenging

Published in Bulletin of the American Meteorological Society, 2022

Despite an urgent demand for reliable seasonal prediction of precipitation in California (CA) due to the recent recurrent and severe drought conditions, our predictive skill for CA winter precipitation remains limited. October hindcasts by the coupled dynamical models typically show a correlation skill of about 0.3 for CA winter (November–March) precipitation. In this study, an attempt is made to understand the underlying processes that limit seasonal prediction skill for CA winter precipitation. It is found that only about 25{\%} of interannual variability of CA winter precipitation can be attributed to influences by El Ni{~{n}}o–Southern Oscillation (ENSO). Instead, the year-to-year CA winter precipitation variability is primarily due to circulation anomalies independent from ENSO, featuring a circulation center over the west coast United States as a portion of a short Rossby wave train pattern over the North Pacific. Analyses suggest that dynamical models show nearly no skill in predicting these ENSO-independent circulation anomalies, thus leading to limited predictive skill for CA winter precipitation. Low predictability of these ENSO-independent circulation anomalies is further demonstrated by a large ensemble of atmospheric-only climate model simulations. While low predictability of the ENSO-independent circulation anomalies could be due to chaotic internal atmospheric processes over the mid- to high latitudes, possible underexploited predictability sources for CA precipitation in models are also discussed. This study pinpoints an urgent need for improved understanding of the formation mechanisms of ENSO-independent circulation anomalies over the U.S. West Coast for a breakthrough in seasonal prediction of CA winter precipitation.

Recommended citation: Jiang, Xianan, Duane E Waliser, Peter B Gibson, Gang Chen and Weina Guan, 2022: Why Seasonal Prediction of California Winter Precipitation Is Challenging, Bulletin of the American Meteorological Society, 103, E2688--E2700, doi:10.1175/BAMS-D-21-0252.1.

More frequent atmospheric rivers slow the seasonal recovery of Arctic sea ice

Published in Nature Climate Change, 2023

In recent decades, Arctic sea-ice coverage underwent a drastic decline in winter, when sea ice is expected to recover following the melting season. It is unclear to what extent atmospheric processes such as atmospheric rivers (ARs), intense corridors of moisture transport, contribute to this reduced recovery of sea ice. Here, using observations and climate model simulations, we find a robust frequency increase in ARs in early winter over the Barents–Kara Seas and the central Arctic for 1979–2021. The moisture carried by more frequent ARs has intensified surface downward longwave radiation and rainfall, caused stronger melting of thin, fragile ice cover and slowed the seasonal recovery of sea ice, accounting for 34{\%} of the sea-ice cover decline in the Barents–Kara Seas and central Arctic. A series of model ensemble experiments suggests that, in addition to a uniform AR increase in response to anthropogenic warming, tropical Pacific variability also contributes to the observed Arctic AR changes.

Recommended citation: Zhang, Pengfei, Gang Chen, Mingfang Ting, L. Ruby Leung, Bin Guan and Laifang Li, 2023: More frequent atmospheric rivers slow the seasonal recovery of Arctic sea ice, Nature Climate Change, doi:10.1038/s41558-023-01599-3.

How Do Different Processes Shape Temperature Probability Distributions? A Percentile-averaged Temperature Tendency Decomposition

Published in Journal of Climate, 2023

Studying temperature probability distributions and the physical processes that shape them is important for understanding extreme temperature events. Previous work has used a conditional mean temperature framework to reveal whether horizontal temperature advection drives temperature to extreme or median values at a specific location as a method to dynamically interpret temperature probability distributions. In this paper, we generalize this method to study how other processes shape temperature probability distributions and explore the diverse effects of horizontal temperature advection on temperature probability distributions at different locations and different temperature percentiles. We apply this generalized method to several representative regions to demonstrate its use. We find that temperature advection drives temperatures towards more extreme values over most land in the midlatitudes (i.e. cold air advection occurs during cold anomalies and warm air advection occurs during warm anomalies). In contrast, we find that horizontal temperature advection dampens temperature anomalies in some coastal summer monsoon regions, where extreme temperatures result from other processes, such as horizontal humidity advection and vertical temperature advection. By calculating the mean of processes conditioned on the temperature percentile, this method enables composite analysis of processes that contribute to events for all percentiles and a range of processes. We show examples of composites at different percentiles for certain processes and regions to illustrate the conditional mean analysis. This general approach may benefit future studies related to temperature probability distributions and extreme events.

Recommended citation: Quan, Heng, Boer Zhang, Stephen Bourguet, Marianna Linz and Gang Chen, 2023: How Do Different Processes Shape Temperature Probability Distributions? A Percentile-averaged Temperature Tendency Decomposition, Journal of Climate, 1--36, doi:10.1175/JCLI-D-22-0556.1.

Extreme stratospheric wave activity as harbingers of cold events over North America

Published in Communications Earth & Environment, 2023

Extreme cold events over North America such as the February 2021 cold wave have been suggested to be linked to stratospheric polar vortex stretching. However, it is not resolved how robustly and on which timescales the stratosphere contributes to the surface anomalies. Here we introduce a simple measure of stratospheric wave activity for reanalyses and model outputs. In contrast to the well-known surface influences of sudden stratospheric warmings (SSWs) that increase the intraseasonal persistence of weather regimes, we show that extreme stratospheric wave events are accompanied by intraseasonal fluctuations between warm and cold spells over North America in observations and climate models. Particularly, strong stratospheric wave events are followed by an increased risk of cold extremes over North America 5–25 days later. Idealized simulations in an atmospheric model with a well-resolved stratosphere corroborate that strong stratospheric wave activity precedes North American cold spells through vertical wave coupling. These findings potentially benefit the predictability of high-impact winter cold extremes over North America.

Recommended citation: Ding, Xiuyuan, Gang Chen, Pengfei Zhang, Daniela I. V. Domeisen and Clara Orbe, 2023: Extreme stratospheric wave activity as harbingers of cold events over North America, Communications Earth & Environment, 4, 187, doi:10.1038/s43247-023-00845-y.

Stratosphere‐Troposphere Coupling of Extreme Stratospheric Wave Activity in CMIP6 Models

Published in Journal of Geophysical Research: Atmospheres, 2023

Extreme stratospheric wave activity has been suggested to be connected to surface temperature anomalies, but some key processes are not well understood. Using observations, we show that the stratospheric events featuring weaker‐than‐normal wave activity are associated with increased North American (NA) cold extreme risks before and near the event onset, accompanied by less frequent atmospheric river (AR) events on the west coast of the United States. Strong stratospheric wave events, on the other hand, exhibit a tropospheric weather regime transition. They are preceded by NA warm anomalies and increased AR frequency over the west coast, followed by increased risks of NA cold extremes and north‐shifted ARs over the Atlantic. Moreover, these links between the stratosphere and troposphere are attributed to the vertical structure of wave coupling. Weak wave events show a wave structure of westward tilt with increasing altitudes, while strong wave events feature a shift from westward tilt to eastward tilt during their life cycle. This wave phase shift indicates vertical wave coupling and likely regional planetary wave reflection. Further examinations of CMIP6 models show that models with a degraded representation of stratospheric wave structure exhibit biases in the troposphere during strong wave events. Specifically, models with a stratospheric ridge weaker than the reanalysis exhibit a weaker tropospheric signal. Our findings suggest that the vertical coupling of extreme stratospheric wave activity should be evaluated in the model representation of stratosphere‐troposphere coupling.

Recommended citation: Ding, Xiuyuan, Gang Chen and Weiming Ma, 2023: Stratosphere‐Troposphere Coupling of Extreme Stratospheric Wave Activity in CMIP6 Models, Journal of Geophysical Research: Atmospheres, 128, doi:10.1029/2023JD038811.

Characterization of the aerosol vertical distributions and their impacts on warm clouds based on multi-year ARM observations

Published in Science of The Total Environment, 2023

Aerosol vertical distribution plays a crucial role in cloud development and thus precipitation since both aerosol indirect and semi-direct effects significantly depend on the relative position of aerosol layer in reference to cloud, but its precise influence on cloud remains unclear. In this study, we integrated multi-year Raman Lidar measurements of aerosol vertical profiles from the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) facility with available Value-Added Products of cloud features to characterize aerosol vertical distributions and their impacts on warm clouds over the continental and marine ARM atmospheric observatories, i.e., Southern Great Plains (SGP) and Eastern North Atlantic (ENA). A unimodal seasonal distribution of aerosol optical depths (AODs) with a peak in summer is found at upper boundary layer over SGP, while a bimodal distribution is observed at ENA for the AODs at lower levels with a major winter-spring maximum. The diurnal mean of upper-level AOD at SGP shows a maximum in the early evening. According to the relative positions of aerosol layers to clouds we further identify three primary types of aerosol vertical distribution, including Random, Decreasing, and Bottom. It is found that the impacts of aerosols on cloud may or may not vary with aerosol vertical distribution depending on environmental conditions, as reflected by the wide variations of the relations between AOD and cloud properties. For example, as AOD increases, the liquid water paths (LWPs) tend to be reduced at SGP but enhanced at ENA. The relations of cloud droplet effective radius with AOD largely depend on aerosol vertical distributions, particularly showing positive values in the Random type under low-LWP condition ({\textless}50 g m−2). The distinct features of aerosol-cloud interactions in relation to aerosol vertical distribution are likely attributed to the continental-marine contrast in thermodynamic environments and aerosol conditions between SGP and ENA.

Recommended citation: Lin, Yun, Yoshihide Takano, Yu Gu, Yuan Wang, Shujun Zhou, Tianhao Zhang, Kuilin Zhu, Jingyu Wang, Bin Zhao, Gang Chen, Damao Zhang, Rong Fu and John Seinfeld, 2023: Characterization of the aerosol vertical distributions and their impacts on warm clouds based on multi-year ARM observations, Science of The Total Environment, 904, 166582, doi:10.1016/j.scitotenv.2023.166582.

Evaluating the Representations of Atmospheric Rivers and Their Associated Precipitation in Reanalyses With Satellite Observations

Published in Journal of Geophysical Research: Atmospheres, 2023

Atmospheric rivers (ARs) are filaments of enhanced horizontal moisture transport in the atmosphere. Due to their prominent role in the meridional moisture transport and regional weather extremes, ARs have been studied extensively in recent years. Yet, the representations of ARs and their associated precipitation on a global scale remains largely unknown. In this study, we developed an AR detection algorithm specifically for satellite observations using moisture and the geostrophic winds derived from 3D geopotential height field from the combined retrievals of the Atmospheric Infrared Sounder and the Advanced Microwave Sounding Unit on NASA Aqua satellite. This algorithm enables us to develop the first global AR catalog based solely on satellite observations. The satellite‐based AR catalog is then combined with the satellite‐based precipitation (Integrated Muti‐SatellitE Retrievals for GPM) to evaluate the representations of ARs and AR‐induced precipitation in reanalysis products. Our results show that the spreads in AR frequency and AR length distribution are generally small across data sets, while the spread in AR width is relatively larger. Reanalysis products are found to consistently underestimate both mean and extreme AR‐related precipitation. However, all reanalyses tend to precipitate too often under AR conditions, especially over low latitude regions. This finding is consistent with the “drizzling” bias which has plagued generations of climate models. Overall, the findings of this study can help to improve the representations of ARs and associated precipitation in reanalyses and climate models.

Recommended citation: Ma, Weiming, Gang Chen, Bin Guan, Christine A Shields, Baijun Tian and Emilio Yanez, 2023: Evaluating the Representations of Atmospheric Rivers and Their Associated Precipitation in Reanalyses With Satellite Observations, Journal of Geophysical Research: Atmospheres, 128, doi:10.1029/2023JD038937.

The role of interdecadal climate oscillations in driving Arctic atmospheric river trends

Published in Nature Communications, 2024

Atmospheric rivers (ARs), intrusions of warm and moist air, can effectively drive weather extremes over the Arctic and trigger subsequent impact on sea ice and climate. What controls the observed multi-decadal Arctic AR trends remains unclear. Here, using multiple sources of observations and model experiments, we find that, contrary to the uniform positive trend in climate simulations, the observed Arctic AR frequency increases by twice as much over the Atlantic sector compared to the Pacific sector in 1981-2021. This discrepancy can be reconciled by the observed positive-to-negative phase shift of Interdecadal Pacific Oscillation (IPO) and the negative-to-positive phase shift of Atlantic Multidecadal Oscillation (AMO), which increase and reduce Arctic ARs over the Atlantic and Pacific sectors, respectively. Removing the influence of the IPO and AMO can reduce the projection uncertainties in near-future Arctic AR trends by about 24{\%}, which is important for constraining projection of Arctic warming and the timing of an ice-free Arctic.

Recommended citation: Ma, Weiming, Hailong Wang, Gang Chen, L Ruby Leung, Jian Lu, Philip J Rasch, Qiang Fu, Ben Kravitz, Yufei Zou, John J Cassano and Wieslaw Maslowski, 2024: The role of interdecadal climate oscillations in driving Arctic atmospheric river trends, Nature Communications, 15, 2135, doi:10.1038/s41467-024-45159-5.

Hydrologic cycle weakening in hothouse climates

Published in Science Advances, 2024

The hydrologic cycle has wide impacts on the ocean salinity and circulation, carbon and nitrogen cycles, and the ecosystem. Under anthropogenic global warming, previous studies showed that the intensification of the hydrologic cycle is a robust feature. Whether this trend persists in hothouse climates, however, is unknown. Here, we show in climate models that mean precipitation first increases with rising surface temperature, but the precipitation trend reverses when the surface is hotter than {~{}}320 to 330 kelvin. This nonmonotonic phenomenon is robust to the cause of warming, convection scheme, ocean dynamics, atmospheric mass, planetary rotation, gravity, and stellar spectrum. The weakening occurs because of the existence of an upper limitation of outgoing longwave emission and the continuously increasing shortwave absorption by H 2 O and is consistent with atmospheric dynamics featuring the strong increase of atmospheric stratification and marked reduction of convective mass flux. These results have wide implications for the climate evolutions of Earth, Venus, and potentially habitable exoplanets.

Recommended citation: Liu, Jiachen, Jun Yang, Feng Ding, Gang Chen and Yongyun Hu, 2024: Hydrologic cycle weakening in hothouse climates, Science Advances, 10, doi:10.1126/sciadv.ado2515.

The Role of Cloud Radiative Effects in the Propagating Southern Annular Mode

Published in Journal of Geophysical Research: Atmospheres, 2024

The Southern Annular Mode (SAM) is the most dominant natural mode of variability in the mid‐latitudes of the Southern Hemisphere (SH). However, both the sign and magnitude of the feedbacks from the diabatic processes, especially those associated with clouds, onto the SAM remain elusive. By applying the cloud locking technique to the Energy Exascale Earth System Model (E3SM) atmosphere model, this study isolates the positive feedback from the cloud radiative effect (CRE) to the SAM. Feedback analysis based on a wave activity‐zonal momentum interaction framework corroborates this weak but positive feedback. While the magnitude of the CRE feedback appears to be secondary compared to the feedbacks from the dry and other diabatic processes, the indirect CRE effects through the interaction with other dynamical and thermodynamical processes appear to play as important a role as the direct CRE in the life cycle of the SAM. The cross‐EOF analysis further reveals the obstructive effect of the interactive CRE on the propagation mode of the SH zonal wind directly through the CRE wave source and/or indirectly through modulating other diabatic processes. As a result, the propagation mode becomes more persistent and the SAM it represents becomes more predictable when the interactive CRE is disabled by cloud locking. Future efforts on inter‐model comparisons of CRE‐denial experiments are important to build consensus on the dynamical feedback of CRE.

Recommended citation: Lu, Jian, Bryce E Harrop, Sandro W Lubis, Samuel Smith, Gang Chen and L Ruby Leung, 2024: The Role of Cloud Radiative Effects in the Propagating Southern Annular Mode, Journal of Geophysical Research: Atmospheres, 129, doi:10.1029/2023JD040428.

Wintertime extreme warming events in the high Arctic: characteristics, drivers, trends, and the role of atmospheric rivers

Published in Atmospheric Chemistry and Physics, 2024

ABSTRACT: . An extreme warming event near the North Pole, with 2 m temperature rising above 0 °C, was observed in late December 2015. This specific event has been attributed to cyclones and their associated moisture intrusions. However, little is known about the characteristics and drivers of similar events in the historical record. Here, using data from European Centre for Medium-Range Weather Forecasts Reanalysis, version 5 (ERA5), we study these winter extreme warming events with 2 m temperature over a grid point above 0 °C over the high Arctic (poleward of 80° N) that occurred during 1980–2021. In ERA5, such wintertime extreme warming events can only be found over the Atlantic sector. They occur rarely over many grid points, with a total absence during some winters. Furthermore, even when occurring, they tend to be short-lived, with the majority of the events lasting for less than a day. By examining their surface energy budget, we found that these events transition with increasing latitude from a regime dominated by turbulent heat flux into the one dominated by downward longwave radiation. Positive sea level pressure anomalies which resemble blocking over northern Eurasia are identified as a key ingredient in driving these events, as they can effectively deflect the eastward propagating cyclones poleward, leading to intense moisture and heat intrusions into the high Arctic. Using an atmospheric river (AR) detection algorithm, the roles of ARs in contributing to the occurrence of these extreme warming events defined at the grid-point scale are explicitly quantified. The importance of ARs in inducing these events increases with latitude. Poleward of about 83° N, 100 {\%} of these events occurred under AR conditions, corroborating that ARs were essential in contributing to the occurrence of these events. Over the past 4 decades, both the frequency, duration, and magnitude of these events have been increasing significantly. As the Arctic continues to warm, these events are likely to increase in both frequency, duration, and magnitude, with great implications for the local sea ice, hydrological cycle, and ecosystem.

Recommended citation: Ma, Weiming, Hailong Wang, Gang Chen, Yun Qian, Ian Baxter, Yiling Huo and Mark W Seefeldt, 2024: Wintertime extreme warming events in the high Arctic: characteristics, drivers, trends, and the role of atmospheric rivers, Atmospheric Chemistry and Physics, 24, 4451--4472, doi:10.5194/acp-24-4451-2024.

North American cooling signature of strong stratospheric wave events depends on the QBO phase

Published in Environmental Research: Climate, 2024

Extreme stratospheric wave activity has been linked to surface cold extremes over North America, but little is known whether the Quasi-biennial Oscillation (QBO) plays a role in this linkage. Here, by comparing strong stratospheric wave events during the westerly phase (wQBO) with those during the easterly phase (eQBO), we show that the cooling signature following strong wave events depends on the QBO phase in observations. During wQBO, strong wave events are followed by an increased risk of North American cold extremes and a vertical structure shift from a westward phase tilt to an eastward tilt. However, strong wave events under eQBO do not change the cold risk nor alter the vertical tilt. We further examine this dependence on QBO in QBO-resolving climate models, finding that the cooling signature of strong wave events in models is largely insensitive to QBO phases. This insensitivity is suggested to be linked to model biases in the stratospheric wave representation.{\&}{#}xD;

Recommended citation: Ding, Xiuyuan, Gang Chen and Gudrun Magnusdottir, 2024: North American cooling signature of strong stratospheric wave events depends on the QBO phase, Environmental Research: Climate, doi:10.1088/2752-5295/ad53f6.

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