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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

Download paper: journal website

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 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…

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…

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.

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.

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…

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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

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.

Facilitating International Collaboration on Climate Change Research

Published in Bulletin of the American Meteorological Society, 2020

ABSTRACT:

He, Hao, Hailong Wang, Zhaoyong Guan, Haishan Chen, Qiang Fu, Muyin Wang, Xiquan Dong, Chunguang Cui, Likun Wang, Bin Wang, Gang Chen, Zhanqing Li and Da-Lin Zhang, 2020: Facilitating International Collaboration on Climate Change Research, Bulletin of the American Meteorological Society, 101, E650--E654, doi:10.1175/BAMS-D-19-0320.1.

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.

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:

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.

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.

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.

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.

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.

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.

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.

talks

teaching

Teaching experience 1

Undergraduate course, University 1, Department, 2014

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Teaching experience 2

Workshop, University 1, Department, 2015

This is a description of a teaching experience. You can use markdown like any other post.