Simulation of the East Asian Subtropical Westerly Jet Stream with GFDL AGCM (AM2.1)

The present study validated the capability of the AM2.1,a model developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL),in reproducing the fundamental features of the East Asian Subtropical Westerly Jet Stream (EASWJ).The main behaviors of the EASWJ are also investigated through the reanalysis of observational NCEP/NCAR data.The mean state of the EASWJ,including its intensity,location,structure,and seasonal evolution is generally well-portrayed in the model.Compared with the observation,the model tends to reproduce a weaker jet center.And,during summer,the simulated jet center is northward-situated.Results also demonstrate the model captures the variability of EASWJ during summer well.The results of the empirical orthogonal function (EOF) applied on the zonal wind at 200 hPa (U200) over East Asia for both the observation and simulation indicate an inter-decadal shift around the late 1970s.The correlation coefficient between the corresponding principle components is as great as 0.42 with significance at the 99% confidence level.

WINTERTIME MIDDLE EAST SUBTROPICAL WESTERLY JET STREAM INTERANNUAL VARIATION CHARACTERISTICS AND ITS POSSIBLE PHYSICAL FACTORS

Using National Centers for Environmental Prediction/Department of Energy(NCEP/DOE) monthly reanalysis data and an extended reconstruction of the sea surface temperature data provided by National Oceanic and Atmospheric Administration, the basic characteristics of the interannual variation in the wintertime Middle East subtropical westerly jet stream(MEJ) and its possible physical factors are studied. The results show that the climatological mean MEJ axis extends southwestward-northeastward and that its center lies in the northwest part of the Arabian Peninsula. The south-north shift of the MEJ axis and its intensity show obvious interannual variations that are closely related to the ElNio-Southern Oscillation(ENSO) and the mid-high latitude atmospheric circulation. The zonal symmetric response of the Asian jet to the ENSO-related tropical convective forcing causes the MEJ axis shift, and the Arctic Oscillation(AO)causes the middle-western MEJ axis shift. Due to the influences of both the zonal symmetric response of the Asian jet to the ENSO-related tropical convective forcing and the dynamical role of the AO, an east-west out-of-phase MEJ axis shift is observed. Furthermore, the zonal asymmetric response to the ENSO-related tropical convective forcing can lead to an anomalous Mediterranean convergence(MC) in the high troposphere. The MC anomaly excites a zonal wave train along the Afro-Asian jet, which causes the middle-western MEJ axis shift. Under the effects of both the zonal symmetric response to the ENSO-related tropical convective forcing and the wave train along the Afro-Asian jet excited by the MC anomaly, an east-west in-phase MEJ axis shift pattern is expressed. Finally, the AO affects the MEJ intensity, whereas the East Atlantic(EA) teleconnection influences the middle-western MEJ intensity. Under the dynamical roles of the AO and EA, the change in the MEJ intensity is demonstrated.

The Asian Subtropical Westerly Jet Stream in CRA-40, ERA5, and CFSR Reanalysis Data:Comparative Assessment

The Asian subtropical westerly jet(AWJ) exerts crucial influences on Eurasian continent weather and climate. This paper analyzes the advantages and limitations of CRA-40, which is China's first generation 40-yr(1979–2018) global atmosphere and land reanalysis product, in describing the characteristics of AWJ, compared with the ECMWF Reanalysis version 5(ERA5) and NCEP Climate Forecast System Reanalysis(CFSR). The results show a close agreement across the three reanalyses on the whole.(1) In terms of climatology, overall differences of 200-h Pa zonal wind across the three reanalyses are within ± 0.5 m s^(-1)(i.e., ± 2%). Large differences with maxima of ± 2 m s^(-1)(±5%) appear over the Iranian Plateau and south of the Tibetan Plateau in the mid–upper troposphere in winter.(2) For seasonal cycle, the position and intensity of the AWJ centers in the three reanalyses are highly consistent, with correlation coefficient over 0.98. But there are some discrepancies in the zonal shift of the western AWJ center during the transition season.(3) On the interannual timescale, intensity of all AWJ centers varies consistently among the three reanalyses, while larger differences appear in their meridional displacement, especially in the eastern AWJ center.(4)For long-term variations, the three reanalyses all present a significant northward movement of the westerly jet axis in winter, and a southward displacement over central Asia(40°–80°E) and a northward migration over East Asia(80°–110°E) in summer. Thus, this study has provided confidence that CRA-40 has comparable performance with ERA5 and CFSR in depicting the characteristics of AWJ.

Applicability Evaluation of NCEP/NCAR Reanalysis Temperature,Geopotential Height and Wind Field Data in the Upper Troposphere and Lower Stratosphere

By using the data in 169 sounding stations over the world,NCEP/NCAR reanalysis data were tested,and the distribution characteristics of standard errors of geopotential height,temperature and wind speed field from the upper troposphere to the lower stratosphere over the world(most were the land zone) were analyzed.The results showed that the standard error distribution of reanalysis wind speed field data was mainly affected by the jet stream zone.There existed the obvious difference between the jet stream zone and the actual wind field.The distribution of standard error in the wind speed field had the obvious seasonal difference in winter,summer,and the average deviation was larger near the coastline.The high value zones of standard errors of reanalysis geopotential height and temperature field mainly concentrated in the low-latitude region in the Eastern Hemisphere(Indian Ocean coast).The distribution of standard error was basically consistent with average error.Therefore,the standard error could be explained well by the average error.The standard errors of reanalysis temperature and geopotential height data in the inland zone were lower.The high value zone mainly distributed along the coastline,and the average error of wind speed field was bigger near the coastline.It closely related to the quality of data in the sounding stations,the regional difference and the fact that the land observation stations were dense,and the ocean observation stations were fewer.

The Response of the East Asian Summer Monsoon to Strong Tropical Volcanic Eruptions

A 600-year integration performed with the Bergen Climate Model and National Centers for Environmental Prediction/National Center for Atmospheric Research （NCEP/NCAR） reanalysis data were used to investigate the impact of strong tropical volcanic eruptions on the East Asian summer monsoon （EASM） and EASM rainfall.Both the simulation and NCEP/NCAR reanalysis data show a weakening of the EASM in strong eruption years.The model simulation suggests that North and South China experience droughts and the Yangtze-Huaihe River Valley experiences floods during eruption years.In response to strong tropical volcanic eruptions,the meridional air temperature gradient in the upper troposphere is enhanced,which leads to a southward shift and an increase of the East Asian subtropical westerly jet stream （EASWJ）.At the same time,the land-sea thermal contrast between the Asian land mass and Northwest Pacific Ocean is weakened.The southward shift and increase of the EASWJ and reduction of the land-sea thermal contrast all contribute to a weakening of the EASM and EASM rainfall anomaly.

Western Pacific Jet Stream Anomalies at 200 hPa in Winter Associated with Oceanic Surface Heating and Transient Eddy Activity

The relationships between the 200-hPa westerly jet stream anomalies over the East Asian coastal waterwestern Pacific （WPJS）, and the oceanic surface heating and synoptic-scale transient eddy （STE） activity anomalies over the North Pacific in wintertime are examined by using ERA-40 and NCEP/NCAR reanalysis data. The analysis demonstrates that the surface heating and the STE anomalies have different patterns, corresponding to the three WPJS anomalous modes, respectively. In the first WPJS anomalous mode, the WPJS main part shows no robust anomaly. The anomalous westerly wind, occurring over the mid-latitude central-eastern Pacific past the date line is associated with the anomalous heating presenting both in the tropical central-eastern Pacific past the date line and the center of the North Pacific basin. Meanwhile, the STE anomaly appears around the region of the anomalous zonal wind. The fluctuation in jet strength shown in the second WPJS mode is strongly related to the heating anomaly in the Kuroshio Current region and the STE anomaly in the jet exit region. The third mode demonstrates a northward/southward shift of the WPJS, which has a statistical connection with a south-north dipolar pattern of the heating anomaly in the western North Pacific separated at 35°N. Meanwhile, the STE spatial displacement is in conjunction with jet shifts in the same direction. The heating anomaly has a close connection with the atmospheric circulation, and thus changes the mid-latitude baroclinicity, leading to the STE anomaly, which then reinforces the WPJS anomaly via internal atmospheric dynamics.

A tripole winter precipitation change pattern around the Tibetan Plateau in the late 1990s

Classical monsoon dynamics considers the winter/spring snow amount on the Tibetan Plateau(TP)as a major factor driving the East Asian summer monsoon(EASM)for its direct influence on the land-sea thermal contrast.Actually,the TP snow increased and decreased after the late 1970s and 1990s,respectively,accompanying the two major interdecadal changes in the EASM.Although studies have explored the possible mechanisms of the EASM interdecadal variations,and change in TP snow is considered as one of the major drivers,few studies have illustrated the underlying mechanisms of the interdecadal changes in the winter TP snow.This study reveals a tripole pattern of change,with decreased winter precipitation over the TP and an increase to its north and south after the late 1990s.Further analyses through numerical experiments demonstrate that the tropical Pacific SST changes in the late 1990s can robustly affect the winter TP precipitation through regulating the Walker and regional Hadley circulation.The cooling over the tropical central-eastern Pacific can enhance the Walker circulation cell over the Pacific and induce ascending motion anomalies over the Indo-Pacific region.These anomalies further drive descending motion anomalies over the TP and ascending motion anomalies to the north through regulating the regional Hadley circulation.Therefore,the positive-negative-positive winter precipitation anomalies around the TP are formed.This study improves the previously poor understanding of TP climate variation at interdecadal timescales.

Alternation of the Atmospheric Teleconnections Associated with the Northeast China Spring Rainfall during a Recent 60-Year Period

Northeast China(NEC)is China’s national grain production base,and the local precipitation is vital for agriculture during the springtime.Therefore,understanding the dynamic origins of the NEC spring rainfall(NECSR)variability is of socioeconomic importance.This study reveals an interdecadal change in the atmospheric teleconnections associated with the NECSR during a recent 60-year period(1961-2020).Before the mid-1980s,NECSR had been related to a Rossby wave train that is coupled with extratropical North Atlantic sea surface temperature(SST),whereas,since the mid-1980s,NECSR has been linked to a quite different Rossby wave train that is coupled with tropical North Atlantic SST.Both Rossby wave trains could lead to enhanced NECSR through anomalous cyclones over East Asia.The weakening of the westerly jet over North America is found to be mainly responsible for the alternation of the atmospheric teleconnections associated with NECSR during two epochs.

Wave Sources, Energy Propagation and Conversion for Anomalous Rossby Wave Activities Along the West Asian Jet Stream

Characteristics of the wave sources, energy propagation and conversion for anomalous Rossby wave activities （RWAs） along the West Asian jet stream （WAJS） in summer are examined based on the NCEP/NCAR （National Centers for Environmental Prediction/National Center for Atmospheric Research） reanalysis data from 1958 to 2003, using the vorticity source equation, the Eliassen-Palm （EP） flux, and the wave energy equation under diabatic heating. The study aims to find the dynamical causes for RWA anomalies along the WAJS and to improve the understanding of mid-high latitude circulation anomalies. The results show that the negative vorticity source and the strong EP flux divergence over the Mediterranean Sea and the North Atlantic Scandinavian Peninsula area act as the wave sources for RWA anomalies along the WAJS. When the intensity and position of the wave sources are anomalous, the excited eastward-propagating RWA along the WAJS also behaves anomalously. In strong （weak） years of RWA, Rossby waves excited by the strong divergence of EP fluxes over the Iceland Scandinavian Peninsula area （east to the Scandinavian Peninsula） propagate eastward and southeastward. The eastward propagating waves become strengthened （weakened） after turning southeastward near the Ural Mountains and then entering the Asian subtropical westerly jet stream （ASWJS） over the Caspian Sea-Aral Sea-Xinjiang. The southeastward propagating waves also strengthen （weaken） after directly entering the ASWJS over the eastern Mediterranean-the Black Sea. Furthermore, the divergence of EP fluxes over the Mediterranean also strengthens （weakens） in the strong （weak） years, so they jointly bring about the strong （weak） RWA along the WAJS. Finally, the perturbation available potential energy （PAPE） along the WAJS （15°- 60°E） produced by diabatic heating, is far greater than the conversion from the kinetic energy of the basic flow into the perturbation kinetic energy and from the available potential energy of the basic flow into PAPE. The RWA along the WAJS looks stronger （weaker） than normal when the PAPEs produced by diabatic heating over the Iranian Plateau and West Asia significantly strengthen （weaken）, and therefore they are also the energy sources of RWA anomalies.

Variations of Winter Precipitation over Southeastern China in Association with the North Atlantic Oscillation

The synoptic-scale winter precipitation variations over southeastem China （22°-32°N, 105°-125°E） and their asso- ciation with the North Atlantic Oscillation （NAO） during 1951-2007 are investigated in this paper. The variability of wintertime precipitation is characterized by meridional displacement of its maximum center. Two precipitation re- gimes, with maximum centers located over the Yangtze and Pearl River basins, are identified via cluster analysis. Time-lagged analyses suggest that the two precipitation regimes are connected with the decaying phases of positive NAO （NAO＋） events of different amplitudes. A strong （medium） NAO＋ event is defined as one when the maximum amplitude of the NAO index exceeds 1.0 （in the range of 0.7-1.0） for at least 4 consecutive days and drops to less than 0.3 within 7 days following the peak index. After the peak of a strong NAO＋, southerly winds expand north- ward to the Yangtze River （about 30°N）, a northeast-southwest-tilted trough migrates to east of Lake Baikal, and cold air intrudes into central eastern China; thus, precipitation is strengthened over the Yangtze River basin where warm and cold air masses converge. In comparison, during the decaying phase of medium NAO＋ events, the south- erly winds are relatively weak, and precipitation tends to be enhanced at lower latitudes （around 25°N）. Further ana- lysis indicates that downstream Rossby-wave propagation may account for the latitudinal expansion of the southerly wind anomalies over the eastern coastal area of China during the decaying phase of NAO＋ events of different strengths.

Interdecadal Variations of Precipitation and Temperature in China Around the Abrupt Change of Atmospheric Circulation in 1976

The interdecadal characteristics of rainfall and temperature in China before and after the abrupt change of the general circulation in 1976 are analyzed using the global 2.5°×2.5° monthly mean reanalysis data from the National Centers for Environmental Prediction of US and the precipitation and temperature data at the 743 stations of China from the National Climate Center of China. The results show that after 1976, springtime precipitation and temperature were anomalously enhanced and reduced respectively in South China, while the reverse was true in the western Yangtze River basin. In summer, precipitation was anomalously less in South China, more in the Yangtze River basin, less again in North China and more again in Northeast China, showing a distribution pattern alternating with negative and positive anomalies （＂ , ＋, -, ＋＂）. Meanwhile, temperature shows a distribution of warming in South China, cooling in the Yangtze and Huaihe River basins, and warming again in northern China. In autumn, precipitation tended to decrease and temperature tended to increase in in South China and warming was most parts of the country. In winter, the trend across all parts of China. precipitation increased moderately The interdecadal decline of mean temperature in spring and summer in China was mainly due to the daily maximum temperature variation, while the interdecadal increase was mainly the result of the minimum temperature change. The overall warming in autumn （winter） was mostly influenced by the minimum （maximum） temperature variation. These changes were closely related to the north-south shifts of the ascending and descending branches of the Hadley cell, the strengthening and north-south progression of the westerly jet stream, and the atmospheric stratification and water vapor transport conditions.

Paleoclimatic proxies from global closed basins and the possible beginning of Anthropocene

Global closed basins,occupying almost one fifth of the world's land area,spatially coincide with arid and semiarid areas.Paleoclimatic proxies can indicate basin-wide environmental change and human activity.However,previous studies have not approached the use of proxies in the same way to reconstruct natural and anthropogenic processes at regional and global scales.Here we present a regional study to investigate the basic processes of paleoclimatic proxies,from a typical closed-basin system in arid China.We use multiple paleoclimatic proxies of surface samples and sediments,as well as groundwater and sediment ages to study environmental change and human activity.We then establish a dataset for paleoclimatic proxies from global closed basins and do a numerical analysis on it.Regional studies verify that human activity greatly impacts paleoclimatic proxies,especially with regard to surface samples,as well as groundwater age,but Holocene sediments are less affected.Results from global studies indicate that the major changing trend of the wet/dry status of closed basins is associated with the movement of the westerly jet streams controlled by long-term changes in winter insolation.There is an abrupt change between 1800 AD and 1900 AD,according to a numerical synthesis of paleoclimatic proxies from global closed basins,which can be linked to human impact.We suggest this time period can be considered as a start point for the Anthropocene based on the sedimentary evidence of closed basins,globally.

Shifting of summertime weather extremes in Western Europe during 2012-2020

Over the past decades,droughts and heatwaves frequently appeared in Western Europe(45°-65°N,10°W-20°E)during boreal summer,causing huge impacts on human society and ecosystems.Although these extremes are projected to increase in both frequency and intensity under a warming climate,our knowledge of their interdecadal variations and causes is relatively limited.Here we show that the droughts and heatwaves in Western Europe have shifted in their trends in the last decade:for 1979-2012,wind speed and precipitation have both strengthened in Western Europe;for 2012-2020,however,Western Europe have experienced declined wind speed,decreased precipitation,and higher air temperature,leading to more frequent droughts and heatwaves there.Recent changes in the WE climate and extremes are related to the variations of the North Atlantic westerly jet stream.In 1979-2012(2012-2020),the westerly jet stream shifted equatorward(poleward),which enhanced(reduced)transportation of water vapor fluxes from the North Atlantic Ocean to the European land areas,resulting in wetter(drier)surface in Western Europe.Further analysis suggests that phase changes in the Pacific Decadal Oscillation could have played a key role in regulating the position of the jet stream,providing important implications for decadal predictions of the Western Europe summertime climate and weather extremes.