Interdecadal Variations Of The South Asian High:Change,Mechanism And Projection

China lies between the largest Eurasian Continent and the largest Pacific Ocean in the world,which is significantly impacted by the East Asian Summer Monsoon(EASM).The EASM brings abundant precipitation to China,which is important for people's life and agriculture.However,it also often leads to climate extreme events and disasters,seriously threatening the ecosystem,society,economy,and Chinese people's life.As an important component of the EASM,the South Asian High(SAH)is characterized by subseasonal,seasonal,interannual,and interdecadal variations,exerting significant influence on the climate of China in summer.Therefore,it is of great meaning for disaster prevention and mitigation in China to investigate the changes in SAH and its associated mechanism.Traditionally,the SAH in most studies is measured by geopotential height,which is significantly affected by the lifting effect under global warming.Consequently,the interdecadal variations of SAH reflected by the geopotential height may be problematical.Thus,this paper will reinvestigate the decadal changes of SAH and associated mechanisms using the eddy geopotential height,which is obtained by subtracting the zonal mean between 0° and 40° N from the geopotential height.The main conclusions are summarized as follows.(1)Decadal changes of SAH under the background of global warming and its contribution to the Yangtze-River-flooding-and-North-China-drought patternThe decadal intensification of the SAH after the late 1970 s,which is determined based on the geopotential height,is suspicious due to the lifting effect upon geopotential height caused by global warming.Based on multiple reanalysis datasets,the SAH reflected by eddy geopotential height,wind and relative vorticity are relatively weak during 1980–2018 compared to that during 1950–1979.This decadal weakening of the SAH after 1979 is barotropic from the mid-to-upper troposphere,which can also be observed in the radiosonde observation data.The SAH could affect precipitation by modulating the East Asian westerly jet(EAWJ)in the upper troposphere and the western Pacific subtropical high(WPSH)in the lower troposphere.Specifically,the weakened SAH after the late 1970 s well matched with the decadal southward shift of the EAWJ.The southward shift of the EAWJ could cause ascending motions over the Yangtze River Valley and descending motions over North China,leading to more rainfall in the Yangtze River Valley and less rainfall in North China after the late 1970 s.Moreover,a weakened and westward shifted SAH is related to the positive relative vorticity advection in its downstream,which may promote ascending motions over the Northwest Pacific and benefit the formation of a cyclonic anomaly in the lower troposphere.This cyclonic circulation is closely connected to the situation that the WPSH weakens and tends to shift eastward after the late1970 s.The weakened EASM is also responsible for the rain band in the Yangtze River Valley.Hence,the decadal weakening of the SAH after the late 1970 s may contribute to the YangtzeRiver-flooding-and-North-China-drought pattern through its connection with other circulation systems of EASM.(2)Contributions of external forcings to the decadal decline of SAHBased on the latest detection and attribution project of CMIP6,the influence of external forcing on the decadal weakening of the SAH is quantitatively assessed using “good” models that reasonably simulated the decadal decline of the SAH.Anthropogenic aerosol played a primary role in the decadal decline of the SAH.Approximately one-third of the decadal decline could be yielded by the aerosol forcing runs.The increased aerosol likely drove a cooling surface over the Tibetan Plateau and East China via its effect on radiation.Consequently,the weakened heat source over the Tibetan Plateau and associated thermodynamic effects over East China would have driven a cooling of eddy temperature and cyclonic anomalies in the upper troposphere,respectively,thereby causing the decline of the SAH.By contrast,greenhouse gas and natural external forcing displayed homogenized and only very slight effects on upper tropospheric temperature,respectively,thus reproducing almost completely insignificant contributions to the decadal weakening of the SAH in the late 1970 s.(3)The possible changes of SAH in the future under different warming scenariosThe capability of CMIP5/6 models for simulating the decadal decline of the SAH is assessed.Both the CMIP5 and CMIP6 models can yield better simulations of eddy geopotential height compared to geopotential height simulations with smaller model spreads.Moreover,CMIP6 models demonstrate higher skills than CMIP5 in the simulation of spatial distribution,spatial variability,and decadal variations of the SAH,which is attributed to the more reasonable simulations of upper-tropospheric eddy temperature by CMIP6.Furthermore,changes in the SAH are projected based on H' simulated by those good CMIP6 models.In the 21 st century under the low emission scenario(SSP1-2.6),the SAH may experience an intensification after the late 2040 s.Under the high emission scenario(SSP5-8.5),the SAH may enter the negative phase from the late 2030 s to the mid-2070 s and enter the positive phase during another period in the 21 st century.The internal climate variability may play a more important role in influencing the decadal variations of SAH compared to the anthropogenic forcing in the future21 st century,although both the external forcing and internal climate variability are important for the historical decadal changes of SAH.(4)Predictability of SAHThe evaluation of experiments from CMIP6 DCPP displays an unsatisfied prediction of the decadal changes of SAH in the historical period.Most models failed to capture the features of SAH decadal change in the late 1970 s.However,the CFSv2 model has good predictive skills in the interannual prediction of SAH.CFSv2 can predict the changes of summer SAH ahead of6 months.Furthermore,Arctic Oscillation(AO)and Antarctic Oscillation(AAO)are the main circulation modes in the Northern and Southern Hemispheres,respectively.Both of them have a significant influence on the variability of SAH.Thus,the capabilities of numerical models for the AO and AAO predictions are evaluated.All of the models can reasonably capture the zonally symmetric pattern of the AO and AAO,but they fail to predict the interannual variations of AO and AAO.To improve the AO and AAO index prediction accuracy of numerical models,a dynamical-statistical model is established utilizing the interannual increment approach and two predictors,namely,the observed preceding sea ice and the concurrent model-predicted SST.This method shows promise for significantly improving the prediction capabilities of the AO and AAO.The above analysis not only helps to understand the change of SAH from the perspective of atmospheric internal variability but also lay a foundation for further predicting the SAH.