Interdecadal Transition Of Extreme Summer Heat In Central Asia In The Mid-1990s And Its Possible Mechanis

Based on the CPC daily maximum temperature,NCEP/NCAR reanalysis dataset,and CMIP6 simulations,this study used the relative threshold to define the frequency(high temperature days,HAD)and intensity(HI)of extreme high temperatures(EHTs)in Central Asia(CA).The interdecadal shift in the mid-1990s and its possible mechanism of CA’s EHTs in last 40 years(1979-2020)were analyzed.The main conclusions are as follows:(1)Natural factors may be the main reason for the lower rate of increase in CA’s EHTs compared to neighbouring regions.The "+-+" latitudinal tripolar structure of summer EHTs(HAD and HI)of Eurasian for 1979-2020 and the interdecadal shift in the mid-1990 s are the major features of natural changes,while its consistent increasing spatial pattern is mainly due to human-induced warming.Under the natural effects,the rates of HAD and HI in CA are-0.07days/a and-0.01 ℃/a,respectively,lower than those in Eastern Europe(EE)and Northern China(NC).Under the influence of global warming,the rates of EHTs for three regions increase to different degrees,especially in CA from negative to positive,but it was still lower than EE and NC.(2)Based on the historical simulations of six CMIP6 models with full-forcing(including anthropogenic and natural forcing)and natural-forcing-only(hist-nat),it is found that the frequency and intensity of CA in the last 40 years are reduced under natural forcing,while in reality the increase of those is due to anthropogenic effects.Using the probability ratio(PR)and the fraction of attributable risk(FAR)metrics,it was found that anthropogenic effects would lead to an approximately four-fold increase in the probability of more than 4 days of HAD and a nearly two-fold increase in the probability of HI exceeding 1 ℃ in CA.By using the random forest feature importance method,the contributions of natural and anthropogenic forcing were found to be 15.77% and 84.23% for HAD,and 8.38% and 91.62% for HI,respectively,for the CA in the last 40 years in the full-forcing simulations.(3)By removing the linear trend from the observed temperatures,it is found that the summer EHTs variation in CA during the last 40 years is generally consistent with the results of hist-nat simulations: the frequency and intensity of EHTs show a decreasing trend(-0.4days/10 a and-0.1℃/10a),and there is an interdecadal shift from high to low values in the mid-1990 s,with a warm period in 1983-1997 and a cold period in 1998-2016.The atmospheric circulation of the two periods reveals that the horizontal wind anomalies at 200 h Pa over both the North Atlantic and Eurasia show alternating anticyclonic and cyclonic wave trains,but the centers of the two periods are opposite.There are cyclonic circulations and negative potential height over the North Atlantic and anticyclonic circulations and positive potential height over CA in the warm period,while there are anticyclonic circulations and positive potential height over the North Atlantic and cyclonic circulations and negative potential height over CA in the cold period.It is further found that the Atlantic multidecadal oscillation(AMO)is closely related to such circulation anomalies,and its influence mechanism is as follows: the warmer sea surface temperature anomalies(SSTA)in the North Atlantic lift the geopotential height and cause anticyclonic circulations over the basin,which leads to the cyclonic anomaly and negative geopotential height anomaly over CA,which is also a loop of the wave train,through the cyclonic-anticyclonic circulation alternating teleconnection wave train over mid-latitude Eurasia,and the local high pressure is weakened,which is not conducive to warming and EHTs in CA.However,the cold North Atlantic SSTA leads to the formation of conditions conducive to the occurrence of CA’s EHTs through the same wave train.Two sensitivity experiments were designed by the CAM5.3 numerical model,and the experimental results verified the observations.Overall,this study concludes that the current AMO(+AMO)has a mitigating role on the EHTs of CA,and the findings also emphasize that if the future AMO shifts to a negative phase,the EHTs of CA may worsen than at present.