Circulation Anomalies Related To The Autumn-to-winter Drought In Southwest China And Possible Impacts Of The North Atlantic SSTs

Drought is one of the most catastrophic natural hazards worldwide due to its large spatial extension,long duration,huge number of population affected and profound damage in economy and ecosystem.Using observation and reanalysis data and the numerical simulation,the autumn-to-winter drought in Southwest China(SWC)has been investigated.Results are as follows:(1)The 6-month standard precipitation index ranging from September to the subsequent February(SPI6?Feb)can well describe the autumn-to-winter drought in SWC.The positive/negative value of SPI6?Feb indicates more/less-than median precipitation.(2)The circulation anomalies regressed on the SPI6?Feb show that when the autumn-to-winter drought occurs,the negative phase of Arctic Oscillation(AO)appears in mid-high latitudes,which decreases the blocking events in Eurasian continent and thus southward intrusion of cold air and thus reduce cold-warm air confluences and precipitation over SWC.There are also apparent Rossby waves propagating southeastward from the mid latitudes of the North Atlantic to SWC and causing anomalous anticyclonic circulation over SWC.The anomalous anticyclone on one way leads to anomalous descending motion over SCW and on the other way reduces the climatological water vapor transport from the tropics,in particular South China Sea to SWC.This can result in drought formation in SWC.(3)The negative phase of AO and the anomalous Rossby waves originated from the North Atlantic that are closely related to drought formation in SWC may be caused by the negative sea surface temperature(SST)anomalies in the North Atlantic around 40?N;there are apparent wave activity fluxes from surface of the North Atlantic heading upward and then northward to the Arctic and southeastward to SWC.In addition,the linear correlation coefficient between the North Atlantic SST and SPI6?Feb is 0.51,significant at the 95%confidence level.(4)Both the linear baroclinic model(LBM)and the CAM5.1 general circulation model are adopted to verify the impacts of North Atlantic SSTs by conducting sensitivity experiments.It seems that without non-linear processes,the LBM can reproduce the Rossby waves propogating southeastward from the North Atlantic and causing anomalous anticyclone over SWC when forced by negative SST forcing centered at 40?N,but fail to simulate the SST impacts on AO.When the negative SST forcing was prescribed in CAM5.1,the modelsuccessfully simulates the negative AO,the southeastward propagating Rossby waves,the anomalous anticyclone over SWC,and thus negative precipitation there.This confirms the impacts of the North Atlantic SSTs on the autumn-to-winter drought formation in SWC via AO and the east-southward Rossby waves.