Mechanisms And Modelling Of The Joint Forcings Of Sea Surface Temperature And Tibetan Snow On Summer Southern China Rainfall Variability

By using the observational re-analysis datasets and Community Atmospheric Model(CAM5.3),this work mainly concentrate on the impacts on East Asian circulation and rainfall anomalies in China based on the mathematical methods such as empirical orthogonal function,regression,correlation analysis,etc.,and numerical modelling.This paper begins with the distinck linkage between winter Tibetan Plateau snow depth and early summer Philippine Sea anomalous anticyclone,and then the early summer southern China rainfall variability and its oceanic drivers has been investigated.At last,the impacts of the North Atlantic sea surface temperature on 2016 and 1998 summer rainfall anomalies was discussed by CAM5.3 numerical modeling.The main conclusions are summarized as follows.(1)Rainfall in southern China reaches its annual peak in early summer(May-June)with strong interannual variability.Using a combination of observational analysis and numerical modeling,the present study investigates the leading modes of this variability and its dynamic drivers.A zonal dipole pattern termed the southern China Dipole(SCD)is found to be the dominant feature in early summer during 1979-2014,and is closely related to a low-level anomalous anticyclone over the Philippine Sea(PSAC)and a Eurasian wave-train pattern over the mid-high latitudes.Linear regressions based on observations and numerical experiments using the CAM5 model suggest that the associated atmospheric circulation anomalies in early summer are linked to decaying El Ni(?)o-Southern Oscillation(ENSO)-like sea surface temperature(SST)anomalies in the tropical Pacific,basin-scale SST anomalies in the tropical Indian Ocean,and meridional tripole-like SST anomalies in the North Atlantic in the previous winter to early summer.The tropical Pacific and Indian Ocean SST anomalies primarily exert an impact on the SCD through changing the polarity of the PSAC,while the North Atlantic tripole-like SST anomalies mainly exert a downstream impact on the SCD by inducing a Eurasian wave-train pattern.The North Atlantic tripole-like SST anomalies also make a relatively weak contribution to the variations of the PSAC and SCD through a subtropical teleconnection.Modeling results indicate that the three-basin combined forcing has a greater impact on the SCD and associated circulation anomalies than the individual influence from any single oceanic basin.(2)Based on the ERA-Interim reanalysis data and numerical modeling(CAMS.3),the present study investigates impact of the North Atlantic Sea surface temperature(S ST)anomalies on China rainfall and the associated physical mechanisms during summer of 2016 and 1998.It is suggested that difference in the North Atlantic SST anomaly patterns between 2016 and 1998 makes important contribution to differences in eastern China summer rainfall and the associated circulation anomalies.The pattern of rainfall anomalies during early summer(June—July)in 2016 was similar to that in 1998,except for more rainfall in the south of Yangtze River in 1998.However,the patterns of rainfall anomalies during late summer(August)were almost opposite between these two years.The differences in summer rainfall anomalies between 2016 and 1998 are directly resulted from circulation anomalies in the Northwest Pacific and the Eurasian mid—high latitudes,which are both closely linked to the North Atlantic SST anomalies.On one hand,the extreme negative phase of the North Atlantic tripole-like SST anomalies can exert a downstream impact on the early summer rainfall of 1998 by inducing a Rossby wave-train over Eurasian mid-high latitudes,which is favorable for more rainfall in southern China,while the response of Eurasian circulation to the North Atlantic SST forcing is relatively weak due to the weak positive phase of tripole-like SST anomalies in 2016.On the other hand,the North Atlantic SST anomalies can also exert an impact on circulation over the Northwest Pacific through a westward subtropical teleconnection.In both early and late summer of 1998,the North Atlantic SST forcing is consistent with that of the tropical Indo-Pacific SST forcing,which result in a strong anomalous low-level anticyclone over the Northwest Pacific.However,the anomalous low-level anticyclone in early summer of 2016 is weaker than that of 1998 due to the opposite impact of the SST anomalies between the North Atlantic and the tropical Indo-Pacific Ocean.In contrast,an anomalous low-level cyclone was observed over the northwest Pacific due to the dominant role of North Atlantic SST forcing in late summer of 2016(3)It is demonstrated that the above-normal Tibetan Plateau snow depth(TPSD)in winter appears to be followed by an intensified Philippine Sea anticyclone in June(PSAC-J),and vice versa.This linkage is clearly independent of the relationship between the El Ni(?)o-Southern Oscillation(ENSO)and the PSAC-J.Moreover,winter TPSD anomalies are typically associated with a PSAC-J pattern that shifts northwards compared with that associated with ENSO.A better understanding of the combined effects of the winter TPSD and ENSO on the PSAC-J could improve our ability to predict both the East Asia summer monsoon and variations in the Meiyu-Changma-Baiu rainbelt.