The Possible Impact Of SST Changes In Key Areas On The Eastward Retreat Of The Western Pacific Subtropical High In The Late 1970s

The Western Pacific Subtropical High(WPSH)is an important component of the East Asian summer monsoon.The area,intensity and location(northern edge or ridgeline)of the WPSH dominant the strength of the East Asian summer monsoon and the large-scale quasi-stationary frontal zones in East Asia.After the late of the 1970 s,the WPSH has undergone a decadal eastward recession.Since the ocean is one of the important factors for the formation and variation of the WPSH,this study focuses on the relative contributions of the key ocean areas to the eastward retreat of the WPSH after the late 1970 s,and explores the possible specific mechanism.Based upon the previous researches,this study focuses on four key ocean areas: the Indian Ocean(30°S-30°N,30°-120°E),the Northwest Pacific(20°-45°N,140°-180°E),the equatorial central and eastern Pacific(20°S-20°N,150°-60°W),and the Pacific/Pacific Decadal Oscillation(PDO,60°S-60°N,120°E-60°W).In this study,CAM4 was used to run the sensitivity experiments of the key ocean areas(the double difference of the summer sea surface temperature between 1979-2000 and 1950-1978).We explore the possible contributions of these four key regions to the eastward retreat of the WPSH by investigating the difference of the eddy geopotential height,horizontal wind and sea level pressure between the sensitivity experiments and the control experiment.The decadal warming of the Indian Ocean after 1979 can cause the westward extension of the WPSH through the circulation teleconnection and Gill-type response related to precipitation anomalies,which are consistent with the results of previous studies.The decadal cooling of the Northwest Pacific Ocean resulted in insignificant precipitation and circulation anomalies.The decadal warming of the equatorial Middle East Pacific caused a slight eastward retreat of the WPSH,but it was not significant.The phase shift of PDO after 1979 simulates the significant eastward retreat of the WPSH.Additionally,in the experiments of positive PDO combined with the warming Indian Ocean,the WPSH still recessed eastward.These results indicate that the Indian Ocean may stay in recharging phase in the decadal scale,the positive PDO maybe the main factor contribute to the eastward recessed WPSH after the late 1970 s.Particularly,associated with the positive phase of the PDO,convective heating from the Indian Peninsula to the western Pacific and over the eastern tropical Pacific has increased,which has subsequently forced a Gill-type response to modulate the WPSH.The resulting cyclonic gyre over the Asian continent and the western Pacific in the lower troposphere is favorable for the eastward retreat of the WPSH.Additionally,the resulting anticyclonic gyre in the upper troposphere is favorable for the strengthening and southward expansion of the East Asian westerly jet,which can modulate the jet-related secondary meridional–vertical circulation over the western Pacific and promote the eastward retreat of the WPSH.