Influence Of Sea Surface Temperature Anomalies On The Tropical Cyclone Genesis Number In The Western North Pacific In The Strong Decaying El Ni?o Years

Based on the Hadley Center sea surface temperature data,NECP/NCAR reanalysis data from 1970 to 2016 and the best-track tropical cyclone(TC)data from the United States Typhoon Joint Warning Center,used the Generalized Balanced Feedback(GEFA)method and numerical modeling simulation(ECHAM4),the possible effects of sea surface temperature anomalies(SSTA)in the strong decaying El Ni?o years,on tropical cyclone(TC)formation and large-scale circulation in the Pacific Northwest in August were studied.The strong El Ni?o events are divided into general strong El Ni?o and special strong El Ni?o according to the difference in duration of the events.2016 as a special decaying El Ni?o year is compared with 1998(general strong decaying El Ni?o years).The main conclusions are as follows.(1)The TC genesis number(TCGN)anomaly of the general strong decaying El Ni?o years from July to October is-2.8 TC(exceeding 95% confident level).The TCGN in August is different between the 1998(3)and 2016(9),which both are decaying El Ni?o years for the strong El Ni?o event.(2)The GEFA analysis shows that the Pacific Northwest TCGN is significantly forced by the first(TP1)and third mode(TP3)of tropical Pacific,tropical Indian Ocean first mode(TIO1)and the North Atlantic first mode(NA1).TIO1 and NA1 contribute 55% and 45% respectively to the reduction of TCGN in the Northwest Pacific,and the tropical Pacific(TP1+TP3)has no positive contribution in the general strong decaying El Ni?o years.Combined with the numerical simulation results,it is shown that in the general strong decaying El Ni?o years,TIO1 and NA1 cause the environmental that is not conducive to TC generation,which is the main reason for the significant reduction of TCGN in the Northwest Pacific.The tropical Pacific Ocean produces a cyclonic circulation located west of 140°E.The China Seas are controlled by cyclonic circulation,which is the main reason for the increase of TCGN on the China Seas.(3)The SSTA,which was almost opposite in the tropical Indian Ocean and the Atlantic Ocean in 1998 and 2016,is one of the main reasons for the significant difference in the TCGN.Tropical and North Pacific SSTA can exert low pressure cyclonic circulation in the Pearl River Delta and south of Japan respectively in August 1998 and August 2016.The anomalous anti-cyclonic response of the tropical Indian Ocean and Atlantic SSTA in 1998 was stronger than the cyclonic anomaly generated by the Pacific SSTA.Therefore,the northwestern Pacific Ocean is subject to anti-cyclonic circulation,which makes the TCGN less.In 2016,under the combined action of three oceans,the anomalous cyclone in the western North Pacific led to more TCGN.The Pacific meridional mode structure forced an east-west overturning circulation anomaly in the subtropical North Pacific,and the response over the western Pacific Ocean was opposite to that observed.Therefore,the Pacific meridional model has no positive contribution to the TCGN in the western North Pacific.