Interdecadal Change Of Tropical Cyclone Activity In The Western North Pacific

In order to study the interdecadal change of tropical cyclone(TC)activity in the Western North Pacific(WNP)by utilizing the frequency of low-level disturbances and the genesis productivity,in this paper,we use Hodges tracking scheme to track the low-level disturbances from 1979 to 2016.The results indicate that compared with 1979 to 2016(PI),from 1998 to 2016(P2),the frequency of nondeveloping low-level disturbances in South China Sea(SCS)has a remarkable increase,while the genesis productivity has a remarkable decrease,so the tropical cyclone genesis frequency(TCGF)in SCS do not change remarkably;the interdecadal change of the nondeveloping low-level disturbances,the genesis productivity and TCGF in the western part of WNP(WWNP)is not distinct;the frequency of the nondeveloping low-level disturbances,the genesis productivity and TCGF in EWNP all decrease remarkably.Therefore,the genesis productivity and TCGF in WNP has a remarkable decrease,while the interdecadal change of the nondeveloping low-level disturbances is not distinct.The most important factor that affects the genesis productivity in SCS is the interdecadal change of the relative humidity,while the most important factor in EWNP is vertical wind shear,which is influenced by the interdecadal change of the zonal circulation.In this paper,we divide 1951 to 2016 into four periods:1951 to 1961,1962 to 1978,PI and P2,and through comparing these four periods,we find that the interdecadal change of the tropical cyclone(TC)activity in WNP and environmental fields maybe affected by the combined effect of the Atlantic Multidecadal Oscillation(AMO),the Pacific Decadal Oscillation and the sea surface temperature anomaly in the North Indian Ocean.The positive(negative)phase of AMO may increase(decrease)TCGF in SCS and EWNP.Influenced by the combined effect of them,there are a remarkable change of Hadley Cell in SCS and WWNP,and a remarkable change of zonal circulation in EWNP,accompanied with the increase belt of vertical averaged 700-500hPa relative humidity in the Southern Hemisphere.Besides,in P2,there are the westward retreat of Monsoon Trough,the increasement of the range of Subtropical High,and the westward movement of Tropical Upper Tropospheric Trough in WNP.These interdecadal change of the environmental fields can lead to the interdecadal change of the genesis productivity in WNP.In WNP,the interdecadal change of TCGF in summer is different from the overall interdecadal change of TCGF from June to November.This is partly because in summer,the TC activity in WNP is influenced by Boreal Summer Intraseasonal Oscillation.