Impacts Of The Pacific Meridional Mode On The Western North Pacific Tropical Cyclone Activities

Pacific Meridional Mode,as a climate mode defined by sea surface temperature(SST)and wind vector,reflects the interaction between atmosphere and sea.This study identifies a significant positive correlation between the Pacific Meridional Mode(PMM)index and frequency of tropical cyclones(TCs)landfalling in China or the frequency of intense TCs over the western North Pacific(WNP)during the peak TC season(June–November).And this interannual association is independent of two types of El Ni(?)o–Southern Oscillation(EP ENSO and CP ENSO).The PMM impacts on the occurrence of TCs?TC genesis?the development environment and the steering flow primarily by modulating large-scale dynamical conditions over the WNP.During the positive PMM phase,anomalous off-equatorial heating over the eastern North Pacific can induce anomalous low-level cyclonic circulation and upper-level anticyclonic circulation over most of the main development region in the WNP,known as the Gill-type Rossby wave response.The resultant larger low-level relative vorticity and weaker vertical wind shear are conducive to the formation of more intense TCs over the main development region.Meanwhile,the anomalous easterly steering flow in the north flank of the anomalous low-level cyclonic circulation is favorable for more TCs moving westward/northwestward and making landfall in China.The physical mechanism for the impact of PMM on large-scale circulation over the WNP is verified by numerical experiments using the International Centre for Theoretical Physics atmospheric general circulation model as well as the Community Atmospheric Model.Therefore,the PMM index is demonstrated to be a crucial predictor for the frequency of intense TCs or landfalling TC frequency in China over the WNP.Meanwhile,The 2018 tropical cyclone(TC)season over the western North Pacific(WNP)underwent an extreme situation: 18 TCs observed during June–August(JJA)and ranked the second most active summer in the satellite era.he extremely active TC activity and northward shift of TC genesis during JJA of 2018 can be attributed to the WNP anomalous low-level cyclone,which is due primarily to El Ni(?)o Modoki and secondarily to the positive phase of the Pacific Meridional Mode(PMM).