Impacts Of Various El Ni(?)o Events On Tropical Cyclone Activities In Western North Pacific

Best track tropical cyclone data from JWTC, HadISST data and atmospheric datafrom NCEP are employed to study on impacts of various El Ni(?)o events on tropicalcyclone activities in Western North Pacific. Firstly, the different impacts on tropicalcyclone genesis location and intensity between eastern Pacific El Ni(?)o and centralPacific El Ni(?)o are separately discussed during summer and autumn. After that, basedon that whether there is a positive IOD event in the same year, the central Pacific ElNi(?)o events are divided into two kinds: p-CPEN and CPEN-IOD, and the differentcharacteristics of cyclone genesis location in these two events’autumn are discussed.During eastern Pacific El Ni(?)o summer, the number of tropical cyclone genesisincreases in north west and decreases in south east. The significant negative SSTA,reduction of low-level vorticity and mid-level humidity are the main factors thatsuppress the tropical cyclone genesis in northern west while reduction of smallvertical and increase of low-level vorticity&mid-level humidity make the southerneastern part more favorable for tropical cyclone genesis. In central Pacific El Ni(?)osummer, the area that tropical cyclone genesis increases extends to the northern westwhere the tropical cyclone genesis number is lowered in eastern Pacific El Ni(?)osummer because there are stronger low-level vorticity and weaker vertical wind shear.In eastern Pacific El Ni(?)o autumn, the active genesis region located in south eastshifts eastward about20degrees compared with that position in summer. The reasonfor the seasonal shift is that the center of negative low-level shifts southward, largervertical wind shear expands and dry air area extends. During central Pacific El Ni(?)oautumn, the northern west is no longer a favorable location for genesis because ofincrease of vertical wind shear and decrease of mid-level humidity. The genesis ineast of the Philippines is suppressed for the same reason.Accumulated Cyclone Energy (ACE) is used to investigate the impacts of twotypes of El Ni(?)o on tropical cyclone intensity. Both eastern Pacific El Ni(?)o index Ni(?)o3and central Pacific El Ni(?)o index EMI have positive correlation with ACEduring summer and autumn, but the correlation between ACE and Ni(?)o3in autumn isnot significant. In summer, the correlation between ACE and Ni(?)o3is stronger thanthe correlation between ACE and Ni(?)o3, but this situation reverses in autumn. Duringsummer, the number of intense tropical cyclone is larger and the total lifetime oftropical cyclone is longer in eastern Pacific El Ni(?)o than in central Pacific El Ni(?)o. Asa result, the former ACE is stronger that the latter one. In autumn, the situationreverses.Tropical cyclone genesis location in CPEN-IOD autumn extends eastward about20°compared with that in p-CPEN autumn. During p-CPEN autumn, there is weakand narrow equatorial westerly wind anomaly which is observed just in the westernarea. However, in CPEN-IOD autumn, the equatorial westerly wind anomaly whichextends across the dateline is much stronger. So the monsoon trough prolongs about20°farther east in CPEN-IOD autumn than in p-CPEN autumn. At the same time,autumn vertical wind shear in the Northwest Pacific is weaker and narrower inp-CPEN than in CPEN-IOD. The differences in position of monsoon trough andvertical wind shear distribution make the tropical cyclone genesis location farther eastin CPEN-IOD autumn than in p-CPEN autumn.