Analyses Of The Near-equatorial Tropical Cyclone Genesis In The Western North Pacific

70 tropical cyclones (TC) were formed in the belt within 0°-5°N in the western North Pacific basin during 1979 to 2012, accounting for 8% of the total TCs. In addition, 64% of them were intensified to Typhoons. However, the studies for such near-equatorial TC are rare. In this study, the best track from Joint Typhoon Warning Center, the reanalysis from European Centre for Medium-Range Weather Forecasts interim, and the National oceanic and atmospheric administration optimum interpolation sea surface temperature are used to diagnose the interannual,interdecadal and seasonal characteristics of the near-equatorial TC. The large-scale environmental fields are mainly investigated. The reason of interseasonal variation of near-equatorial TC is opposite to that of total TC over WNP is discussed. And classification and evolution of synoptic scale disturbance before the formation of near-equatorial TC is also investigated. The main conclusions are as folllows:the near-equatorial TC have obvious interannual and interdecadal variations and opposite seasonal variations contrast with the total TCs over the western North Pacific. The large-scale environmental fields of near-equatorial TC genesis are characterized by a cyclonic circulation within the northeasterly and northwesterly wind. The dynamic term of absolute vorticity in the lower troposphere and thermal term of relative humidity in the middle troposphere are the main contributors to the formation of the near-equatorial TC. The higher positive vorticity in the lower troposphere and relative humidity in the middle troposphere are required comparing with the TC formed in the belt within 5°-10°N.For the cyclonegenesis in WNP, the peak active season from August to September (AS) has the minimum frequency of near-equatorial (2 TCs per month),while the inactive TC season during December through May (DJFMAM) occurred the maximum frequency (7 TCs per month). The mechanism of near-equatorial TC frequency increase in DJFMAM are investigated. The results indicate that the significant increase of atmospheric low-level absolute vorticity in the near-equatorial region significantly contribute to the increase of near-equatorial TC frequency. The average contribution of VWS term is relatively smaller due to the inclusion of both negative and positive effect although the decreased (increased) VWS lead to positive(negative) effect for the east (west) of the genesis domain. The study suggests that dynamic variable are more important in near-equatorial TC genesis in the WNP.The large scale circulation background field and synoptic scale disturbance of near-equatorial cyclonegenesis is different to that of total TC in active TC season over WNP.The near-equatorial cyclonic vorticity provides a favorable large-scale environmental field for near-equatorial cyclonegenesis in boreal winter in the WNP ,and most of the near-equatorial TC is generated near this cyclonic vorticity center. In the northwest Pacific near-equatorial TC, synoptic scale disturbance field can be divided into westward propagation type, eastward propagation type, cross-equatorial northward propagation type, Slowly propagation type. The westward propagation type is the most disturbed, mainly including the northwest propagation synoptic wave trains and westward propagation easterly wave. The cross-equatorial north-type weather scale is generated in the southern hemisphere and is developed into TC in the south of the large-scale cyclone circulation southwest of the more equatorial southwest airflow. East-style has two different intensity of large-scale cyclonic background circulation, the synoptic scale disturbance formed between the two circulation, after the East to the eastern circulation center near the development of TC. The slow propagation of TC is mainly due to the propagation of the large-scale cyclonic circulation center around the large-scale circulation center.