Structural Evolution And Environmental Field Analysis Of Extratropical Transition Of Tropical Cyclones In The Northwest Pacific At Different Latitudes

Based on CMA database and ERA-Interim high resolution grid data,the extratropical transition(ET)of tropical cyclones(TCs)at different latitudes in the Northwest Pacific from1979 to 2017 is studied in this paper.The main conclusions are as follows:The frequency of ET TCs shows obvious characteristics of interdecadal variation.Most TCs undergo ET in August,September,and October,of which the most are in September.The number of low-latitude ET TCs is small,mainly appearing in winter and spring;the percentage of ET TCs in mid-latitude is high,both exist in April-November;the high-latitude ET TCs are mainly distributed in summer and autumn.In march,the average ET latitude is the southernmost,which is 23°N;then it moves to the north;northernmost in August,then southward.The latitude zone most prone to ET is 37.5°N?42.5°N.About 68.6% of TCs die out quickly after ET,and only a few TCs persist for several days after ET.The intensity of TC ET increases with increasing latitude.For the same latitude,the intensity of TC ET at sea is greater than that on land or offshore.The dynamic composite analysis of TCs with different latitudes shows that whether TCs undergo ET depends largely on the structural configuration between the SST and the Eady baroclinic growth rate.The ET TCs at different latitudes are all connected with the southwest low-level jet stream,but the mid-high latitude TCs are stronger than the low-latitude TCs,and the water vapor supply is more abundant.The low-latitude TCs steering flow have a small northward component,and the subtropical high of the mid-high latitude TCs have an obvious block structure.Under the steering of strong southwest flow at the leading edge of the subtropical high,TCs accelerate northward and interact with the eastward westerly trough.The low-latitude TCs frontogenesis is not obvious,and the middle-high latitude TCs have obvious cold frontogenesis on the southwest side and the northeast quadrant is a warm front.The low-latitude TCs are always located on the south side of the jet stream entrance.On the north side of the mid-high latitude TCs,there is a significant upper-level trough activity,and there is a southwest upper-level jet stream in front of the trough.The strong divergence at the entrance of the jet provides a favorable suction environment for the maintenance anddevelopment of TC low-pressure circulation.TCs in each latitude are always on the southwest side of the large value area of vertical wind shear,and the negative value area of vorticity difference field also has a good correspondence with the large value of wind shear meridional gradient.The ET of typhoon Faxai(1403)at low latitude is selected to study and it is found that the process is mainly developed by the combination of the weakened typhoon circulation and the short-wave trough on the northwest side of TC.The TC low-layer thickness field in the ET stage changes from a uniform symmetrical distribution to a non-uniform distribution,and the B value after ET is up to 30,which is a weak baroclinic asymmetric structure.There is almost no obvious cold frontogenesis in the west side of Faxai during the whole ET stage,but only a warm frontogenesis in the northeast side of the circulation.Cold air sinks and invades from the middle and lower troposphere on the western side of tropical depression,TC changes from a symmetric structure of warm core to an asymmetric baroclinic structure with cold left and warm right,and the temperature increases in the middle layer may be related to the strong downdraft behind the trough.The divergence field dominates the scalar frontogenesis around the cyclone,and the tilting term has a secondary contribution.The vorticity field is the main factor causing the rotation frontogenesis,and the other two are negligible.