Interannual Variation Of Tropical Cyclones Genesis Frequency Over The Western North Pacific Associated With The Objective-Identified Large-Scale Pattern

The variation of large-scale low-level circulation is one of the key factors in controlling the tropical cyclones genesis(TCG)over the western North pacific in summer.Thus,The influence of the large-scale low-level circulation on the interannual variability of TC over the western north pacific is worth studying and discussing.In this paper,the environment of 796 TCG from summer(May to October)of 1979 to 2013 is assessed via categorization researched into five patterns,using the reanalysis data and the best track.Through statistical method and data analysis,the climatic characteristics of five major large-scale circulation patterns and the interannual and Interdecadal variations of TC frequency with the objective-identified large-scale pattern are studied,and the possible physical causes are preliminarily given.The follows are the main results:(1)the environment of 796 tropical cyclones genesis(TCG)from 1979 to 2013 is assessed via categorization researched into five flow patterns:monsoon(SL),monsoon confluence region(CR),monsoon gyre(GY),easterly wave(EW)and preexisting tropical cyclone(PTC).The number of TCG under the SL circulation pattern is the most,that the proportion is 42%,and the number of TCG under the GY circulation pattern is the least.About 67%of the TC is under the circulation pattern relatived to the monsoon trough(SL+CR+GY),which is most generated in the the South China sea,west and southeast of WNP,and the TC counts in northeast region is very small.16%of TC is under the EW circulation pattern,which mainly occurs in the east of 120 °E,and TC coubts in the South China Sea is the least.12%of the TC is under the PTC circulation pattern,most of which is generated in the southeast region of WNP,followed by the southwest region.Compared with the other circulation patterns,the TC is obviously reduced in the west of 150°E.especially South China Sea,where is no TC,which is related to the direction of the propagation with Rossby wave is in the Southeast.(2)the environment of tropical cyclones genesis(TCG)is assessed via categorization researched into five flow patterns:monsoon(SL),monsoon confluence region(CR),monsoon gyre(GY),easterly wave(EW)and preexisting tropical cyclone(PTC).The TC frequency with the SL circulation pattern is the most,that the proportion is 42%,and the TC frequency with the GY circulation pattern is the least.About 67%of all the TC is under the circulation pattern relatived to the monsoon(SL+CR+GY),which is most generated in the the South China sea,west and southeast of WNP,and TC frequency in the northeast region is very small.16%of all the TC frequency is under the EW circulation pattern,which mainly occurs to the east of 120 °E,and TC frequency in the South China Sea is the least under this pattern.12%of all the TC frequency is under the PTC circulation pattern,most of which is generated in the southeast region of WNP,followed by the southwest region.Compared with the other circulation patterns,the TC frequency is obviously reduced to the west of 150°E,especially the South China Sea,where is no TC under PTC pattern,because the wave trains are oriented in a northwest-southeast direction.The distribution of climate field of each pattern is better explained the characteristics of TCG location distribution.(3)Comparing the time series of TC frequency with the pattern related monsoon and the total TC frequency,it is found that the correlation between them has changed and the correlation coefficient has been increased significantly after 1998.The statistical results shows that the reason may be the interdecadal variation of TC frequency with the PTC pattern,the 850 hPa circulation synthesis with PTC pattern of two stage and the difference field of each element between 1979-1997 and 1998-2013 confirm that the two-stage PTC pattern does exist difference.The location distribution of TC generation of two stage is explained at the same time.