The Climatologic Research Of Summer Monsoon Over The Western North Pacific And Its Influence On Typhoon Genesis

The climate characters of western North Pacific summer monsoon (WNPSM) and their influences on typhoon genesis are studied by using the long-term climate mean data. The emphases are the onset, three cycles of seasonal evolution and climatological intraseasonal oscillations (CISO). Besides, the effects on typhoon genesis of western north Pacific monsoon trough, active-break cycles and CISO are also analyzed.The major conclusions of this paper are as follows:(1) WNPSM onsets at pentad 31 (early June) or so, ends at pentad 60 (late October), lasts a period of five months. The onset time is about 20 days later than SCS summer monsoon. The characters after WNPSM onset are that precipitation, convection and low-level west wind strengthen and expand over western north Pacific.(2) The seasonal evolution, precipitation, convection, low-level west wind and SST all have a trend of spreading northeastward. At pentad 31 or 32 (early June) and after monsoon onsets, precipitation, convection, low-level west wind and SST extend northeastward, and at nearly pentad 41 (late July) they arrive at the most northeast zone quickly, and then their intensities decrease, but they still have a trend of extending eastward. Precipitation, convection, and west wind begin to retreat southwestward at pentad 60 (late October).(3) Three cycles constitute seasonal evolution of WNPSM. In the first cycle (early June -mid July), precipitation and west wind mainly locate between 5°N- 10°N. In the second cycle (late July-late September), precipitation and west wind extend northeastward, precipitation become strong and this time is the most prosperous period of WNPSM. The third cycle represents weak period, precipitation and westwind are weaker than them in the second cycle. The end of this cycle indicates the end ofWNPSM.(4) When WNP monsoon trough is strong, vertical wind shear, divergence wind in upper and lower level, humidity and sea surface temperature are propitious to the genesis of typhoon in the WNP. The number of typhoon is obviously more than that when monsoon trough is weak; and the genesis location is eastern. When monsoon trough is weak, these conditions are reverse.(5) In the active phase of WNPSM, namely, WNPSM is strong, typhoon number is great, but if monsoon breaks, that is summer monsoon is weak, typhoon number is few.(6) The CISO of convection and 850hPa zonal wind over WNP are composed of 30-60d and 10-20d oscillations. They have important effect on typhoon. The number of typhoon is obviously more in wet-phase period of MJO convection and westerly phase; moreover it is less in dry-phase period and easterly phase.(7) Temporal-spatial group occurrence of typhoon has a close correlation with general circulation in tropical WNP. In more typhoon periods (years), there are strong monsoon trough, weak vertical wind shear and enough water vapor supplying. However, this is reverse in few typhoon periods (years).