NUMERICAL EXPERIMENTS ON THE FORMATION OF THE EASTERN PACIFIC SUMMER MONSOON

In this paper,we use a two-dimensional primary equation model which contains (1) heating of radiation,(2) heating of condensation,and (3) transfers of sensible and latent heat between air and the underlying surface.To investigate the causes for the formation of the eastern North Pacific summer monsoon,the data at 110°W are obtained and winds at underlying surface and at 200 hPa are modified under the conditions (1) removing topography and (2) changing meridional sea surface temperature (SST) gradient.In the numerical modification,we find that by removing the topography,the center's location of the eastern North Pacific summer monsoon does not change,but the intensity of the summer monsoon is weakened.Also the onset of the summer monsoon is delayed to the end of May.The tropical easterly jet is weakened obviously,even changes to westerly wind.On the other hand,we find that the SST gradient along 110°W influences the eastern North Pacific summer monsoon distinctly.If the SST gradient is decreased,the center of the southwest wind near 12°N does not exist any more,the intensity of the whole summer monsoon becomes very weak and the circulation pattern of the summer monsoon also changes a lot.Finally,we indicate that both topography and meridional SST gradient play important roles in the occurrence of the eastern North Pacific summer monsoon.The meridional SST gradient is the most important factor that triggers the summer monsoon and the topography along 110°W influences the intensity and the onset time of the summer monsoon there mostly.