The characteristics of the upper (200 mb) and the lower (850 mb) tropospheric wind fields for the period between July and October 1980 over the tropical western Pacific are analyzed, focusing on typhoon formation. GMS satellite wind data and rawinsonde data are used to produce the grid point wind fields. The MASCON model (Dickerson, 1978) was used to adjust the interpolated lower wind fields.
Most of the typhoons and tropical storms that occurred during 1980 originated from easterly wave disturbances in the ITCZ. The easterly wave disturbances move in a westward direction before recurving and propagating north and northeastward along the region of large high cloud amounts in which they develop into storm intensity. The origin of the wave disturbances is traced back to as far as near or east of the international date line. The intensity of a given wave disturbance in its early stage is not related to the intensity at its most developed stage. Whether the disturbance develops to tropical storm intensity or not is related to the large scale wind fields surrounding the disturbance. Development of easterly wave disturbances into tropical depressions and storms occurs around the area where the easterlies from the east are confluent with both the westerlies from the west and the southerly winds form the southern hemisphere. This confluence area moves eastward and westward over a period of 10 to 30 days and most of the typhoons and tropical storms appear when the westerly wind region expands eastward.