The Origin of Tropical Disturbances in the Equatorial Pacific

Abstract

Tropical wave disturbances, which later developed into tropical storms over the equatorial Pacific (11OE-9OW, 3OS-3ON), are analyzed using the satellite and rawinsonde wind data in July and September, 1980. Daily wind fields at upper (200mb) and lower (850mb) levels on grid of 1°×1° are composed and analyzed in relation to tropical storm development. There were 10 tropical storms during these two months in the western Pacific, and 5 of them are analyzed, their initial wave disturbances coming from a far-eastern area in the western hemisphere, except one which appears in higher latitudes. The four tropical storms developed from the easterly wave disturbances, which travel with easterly wave of about 5 day in period along 10N line from the eastern Pacific, around 150W. With the westward movement of easterly wave disturbances, the upper cold lows are cut off from the Mid Pacific Trough in the middle latitudes at about 150W and move westward along 20N. The upper cold low is located north to north-east of the easterly wave disturbances in the early stage, but it is seen in the north-west portion of the easterly wave disturbance in the western Pacific.
These tropical disturbances in the western Pacific, as well as those in the eastern Pacific around 110W, develop into tropical storms when they encounter the ascending motion of the zonal circulation cells, which are seen around 140E and 110W. The position and the strength of the tonal circulation cell changes with some periodicity in relation to intraseasonal variation. The subsidence motion is suggested around 140W for the western cell, and the initial wave disturbances of typhoons are first recognized to the west of this subsidence.