Abstract
Several types of westward-propagating equatorial disturbances within a 4-to-20-day-period range in the equatorial Pacific are detected from an objective analysis data during November and December 1992. Their horizontal structures are determined by composite analysis. A signal of 4-to-5-day period is detected in the 850hPa meridional wind field and a signal of a 7-day period is detected in the 200hPa meridional wind field. The associated structure of both signals has a characteristic of the mixed Rossby-gravity wave. Further, a 15-to-20-day signal is detected in equatorial zonal wind variation, both at 200hPa and 850hPa. The structure and westward phase speed can be interpreted as the equatorial Rossby wave of n=1. The signals at the 200hPa and 850hPa levels are coherent and baroclinic near the equator but nearly barotropic in the subtropics. These disturbances have a relation to the cloud activity and have a equivalent depth of the order of 10m. However, the equivalent depth of the 15-day signal in the 200hPa level is estimated to be an order of magnitude larger.
It is suggested that the development of tropical cyclones in the tropical western Pacific is affected by both the 4-5-day mixed Rossby-gravity type waves and 15-day Rossby type waves. A possible excitation mechanism of the 15-day Rossby type waves through an intensification of 200hPa trough by midlatitude forcing is also suggested.
