4-5-DAY-PERIOD VARIATION AND LOW-LEVEL DRY AIR OBSERVED IN THE EQUATORIAL WESTERN PACIFIC DURING THE TOGA-COARE IOP

A significant 4-5-day-period variation was observed in surface and aerological data obtained at the equator, 156E during November 1992. The time-height structure of the variation is determined by use of spectral and composite analysis. The 4-5-day period variation can be basically explained by westward-propagating mixed Rossby-gravity wave disturbances. However, the structure of the circulation was significantly distorted by the presence of a tropical depression. Further, two distinctive events of extremely dry air in the lower troposphere were observed. The vertical profiles of specific humidity as well as the equivalent potential temperature had distinct minima at about the 3 km level. The trajectory analysis revealed that the dry-air parcels were brought from a subtropical area by tropical disturbances, especially by the intense meridional how associated with 4-5-day period disturbances. These events of dry-air intrusion are detected in the precipitable water data derived from SSM/I satellite imageries and the zonal scale of the intrusion is estimated to be about 1000 km. Convection was strongly suppressed over the dry air, and the latter was maintained to about 4 days due to very weak vertical mixing.