3-5 Day-Period Disturbances Coupled with Convection over the Tropical Pacific Ocean

Abstract

Convection-associated disturbance systems that propagate westward over the equatorial Pacific with a period of 3-5 days are studied utilizing the GMS IR equivalent blackbody temperature (T_BB) data and the ECMWF global analysis data. The period of analysis is JJA 1980-1989 excluding 1984. Spectral analysis, lag-correlation analysis and composite analysis are utilized for the study.
The dominance of 3-5 day variations with convective activity is statistically confirmed in the equatorial Pacific ITCZ region for the boreal summer season. It is shown, in a climatological sense, that these convective variations are associated with mixed Rossby-gravity (MRG)-wave-type disturbances in the central Pacific near the dateline, while they are associated with tropical depression (TD)-type disturbances in the tropical western north Pacific. It is suggested that the changes of dominant disturbance types are attributed to variations in large-scale environmental conditions, such as the mean vertical wind shear, the speed of mean zonal wind as well as the SST distribution. This hypothesis is supported by comparing disturbance structures in an El Niño year with those in a La Niña year at the same longitude.
Three-dimensional structures and characteristic values are examined for these two types of disturbances associated with organized convection. The characteristics of MRG-wave-type disturbances correspond well with MRG waves with the equivalent depth of ∼30m. A fairly large amplitude is observed in a limited longitudinal region of about a half-wavelength width (∼4000km) near the dateline. The convection and wave convergence exhibit relatively “loose” coupling. The vertical phase structure for MRG waves coincides with the result of Yanai et al. (1968) and what was proposed by Hayashi (1970). However, it is not clear that the system can be described in the framework of wave-CISK. The characteristics of TD-type disturbances, on the other hand, coincide with the classical “easterly waves” as described by Reed & Recker (1971) and exhibit “tight” coupling with convection. They do not correspond to any linear equatorial waves. It is shown that both types of disturbances obtain the kinetic energy through energy conversion from the available potential energy.