The Flow during TOGA COARE as Diagnosed by the BMRC Tropical Analysis and Prediction System

Abstract

The evolution of the large-scale flow through the four-month intensive observing period of TOGA COARE is documented from large-scale numerical analyses and GMS cloud imagery produced by the Australian Bureau of Meteorology and transmitted to the field stations during the experiment. The evolution of the flow is dominated by the following phenomena: 1) the normal seasonal evolution of the tropical flow over this region, including a southward and eastward progression of the tropical convective heat source as the Southern Hemisphere monsoon developed and matured; 2) a more eastward than normal progression of this monsoon circulation, associated with a warm event of the ENSO phenomenon; 3) the existence of a major westerly?easterly?westerly cycle of the Madden?Julian low-frequency wave occurring during the latter half of the experimental period, and 4) the development and subsequent movement of tropical cyclones in both (northern and southern) hemispheres. The Madden?Julian event consisted of two eastward progressions across the domain of satellite-observed cloud, south of the equator. The horizontal scale of the cloud regions is approximately 10° latitude ? 40° longitude and the eastward phase speed is approximately 3.7 m s?1. Linear correlation studies substantiate the eastward movement of both cloud and zonal wind across the domain. The correlation analysis reveals a strong relationship between cloud and low-level zonal wind, with the cloud variations leading those in wind by approximately five days. Time-longitude sections of relative vorticity show that the synoptic activity also progressed eastward with the cloud, and its structure is suggestive that the controlling dynamics (for the synoptic activity) may be the energy dispersion mechanism of Davidson and Hendon. The development of each westerly event was accompanied by a major change in the Southern Hemisphere deep-layer mean flow from easterly to westerly. Examination of flow fields and satellite imagery for individual days shows that the peak of the first westerly event is associated with the flow patterns surrounding two Southern Hemisphere tropical cyclones. The subsequent rapid evolution to an easterly state occurs as the cyclones move eastward and southward, and the monsoon flow collapses in their wake. There is an accompanying ridging at low levels in the subtropics and the establishment of the Southern Hemisphere subtropical jet. The subsequent reestablishment of the monsoon (the second westerly event) occurs from west to east with the eastward moving cloud bands. There is also a suggestion that an equatorward extension of a Southern Hemisphere upper-level trough may have played a role. Major active and break periods are identified over four tropical subdomains over the TOGA COARE region. These are most easily defined in the Southern Hemisphere subdomains. They are characterized by a slowly ,varying signal in the satellite-observed average cloud-top temperature. Superimposed on this is a rapid transition between the active and break states.