On large scale convection in the atmosphere over the tropical Western Pacific and Eastern Indian Ocean

The motivation for this study came from the variation of deep convection during the anomalous climatological events known as El Nino/Southern Oscillation (ENSO). Various environmental variables, mainly sea surface temperature (SST), wind and precipitation affect various stages of the convection process. Despite the considerable work that has been done in this area, there are gaps in our knowledge of deep convection and its relationship to SST. The study encompassed an area from 70°E to 180°E and from 45°S to 45°N, but concentrated on the tropical region which is considered to be of primary importance to the development of ENSO events. The data sets used consistd of satellite-derived SST fields, wind fields from Australian Bureau of Meteorology, precipitable water vapour from NOAA/TOVS and outgoing longwave radiation (OLR), all for the period 1984 to 1986. This period lies between the 1982/83 and 1986/87 ENSO events, and therefore provides information on the character of convection prior to and at the early stages of the 1986/87 event. The annual variation of SST, wind, OLR and \\precipitable water were mapped during the study period. A wind divergence analysis at 200 mb revealed a zone of convergence over northern Australia from June to November. Over the ocean, the maximum convection is generally located west of the SST warm centre, and convection follows changes in the location of the warm centre. A conceptual model relating OLR to SST and upper level wind divergence has been developed. It successfully explains the relationship between these three variables, especially above the threshold temperature of 27.5°C, and points to the importance of upper level divergence in the development of deep convection. Some interesting features were found to be related to the 1986/87 ENSO event. A positive SST anomaly developed over the tropical mid-Pacific during the study period. By contrast a negative anomaly developed simultaneously over the south Indian Ocean west of Australia. Evidence of a weak convection signal, much before the ENSO anomaly was fully developed, was observed in the Philippines Sea region from May 1986 onward.