The tropical circulation in the Australian and Asian region - May to October 2009

Abstract

A summary of the broadscale tropical circulation from 70º E to 180º, for the six months May to October 2009, is presented. During this period the El Niño– Southern Oscillation (ENSO) state shifted gradually from neutral conditions to El Niño-like conditions. The Southern Oscillation Index (SOI) remained mostly between +5 and -5, except during October where it plunged to -15. The mean SOI for the season was -3.5. Mean sea-level pressure anomalies remained weak in the Darwin Regional Specialised Meteorological Centre (RSMC) analysis area except over parts of Australia, where pressure remained higher than normal. Sea-surface temperatures in the equatorial Pacific and tropical Indian Ocean were warmer than their long-term mean. Convection over the tropical Indian Ocean, Australia and parts of the Maritime Continent was below average. The tropical western Pacific and the South Pacific convergence zone areas remained active during the season. Several active convective phases of the Madden-Julian Oscillation were observed in the region during the season with a periodicity of 30 to 35 days. A total of 22 tropical cyclones (including 13 typhoons/severe cyclones) were analysed during the period, less than the mean of 30 for the Darwin RSMC analysis area.