The tropical circulation in the Australian/Asian region - May to October 1997

A summary of the broadscale tropical circulation from 70°E to 180°, for the six months May to October 1997, is presented. Cool-ENSO conditions prevailed during the previous southern hemisphere summer period. By the start of this summary period there were indicators (e.g., winds and sea-surface temperatures of the near-equatorial Pacific and the southern oscillation index) of a dramatic swing to warm-ENSO conditions. These intensified in the first two months and persisted throughout the period. The northern hemisphere southwest monsoon system became established over much of Indo-China and the Bay of Bengal during an active phase of the 30 to 60-day intraseasonal oscillation (ISO) in May and advanced over India during June. It had retreated from most mainland areas early in October. Outgoing long wave radiation and low-level wind anomalies indicate a near-average monsoon season over India and Indo-China, but areas around Indonesia and Papua New Guinea were much drier than normal, indicative of the strong warm-ENSO event. Two coherent active phases of the ISO were evident early in the period, near the middle of May and the end of June. After this time the ISO signal became difficult to interpret, possibly due to the warm-ENSO event suppressing convection in the central longitudes of the region. There may have been up to six cycles of the ISO during the summary period. A near-average number of tropical cyclones developed during the period. More than 65 per cent of those that affected the northwest Pacific formed east of 145°E, with several also forming in the southwest Pacific, again indicative of the warm-ENSO event.