Climate developments in September 2009

Climate developments in September 2009

Enhanced South Pacific Convergence Zone (SPCZ) anomalies were largely absent from the region last month. High rainfall occurred along the Equator near Papua New Guinea, the Solomon Islands and Western Kiribati during September. Northeast Australia was very dry, with Cairns and Townsville reporting very low rainfall during the month. Suppressed convection existed southeast of Western Kiribati last month near Tuvalu, Tokelau, and the Northern Cook Islands. The regional circulation in September was characterised by more frequent low pressure north of the Equator to the east of Eastern Kiribati and higher than normal pressure to the east of New Zealand. This pattern resulted in more frequent easterly anomalies across the southwest Pacific, particularly in the northern Tuamotu Archipelago region and the Marquesas.

Very high rainfall was recorded in the Solomon Islands, with many stations receiving more than 150mm for the month of September. In Western Kiribati, 266mm (211% of normal) of rain fell during the month, and this is the fourth month in a row with considerable rainfall at that location. Positive rainfall anomalies in Western Kiribati are now more pronounced than in central and eastern parts of Kiribati. In Fiji, well above normal rainfall occurred for 50% of stations that reported. Frontal systems delivered more than 100mm of rainfall on two separate occasions during September, and a daily record was broken at Labasa. Similarly, in the Society Islands and Austral Islands, rainfall was well above normal. Many parts of southern French Polynesia exceeded 125% to 200% of normal rainfall for the month.

In contrast, dry conditions occurred over most of Vanuatu during September, with many stations stating less than 40% of normal rainfall had occurred. Northern New Caledonia and northern Australia were also very dry. Niue received between 40–60% of normal rainfall for the month, which was in contrast to neighbouring Tonga, which had stations that received in excess of 250% of normal rainfall during September.

Warmer than normal conditions occurred as a whole across French Polynesia during September, with +1.0°C above normal temperatures registered in the Marquesas. Elsewhere in the southwest Pacific, temperatures were also above normal, with an average anomaly of +0.7°C for New Caledonia, and up to +3°C above normal in northern Queensland.

Outgoing Long-wave Radiation (OLR) anomalies, in Wm2 are represented by hatched areas. High radiation levels (yellow) are typically associated with clearer skies and lower rainfall, while cloudy conditions lower the OLR (blue) and typically result in higher rainfall. The September 2009 position of the South Pacific Convergence Zone (SPCZ) was near normal and contracted toward Papua New Guinea. The average position of the SPCZ is identified by the dashed green line, which is based on mean January rainfall for the South Pacific. Mean sea level (MSL) pressure anomalies (in hPa) are shown as solid and dashed black lines.