Feature article

The Southwest Pacific climate in 2006

Stuart Burgess and Dr Jim Salinger, NIWA

Figure 1. The Southern Oscillation Index (SOI). (Click for enlargement)

Figure 2: Sea Surface temperature (SST) anomalies (departure from average,°C).(Click for enlargement)

Figure 3. Outgoing Long-wave Radiation (OLR) anomalies (departure from average).(Click for enlargement)

This year saw a change from a La Niña to an El Niño climate pattern in the Pacific. The first quarter of the year was dominated by weak La Niña-like (cold episode) characteristics, followed by a neutral period. Weak to moderate El Niño conditions were in place by September, which peaked in December (Fig.1). A higher frequency of surface equatorial westerlies occurred near the Date Line from August through November (the highest persistence since the 2002 El Niño event). Trade winds generally were near normal in strength at other times of the year. There was also a La Niña-like infl uence on the location of the South Pacific Convergence Zone (SPCZ) which was further south than usual from January through May. It was near its normal location throughout much of the remainder of the year. Below average equatorial sea surface temperatures (SSTs) occurred around Western and Eastern Kiribati with the cold episode (-1 °C anomalies), returning to normal by April. These warmed to a constant +1.5 °C above average from August/September onwards. Negative SST anomalies occurred around New Caledonia from August through December, being more than 1 °C below average in October. From January through June outgoing longwave radiation (OLR) anomalies showed enhanced convection over the Solomon Islands and Papua New Guinea. Suppressed convection occurred over Western and Eastern Kiribati from January through May, with a reversal to enhanced convection from August through December. Mean sea level pressures were below average east of the Date Line. However, positive anomalies prevailed in the western Pacific over Australia and the Tasman Sea from May onwards.

For 2006, above average SSTs occurred throughout much of the tropical Southwest Pacific (Fig. 2). These were at least +0.7 °C above average throughout Central and Southern French Polynesia, the Southern Cook Islands and Pitcairn Island. SSTs were at least +0.5 °C above average in many other tropical island nations, and near average between Papua New Guinea and New Zealand, including New Caledonia.

Southwest Pacific island surface air temperature anomalies for 2006 were consistent with the SST anomalies. It was an extremely warm year in Fua&38217;amotu, Tonga (warmest since measurements commenced in 1980) and Tahiti-Faaa, with mean temperatures of 24.6 and 26.9°C, 0.8 °C and 0.7°C respectively above average.

2006 OLR anomalies (Fig.3) showed a region of enhanced convection over Papua New-Guinea, extending southeast to the Solomon Islands and northern part of Vanuatu. Enhanced convection also occurred over Niue and the Southern Cook Islands. Convection was suppressed in a horse-shoe like pattern over Eastern Kiribati and parts of Western Kiribati, extending to the Northern Cook Islands and southeast over the Tuamotu Islands of French Polynesia to Pitcairn Island.The year’s rainfall anomalies were similar to those of the OLR, with above average (at least 110% of normal) rainfall over the much of the Solomon Islands, northern Vanuatu, Niue and the Southern Cook Islands, and parts of central and southern French Polynesia. Rainfall was below average (less than 90% of normal) in the North Tasman and over New Caledonia, and parts of Eastern Kiribati and the Northern Cook Islands. One location, Rotuma Island, Fiji, recorded an extremely high 2006 rainfall of 4378 mm (128% of normal). Two locations recorded well below average annual totals. These were Noumea, New Caledonia, with 735 mm (75% of normal), and Raoul Island, New Zealand, with 948 mm (61% of normal).