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Feature article

The Southwest Pacific Climate in 2005

Stuart Burgess and Dr Jim Salinger, NIWA

Figure 1. The Southern Oscillation Index (SOI). The index displayed neutral values for the first half of the year then drifted negative into the weak El Niño range for the remainder of the year.

Figure 2: Sea Surface temperature (SST) anomalies (departure from average). Yellow or orange areas represent warmer than average sea surface.

Figure 3. Outgoing Long-wave Radiation (OLR) anomalies (departure from average), in Wm-2 are represented by coloured areas, and rainfall percentage of average, shown by numbers. High radiation levels (yellow or orange) are typically associated with clearer skies and lower rainfall, while cloudy conditions lower the OLR (blue) and typically mean higher rainfalls.

The year 2005 saw El Niño/Southern Oscillation (ENSO) conditions evolve from a weakly negative state in the first quarter of the year, to a situation with several La Niña-like features in the central equatorial Pacific in the latter part of the year (Fig. 1).

A high frequency of surface equatorial westerlies occurred near the Date Line in February (the most since the last El Niño in 2002). However, these conditions were only temporary, trade winds generally being near normal in strength at other times of the year. There was also some ENSO influence on the location of the South Pacific Convergence Zone (SPCZ) during the year. West of the Date Line, the SPCZ was further north than usual from January through August, and further south than usual from November through December. East of the Date Line, the SPCZ was very weak from March through August. It was further south than usual from October through December. Above average equatorial sea surface temperatures (SSTs) occurred with the weakly negative El Niño; however, the region of positive SST anomalies drifted west as the El Niño faded. Some La Niña-like features were seen in the central equatorial Pacific Outgoing Long-wave Radiation (OLR) anomalies from August through December, with enhanced convection over Papua New Guinea, and suppressed convection during those months over Western and Eastern Kiribati, Tokelau, Tuvalu, and the Northern Cook Islands. For much of the year, mean sea level pressures were above average west of the Date Line, and below average in the east.

Above average SSTs occurred throughout much of the tropical Southwest Pacific during 2005 (Fig. 2), being about +1.0 °C above average around Western Kiribati, and at least +0.5 °C above average in many other island nations, especially those north of 20 °S. New Caledonia, the Southern Cook Islands, the Austral Islands, and Pitcairn Island were surrounded by near average SSTs. Southwest Pacific island surface air temperature anomalies for 2005 were consistent with the SST anomalies throughout the region. It was an extremely warm year in Tahiti-Faaa, central French Polynesia, where the mean temperature was 27.0 °C (+0.8 °C above the 1971-2000 normal), and equal highest since measurements commenced in 1957.

Southwest Pacific 2005 OLR anomalies (Fig. 3) showed a region of enhanced convection over Papua New Guinea extending towards the Solomon Islands. There was also an area of weakly enhanced convection over Niue and the Southern Cook Islands, as well as Pitcairn Island. Convection was suppressed in 2005 over Western and Eastern Kiribati, Tokelau, Tuvalu, Wallis and Futuna, the Northern Cook Islands, and the Marquesas Islands. The year’s rainfall distribution indicates similarities to the OLR pattern. However, for rainfall there were not many significant anomalies. Rainfall was at least 110% of normal affecting a region extending from Niue to north and east of Fiji, and also parts of southern French Polynesia. In contrast, rainfall was less than 90% of average throughout much of New Caledonia.

Nine tropical cyclones affected the Southwest Pacific during the 2004-05 season, the same as the average number that are normally expected. About half reached major hurricane strength, with sustained wind speeds of at least 168 km/h. All but one of the Southwest Pacific tropical cyclones originated east of the Date Line, and all occurred within the December 2004 to April 2005 period.