Rainfall outlook for October to December 2003
Suppressed convection in the equatorial region of Western and Eastern Kiribati Average or above average rainfall is expected in Samoa Average or below average rainfall in Vanuatu, New Caledonia, and the Marquesas Islands
Average or above average rainfall is expected in Samoa. However, a large area of suppressed convection in the equatorial region should extend south, to affect areas west of the Date Line, with below average rainfall likely in Western and Eastern Kiribati, and average or below average rainfall in New Caledonia, and Vanuatu, as well as the Marquesas Islands. Average rainfall is expected elsewhere in the region. The model skill is moderate to low for this time of the year.
Rainfall outlook map for October to December 2003
Tropical Cyclones
The South Pacific tropical cyclone (TC) season coincides with the southern hemisphere wet season, usually from November through April. Peak cyclone occurrence is normally during January, February and March. On average, the highest numbers occur in the region around Vanuatu, New Caledonia, and the adjacent Coral Sea.
In the period 1939-2003, cyclones have been recorded only occasionally earlier than November. The earliest recorded was on 4 October 1997.
Other early cyclones were 19 October 1972 and 31 October 1982, and all of these were during El Niño periods – 1982/83 (16 TCs) and 1997/98 (19 TCs) being the most active TC seasons on record.
However, this tropical cyclone season is expected to be neutral for ENSO. In 37% of the past 30 seasons, the first TC occurred before 1 December, increasing to 77% for at least one occurrence before 1 January. December normally has an average of one occurrence.
Probabilities of rainfall departures from average
Broad-scale rainfall patterns and anomalies in the southern tropical Pacific area are estimated from the state of large-scale regional climate factors, such as La Niña or El Niño, their effect on the South Pacific and Tropical Convergence Zones, surface and sub-surface sea temperatures, and computer models of the global climate.
Rainfall estimates for the next three months for Pacific Islands are given in the adjacent table. The tercile probabilities (e.g. 20:30:50) are derived from the interpretation of several global climate models. They correspond to the odds of the observed rainfall being in the lowest (driest) one third of the rainfall distribution, the middle one third, or the highest (wettest) one third of the distribution. On the long-term average, rainfall is equally likely (33% chance) in any tercile.
The probabilities shown express the expected shift in the distribution from the long-term average, based on predictions of oceanic and atmospheric conditions. The amount of inter-model forecast consistency is indicated by the levels of confidence expressed in the table.
| Island Group | Rainfall Outlook | Confidence in the Outlook | |
|---|---|---|---|
| Samoa | 15:45:40 | (Average or above average) | Low |
| Papua New Guinea | 30:50:20 | (Near average) | Moderate – Low |
| Solomon Islands | 15:50:35 | (Near average) | Moderate – Low |
| Wallis and Futuna | 20:55:25 | (Near average) | Moderate – Low |
| Tuvalu | 30:55:15 | (Near average) | Moderate – Low |
| Tokelau | 25:50:25 | (Near average) | Moderate – Low |
| Tonga | 25:55:20 | (Near average) | Moderate – Low |
| Niue | 25:55:20 | (Near average) | Moderate – Low |
| Northern Cook Islands | 15:50:35 | (Near average) | Low |
| Southern Cook Islands | 35:55:10 | (Near average) | Moderate |
| Society Islands | 30:50:20 | (Near average) | Moderate |
| Austral Islands | 15:60:25 | (Near average) | Moderate – Low |
| Tuamotu Islands | 15:50:35 | (Near average) | Moderate – Low |
| Pitcairn Island | 30:45:25 | (Near average) | Low |
| Fiji | 30:40:30 | (Near average) | Moderate – Low |
| Vanuatu | 40:50:10 | (Average or below average) | Moderate – Low |
| New Caledonia | 40:40:20 | (Average or below average) | Moderate – Low |
| Marquesas Islands | 50:40:10 | (Average or below average) | Moderate – Low |
| Eastern Kiribati | 50:20:30 | (Below average) | Moderate |
| Western Kiribati | 45:25:30 | (Below average) | Moderate |
Rainfall outcomes as estimated from models and historical records. The third column indicates the probability of bottom (below), middle (average) or top (above) tercile rainfall, where a percentage is given. The rainfall outlook (second column) is subjectively estimated from the probabilities of bottom, middle and top terciles.