Tropical rainfall outlook: November 2004 to January 2005
November heralds the start of the rainy and tropical cyclone season for the South Pacific. Enhanced convection is expected in the equatorial Pacific Ocean where above average rainfall is likely in Western and Eastern Kiribati and Tuvalu. Rainfall is expected to be near or above average over Tokelau and the Northern Cook Islands.
Suppressed rainfall is expected from Papua New Guinea southeast to the Southern Cook Islands, including the Solomon Islands, Fiji, Vanuatu, New Caledonia, Niue and Tonga. Rainfall is also expected to be near average or below over the Marquesas Islands.
The model forecast skill confidence ranges from low to moderate for this seasonal forecast period.
Rainfall outlook map for November 2004 to January 2005
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 | Outlook confidence |
|---|---|---|
| Western Kiribati | 25:30:45 (Above) | Moderate |
| Eastern Kiribati | 20:30:50 (Above) | Moderate |
| Tuvalu | 20:35:45 (Above) | Moderate |
| Tokelau | 20:40:40 (Near average or above) | Moderate |
| Northern Cook Islands | 20:40:40 (Near average or above) | Moderate |
| Wallis and Futuna | 35:45:20 (Near average) | Moderate |
| Samoa | 35:45:20 (Near average) | Low |
| Niue | 35:45:20 (Near average) | Moderate |
| Society Islands | 20:45:35 (Near average) | Low – Moderate |
| Austral Islands | 30:50:20 (Near average) | Low – Moderate |
| Tuamotu Islands | 25:45:30 (Near average) | Moderate |
| Pitcairn Island | 35:45:20 (Near average) | Moderate |
| Vanuatu | 40:45:15 (Near average or below) | Moderate |
| Fiji | 40:45:15 (Near average or below) | Low – Moderate |
| Southern Cook Islands | 40:40:20 (Near average or below) | Moderate |
| Marquesas Islands | 40:40:20 (Near average or below) | Moderate |
| Solomon Islands | 40:45:15 (Below or Near average) | Low – Moderate |
| Papua New Guinea | 45:40:15 (Below) | Moderate |
| New Caledonia | 45:40:15 (Below) | Moderate – High |
| Tonga | 45:40:15 (Below) | 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.