Tropical rainfall outlook: April to June 2005
Continuing incoherence between the atmospheric and oceanic conditions because of the decaying El Niño conditons in the equatorial Pacific, and April to June being transition months from the wet to the dry season, resulted in lack of normal rainfall patterns in the Pacific region.
Enhanced convection is expected over Western Kiribati and the Northern Cook Islands, where the rainfall is forecast to be near or above average.
Dry conditions are expected over the Marquesas Islands of French Polynesia, where the expected rainfall is below average.
Average or below average rainfall is forecast for Eastern Kiribati, Samoa, Fiji, and the Austral Islands.
The rainfall model skill for this forecast period is low to moderate because of the transition from the wet to the dry season.
Rainfall outlook map for April to June 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 | 20:40:40 (Near average or above) | Moderate |
| Northern Cook Islands | 15:45:40 (Near average or above) | Moderate |
| Papua New Guinea | 30:45:25 (Near average) | Moderate |
| Solomon Islands | 30:45:25 (Near average) | Moderate |
| Vanuatu | 25:50:25 (Near average) | Moderate |
| New Caledonia | 30:40:30 (Near average) | Low – moderate |
| Tokelau | 30:45:25 (Near average) | Moderate |
| Tuvalu | 25:50:25 (Near average) | Moderate |
| Wallis & Futuna | 25:50:25 (Near average) | Moderate |
| Tonga | 30:50:20 (Near average) | Moderate |
| Niue | 25:50:25 (Near average) | Moderate |
| Southern Cook Islands | 30:50:20 (Near average) | Moderate |
| Society Islands | 30:50:20 (Near average) | Moderate |
| Tuamotu Islands | 25:50:25 (Near average) | Low – moderate |
| Pitcairn Island | 25:50:25 (Near average) | Moderate |
| Eastern Kiribati | 40:40:20 (Near average or below) | Low – moderate |
| Samoa | 40:40:20 (Near average or below) | Low |
| Fiji | 40:45:15 (Near average or below) | Low – moderate |
| Austral Islands | 40:45:15 (Near average or below) | Moderate |
| Marquesas Islands | 40:35:25 (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.