Three-month outlook
Tropical rainfall outlook: September to November 2005
Enhanced convection is likely over the Solomon Islands, where rainfall is expected to be above average. A region of near or above average rainfall is expected over Papua New Guinea. Another region of near or above average rainfall extends from the Marquesas Islands west to Samoa including the Tuamotu Islands, the Southern Cook Islands, and Niue.
Suppressed convection is expected from Vanuatu to Fiji, including Tuvalu, and also over Eastern Kiribati. Near average rainfall is expected elsewhere in the region.
The seasonal forecast skill is in the low to moderate range as the forecast period coincides with the transition from the dry season to the wet season.
Rainfall outlook map for September to November 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 |
|---|---|---|
| Solomon Islands | 20:35:45 (Above average) | Moderate |
| Papua New Guinea | 20:40:40 (Near or above average) | Moderate |
| Samoa | 20:40:40 (Near or above average) | Moderate |
| Niue | 15:40:45 (Near or above average) | Low – moderate |
| Southern Cook Islands | 20:40:40 (Near or above average) | Low – moderate |
| Tuamotu Islands | 15:45:40 (Near or above average) | Moderate |
| Marquesas Islands | 15:45:40 (Near or above average) | Moderate |
| Western Kiribati | 20:45:35 (Near average) | Low – moderate |
| Wallis & Futuna | 25:45:30 (Near average) | Low – moderate |
| Tokelau | 25:50:25 (Near average) | Moderate |
| New Caledonia | 35:40:25 (Near average) | Low – moderate |
| Northern Cook Islands | 25:50:25 (Near average) | Moderate |
| Tonga | 30:45:25 (Near average) | Moderate |
| Society Islands | 20:45:35 (Near average) | Moderate |
| Austral Islands | 30:50:20 (Near average) | Low – moderate |
| Pitcairn Island | 25:45:30 (Near average) | Low – moderate |
| Eastern Kiribati | 40:40:20 (Near or below average) | Moderate |
| Vanuatu | 40:40:20 (Near or below average) | Moderate |
| Tuvalu | 40:45:15 (Near or below average) | Moderate |
| Fiji | 40:40:20 (Near or below average) | Low – 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.