Rainfall outlook for August to October 2004
Above average rainfall likely in Eastern and Western Kiribati
Suppressed convection in Papua New Guinea and the Coral Sea is expected
The continuing lack of coherence between the atmosphere and the ocean in the equatorial Pacific Ocean means that the mix of global model seasonal rainfall forecast guidance is still inconsistent. Therefore, local climate effects are important for the upcoming three months.
Enhanced convection in the equatorial Pacific Ocean is likely to result in above average rainfall in Eastern and Western Kiribati, trending south-south east to include Tuvalu, the Cook Islands and the Austral and Tuamotu Islands of French Polynesia.
Average or below average rainfall is forecast for Papua New Guinea, New Caledonia, Vanuatu and Fiji. Rainfall is expected to be near average elsewhere in the region. The forecast model skill ranges from low to moderate at this time of year.
Rainfall outlook map for August to October 2004
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 | |
|---|---|---|---|
| Western Kiribati | 20:30:50 | (Above) | Moderate |
| Eastern Kiribati | 20:30:50 | (Above) | Moderate |
| Tuvalu | 20:40:40 | (Average or above) | Moderate |
| Northern Cook Islands | 20:40:40 | (Average or above) | Low – Moderate |
| Southern Cook Islands | 15:45:40 | (Average or above) | Low – Moderate |
| Austral Islands | 20:40:40 | (Average or above) | Moderate |
| Tuamotu Islands | 20:35:45 | (Average or above) | Low |
| Solomon Islands | 20:50:30 | (Near average) | Low – Moderate |
| Wallis and Futuna | 25:45:30 | (Near average) | Moderate |
| Tokelau | 20:45:35 | (Near average) | Moderate |
| Samoa | 20:50:30 | (Near average) | Low – Moderate |
| Tonga | 30:50:20 | (Near average) | Low – Moderate |
| Niue | 25:50:25 | (Near average) | Low – Moderate |
| Society Islands | 30:50:20 | (Near average) | Moderate |
| Marquesas Islands | 30:40:30 | (Near average) | Low |
| Pitcairn Island | 30:50:20 | (Near average) | Moderate |
| Papua New Guinea | 40:35:25 | (Average or below) | Moderate |
| Vanuatu | 40:40:20 | (Average or below) | Low – Moderate |
| New Caledonia | 35:40:25 | (Average or below) | Low – Moderate |
| Fiji | 35:40:25 | (Average or below) | 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.