Rainfall outlook for July to September 2004
Above average rainfall over the Austral Islands
Suppressed convection over Vanuatu, New Caledonia and Fiji
Variability in the ENSO system, and the present lack of coherent large-scale forcing of the tropical Pacific climate system mean that global model seasonal rainfall guidance is quite inconsistent for most Pacific Island countries. Hence, it is likely that rainfall patterns will be dominated by local effects and by episodic events for the upcoming three months.
Enhanced convection is expected over the Austral Islands of French Polynesia where rainfall is forecast to be above average. Rainfall is also expected to be average or above average in a region extending south southeast from Western and Eastern Kiribati to the Society Islands, including Tuvalu, Samoa, Niue and the Southern Cook Islands.
A region of suppressed convection is forecast just west of the Date Line, where rainfall is expected to be near average or below average over Vanuatu, New Caledonia and Fiji. Rainfall is expected to be near average elsewhere in the region. The consensus for model forecast skill is low to moderate for this time of year.
Rainfall outlook map for July to September 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 | |
|---|---|---|---|
| Austral Islands | 20:30:50 | (Above) | Moderate |
| Western Kiribati | 25:35:40 | (Average or above) | Low |
| Eastern Kiribati | 25:35:40 | (Average or above) | Low |
| Tuvalu | 25:35:40 | (Average or above) | Moderate |
| Samoa | 20:40:40 | (Average or above) | Low – Moderate |
| Niue | 25:35:40 | (Average or above) | Low – Moderate |
| Southern Cook Islands | 20:40:40 | (Average or above) | Moderate |
| Society Islands | 20:40:40 | (Average or above) | Moderate |
| Papua New Guinea | 25:45:30 | (Near average) | Low |
| Solomon Islands | 25:50:25 | (Near average) | Low – Moderate |
| Tokelau | 30:50:20 | (Near average) | Moderate |
| Wallis and Futuna | 25:45:30 | (Near average) | Low – Moderate |
| Tonga | 30:45:25 | (Near average) | Low – Moderate |
| Northern Cook Islands | 25:45:30 | (Near average) | Moderate |
| Pitcairn Island | 20:45:35 | (Near average) | Low – Moderate |
| Marquesas Islands | 30:40:30 | (Near average) | Moderate |
| Tuamotu Islands | 25:50:25 | (Near average) | Moderate |
| Vanuatu | 45:40:15 | (Average or below) | Moderate |
| New Caledonia | 35:40:25 | (Average or below) | Low – Moderate |
| Fiji | 35:40:25 | (Average or below) | Low |
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.