Tropical rainfall outlook: October to December 2004
The current incoherent El Niño/Southern Oscillation (ENSO) state in the equatorial Pacific is expected to influence rainfall to some extent across the region for the coming three months.
Enhanced convection is expected in the Enhanced equatorial region of Western and Eastern Kiribati, where rainfall is likely to be above Kiribati, where rainfall is likely to be above average. Near or above average rainfall is expected from the Solomon Islands trending eastwards to Tokelau including Tuvalu. Another region of near average or above average rainfall is expected over the Austral and Tuamotu Islands of French Polynesia and Pitcairn Island.
Suppressed convection is expected over New Caledonia, Samoa and the Southern Cook Islands, where rainfall is likely to be below average. Near or below average rainfall is forecast for Papua New Guinea, Vanuatu, Tonga and the Marquesas Islands. Rainfall is likely to be near average elsewhere in the region.
The seasonal rainfall model skill is low to moderate for most island countries in the region.
Rainfall outlook map for October to December 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 average | Moderate |
| Eastern Kiribati | 25:30:45 | Above average | Low – Moderate |
| Solomon Islands | 20:40:40 | Near average or above | Moderate – High |
| Tuvalu | 25:35:40 | Near average or above | Low – Moderate |
| Tokelau | 20:40:40 | Near average or above | Moderate |
| Austral Islands | 20:40:40 | Near average or above | Moderate |
| Tuamotu Islands | 25:35:40 | Near average or above | Low – Moderate |
| Pitcairn Island | 15:40:45 | Near average or above | Moderate |
| Wallis & Futuna | 30:45:25 | Near average | Moderate |
| Fiji | 35:40:25 | Near average | Low – Moderate |
| Niue | 30:50:20 | Near average | Moderate |
| Northern Cook Islands | 25:50:25 | Near average | Low – Moderate |
| Society Islands | 30:40:30 | Near average | Low – Moderate |
| Papua New Guinea | 40:40:20 | Near average or below | Moderate – High |
| Vanuatu | 40:40:20 | Near average or below | Low – Moderate |
| Tonga | 40:45:15 | Near average or below | Low – Moderate |
| Marquesas Islands | 40:40:20 | Near average or below | Low |
| New Caledonia | 50:35:15 | Below average | Moderate |
| Samoa | 45:30:25 | Below average | Low – Moderate |
| Southern Cook Islands | 50:30:20 | Below average | 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.