Tropical rainfall outlook: September to November 2004
Continuing variability in the equatorial Pacific is expected to influence the Pacific rainfall for the coming three months. Even though this El Niño event is expected to be weak, the climate impacts will vary and can be extreme for some island countries.
Rainfall is expected to be above average in the equatorial region of Western and Eastern Kiribati. Another region of enhanced convection is expected near the Tuamotu Islands extending southeast to Pitcairn Island, where rainfall is likely to be near average or above average.
Suppressed convection is expected in the south of the Coral Sea extending east to the Date Line, where rainfall is forecast to be near average or below average for New Caledonia, Fiji and Tonga. Rainfall is also expected to be near average or below average over 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 September to November 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 | Above | 20:30:50 | Moderate – high |
| Eastern Kiribati | Above | 20:30:50 | Moderate – high |
| Tuamotu Islands | Average or Above | 20:40:40 | Moderate |
| Pitcairn Island | Average or Above | 15:40:45 | Moderate |
| Papua New Guinea | Average | 25:50:25 | Moderate |
| Solomon Islands | Average | 20:45:35 | Moderate |
| Vanuatu | Average | 35:45:20 | Moderate |
| Tuvalu | Average | 25:50:25 | Moderate |
| Wallis and Futuna | Average | 20:50:30 | Low – moderate |
| Tokelau | Average | 30:40:30 | Low |
| Niue | Average | 20:50:30 | Low – moderate |
| Samoa | Average | 30:45:25 | Moderate |
| Northern Cook Islands | Average | 20:50:30 | Moderate |
| Southern Cook Islands | Average | 25:50:25 | Moderate |
| Society Islands | Average | 30:40:30 | Moderate |
| Austral Islands | Average | 25:50:25 | Low |
| New Caledonia | Average or Below | 45:40:15 | Low – moderate |
| Fiji | Average or Below | 35:40:25 | Low – moderate |
| Tonga | Average or Below | 40:35:25 | Low – moderate |
| Marquesas Islands | Average or Below | 45:40:15 | 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.