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Three-month outlook

Tropical rainfall outlook: January to March 2005

The tropical Pacific Ocean is still in a weak El Niño state and this will continue to influence rainfall patterns in the Pacific region over the next three months.

Enhanced convection is expected in the equatorial region of Western and Eastern Kiribati and Tuvalu where rainfall is forecast to be above average. Rainfall is likely to be near or above average in Tokelau, the Northern Cook Islands, and the Society Islands of French Polynesia.

Suppressed convection is expected over New Caledonia and the Marquesas Islands, where rainfall is likely to be below average.

The region of near or below average rainfall is expected to be exceptionally large for the coming three months from Papua New Guinea extending east southeast to the Southern Cook Islands, including Vanuatu, Fiji, Tonga, Niue, and Samoa.

Near average rainfall is likely for the rest of the forecast region.

Rainfall outlook map for January to March 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
Western Kiribati 20:20:60 (Above) High
Eastern Kiribati 20:30:50 (Above) Moderate – high
Tuvalu 20:30:50 (Above) Moderate – high
Tokelau 15:40:45 (Near average or above) Moderate – high
Northern Cook Islands 15:45:40 (Near average or above) Moderate
Society Islands 15:45:40 (Near average or above) Moderate
Solomon Islands 35:45:20 (Near average) Low – Moderate
Wallis and Futuna 35:45:10 (Near average) Moderate
Austral Islands 30:50:20 (Near average) Moderate
Tuamotu Islands 20:45:35 (Near average) Moderate
Pitcairn Island 30:50:20 (Near average) Moderate
Papua New Guinea 40:40:20 (Near average or below) Moderate
Vanuatu 40:45:15 (Near average or below) Moderate
Fiji 40:45:15 (Near average or below) Moderate
Tonga 40:45:15 (Near average or below) Moderate
Niue 40:40:20 (Near average or below) Moderate
Samoa 40:45:15 (Near average or below) Low – moderate
Southern Cook Islands 40:45:15 (Near average or below) Low – moderate
New Caledonia 50:35:15 (Below) Moderate
Marquesas Islands 50:30:20 (Below) 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.