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Three-month article

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.