Tropical rainfall and SST outlook: September to November 2011

Suppressed convection is expected in the southwestPacific near Tuvalu, Tokelau, the Tuamotu Archipelago, the Northern Cook Islands, and Samoa.

These islands are forecast to receive below normal rainfall in the coming three months. Average or below average rainfall is expected for the Society Islands, Pitcairn Island, and Wallis & Futuna. Above normal rainfall is forecast for Papua New Guinea, Vanuatu, Fiji, Tonga and Niue. Near or above average rainfall is forecast for the Solomon Islands, the Austral Islands, New Caledonia, and the Southern Cook Islands. Near normal rainfall is expected for the Marquesas and Eastern Kiribati. No clear guidance is offered for Western Kiribati.

The global model ensemble continues to show changes in the near equatorial Pacific sea surface temperature cold anomalies relative to past months. For the coming three months, some cold sea surface anomalies are expected south of the Equator and east of the Dateline near the Northern Cook Islands and the northern part of French Polynesia.

Below normal SSTs are expected for the Marquesas. Normal or below normal SSTs are forecast for Western Kiribati, Eastern Kiribati, the Tuamotu Archipelago, the Northern Cook Islands, the Society Islands, Tuvalu, and Tokelau. Above normal SSTs are forecast for Niue and Tonga. Near normal or above normal sea surface temperatures are forecast for Papua New Guinea, the Solomon Islands, New Caledonia, Vanuatu, Fiji, the Southern Cook Islands, and the Austral Islands. Near normal SSTs are forecast for Wallis & Futuna, Samoa, and Pitcairn Island.

The confidence for the rainfall outlook is moderately high. The average region–wide hit rate for rainfall forecasts issued in September is 66%, three percent higher than the long– term average for all months combined. The SST forecast confidence is mostly high, with the greatest uncertainty near Eastern Kiribati and the Marquesas. 

Rainfall anomaly outlook map for September to November 2011
SST anomaly outlook map for September to November 2011
NOTE: Rainfall and sea surface termperature estimates for Pacific Islands for the next three months are given in the tables above. The tercile probabilities (e.g., 20:30:50) are derived from the averages of several global climate models. They correspond to the odds of the observed rainfall or sea surface temperatures being in the lowest one third of the distribution, the middle one third, or the highest one third of the distribution. For the long term average, it is equally likely (33% chance) that conditions in any of the three terciles will occur. *If conditions are climatology, we expect an equal chance of the rainfall being in any tercile.