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Tropical rainfall and SST outlook: October to December 2011

1 November 2011

A La Nina pattern is re-emerging in the ensemble ofglobal climate forecasts.  

A La Nina pattern is re-emerging in the ensemble ofglobal climate forecasts. Suppressed convection is expected in the southwest Pacific near Tuvalu, Tokelau, the Tuamotu Archipelago, the Northern Cook Islands, Samoa, Pitcairn Island, and Western Kiribati which are forecast to receive below normal rainfall in the coming three months. Average or below average  rainfall is expected for the Society Islands, Eastern Kiribati, 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.

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

The confidence for the rainfall outlook is moderately high. The average region–wide hit rate for rainfall forecasts issued in October is 67%, four 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 Samoa. 

Rainfall anomaly outlook map for October to December 2011
SST anomaly outlook map for October to December 2011
NOTE: Rainfall and sea surface termperature estimates for Pacific Islands for the next three months are given in the tables below. 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.