Tropical rainfall and SST outlook: August to October 2011

Signals in the global climate models continue to showeasing of the rainfall anomalies associated with La Nina that existed earlier in the year. During August – October suppressed convection is expected in the southwest Pacific near Tuvalu, the Northern Cook Islands, and Pitcairn Island, which are expected to receive near normal or below normal rainfall. Above normal rainfall is forecast for Tonga, Niue, and Papua New Guinea. Near or above average rainfall is forecast for Vanuatu, Fiji, the Solomon Islands, the Austral Islands, New Caledonia, and the Southern Cook Islands. Near normal rainfall is expected for Eastern Kiribati, Western Kiribati, Samoa, the Marquesas and Wallis & Futuna. No clear guidance is offered for the Society Islands or Tuamotu.

Equatorial Pacific sea surface temperature cold anomalies relative to past months. Some models suggest development of a warm equatorial sea surface anomaly during austral spring. For the coming three months, some negative sea surface temperatures (SSTs) 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 Northern Cook Islands. Normal or below normal SSTs are forecast for the Tuamotu Archipelago, the Marquesas, the Society Islands, Tuvalu, and Tokelau. Near normal or above normal sea surface temperatures are forecast for Papua New Guinea, the Solomon Islands, New Caledonia, Vanuatu, Fiji, Niue, Tonga, the Southern Cook Islands, and the Austral Islands. Near normal SSTs are forecast elsewhere, except for Eastern Kiribati where no clear SST guidance is offered. 

The confidence for the rainfall outlook is moderately high. The average region–wide hit rate for rainfall forecasts issued in August is 67%, equal to the long–term average for all months combined. The SST forecast confidence is mostly high, with the greatest uncertainty near southern French Polynesia.

Rainfall anomaly outlook map for August to October 2011
SST anomaly outlook map for August to October 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.