Tropical rainfall and SST outlook: May to July 2011

During May – July 2011, region of suppressed convection is expected in the southwest Pacific encompassing...

During May – July 2011, region of suppressed convection is expected in the southwest Pacific encompassing Tuvalu and the Northern Cook Islands. Below average rainfall is expected for those island groups. Average or below average ainfall is expected for Samoa, the Marquesas, the Society Islands, the Tuamotu Archipelago and Tokealu. Enhanced convection is likely along the Southwest Pacific Convergence Zone, which is expected to return to a position close to normal. Tonga, Fiji, Niue and the Austral Islands are expected to receive above normal rainfall for the coming three month period. Near or above average rainfall is forecast for Western Kiribati and Eastern Kiribati, Papua New Guinea, New Caledonia, the Solomon Islands, Vanuatu, and the Southern Cook Islands. Near normal rainfall is expected for Pitcairn Island. No clear rainfall guidance is offered for Wallis & Futuna.

Many global models continue to show weakening and contraction of the equatorial Pacific SST cold anomalies close to the International Dateline in the coming months. For May – July, above average SSTs are forecast for the Austral Islands and Tonga. Average or above average sea surface temperatures are forecast for Papua New Guinea, Vanuatu, Fiji, Pitcairn Island and the Southern Cook Islands. Near normal or below normal SSTs are forecast for Wallis & Futuna, Samoa, the Society Islands, and the Tuamotu Archipelago. The Northern Cook Islands, Tuvalu, Tokelau, Eastern Kiribati and the Marquesas are expected to experience below normal SSTs. Near normal SSTs are forecast for Niue, New Caledonia, Western Kiribati and the Solomon Islands.

The forecast confidence for the rainfall outlook is moderately high. The average region–wide hit rate for rainfall forecasts issued in May is 53%, 8% lower than all months combined. The SST forecast confidence is mostly high, with uncertainty localised near the Society Islands and the Marquesas.

Rainfall anomaly outlook map for May to July 2011

SST anomaly outlook map for May to July 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.