Tropical rainfall and SST outlook: April to June 2010

Tropical rainfall and SST outlook: April to June 2010

During April – June 2010, a region of suppressed convection is likely in the southwest Pacific encompassing the Marquesas and Vanuatu. Below average rainfall is expected for those island groups.Near or below normal rainfall is expected for Tonga, Niue, Fiji, and Pitcairn Island. Enhanced convection is likely along the Equator extending from Western to Eastern Kiribati, with an expectation of above average rainfall for those islands. Near or above average rainfall is forecast for Papua New Guinea, the Northern Cook Islands, Tuvalu, Tokelau, Samoa, and the Society Islands. Near normal rainfall is forecast for the Tuamotu Archipelago, the Southern Cook Islands, New Caledonia, and Wallis & Futuna. No clear precipitation guidance is offered for the Solomon Islands.

The global models are continuing to show elevated temperatures in the near equatorial Pacific. Cold anomalies that existed around Tonga and Niue in previous months are expected to persist in the coming three month period. Above average sea surface temperatures are forecast for Eastern and Western Kiribati. A region of near or above average sea surface temperatures is forecast around Tokelau, Tuvalu, the Northern Cook Islands, and the Marquesas. Average or below average SSTs are forecast for Tonga, Niue, and Pitcairn Island. No clear SST guidance is offered for Fiji. Near normal SSTs are forecast for the remainder of the southwest Pacific.

The confidence in the multi-model ensemble forecast skill for this seasonal rainfall outlook is moderate to moderately high. In the past, the average region-wide hit rate for rainfall forecasts issued in April is 55%, 6% lower than the long-term average for all months combined. The SST forecast confidence is mostly high, but the greatest uncertainty is localised around the Marquesas, Fiji, and Eastern Kiribati.

Rainfall outlook map for April to June 2010

SST outlook map for April to June 2010

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.