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Tropical rainfall and SST outlook: April to June 2012

15 May 2012

While ENSO is expected to be in a neutral state over the forecast period, some remnant anomalies from the event are expected to persist, including increased rainfall in a broad horseshoe-shaped zone affecting the Tropical western Pacific west of the Dateline and parts of the southwest Pacific region.

Relatively dry conditions are expected for the eastern part of the Maritime Continent and the SPCZ is still expected to be displaced slightly southwest of normal during the forecast period. As of late, indications are that the SPCZ will probably return to its climatological position by winter.

Above normal rainfall is forecast for Papua New Guinea. Near or above average rainfall is expected for Fiji, New Caledonia, Tonga, Vanuatu and Niue.

Near normal rainfall is forecast for the Austral islands, Western Kiribati, the Marquesas, Pitcairn, The Solomon Islands, the Tuamotu Archipelago and Wallis & Futuna.

Average or below average rainfall is expected for Samoa, the Society Islands, the Northern Cook Islands, Tokelau and Tuvalu. No clear guidance is given for the Southern Cook
Islands and Eastern Kiribati.

La Niña SST anomaly signals that have existed in past months have eased. For the coming three months, below normal sea surface anomalies are expected to the east of the Dateline near Eastern Kiribati.

Normal or below normal SSTs are forecast for Western Kiribati, Wallis & Futuna, Tuvalu, the Tuamotu Archipelago, the Society Islands, the Northern Cook Islands, Tokealu, the Marquesas and Samoa.

Near normal or above normal sea surface temperatures are forecast for Papua New Guinea, Vanuatu, Fiji, Tonga, the Austral Islands and Niue. Near normal SSTs are forecast for the Solomon Islands, New Caledonia, the Southern Cook Islands and Pitcairn Island.

The confidence for the rainfall outlook is moderately high. The average region–wide hit rate for rainfall forecasts issued in April is 57%, six percent lower than the long–term average for all months combined. The SST forecast uncertainty is greatest near the Marquesas and Eastern Kiribati. 

SST anomaly outlook map for March to May 2012.
Rainfall anomaly outlook map for March to May 2012.
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.