Tropical rainfall and SST outlook: July to September 2010

During July – September 2010, suppressed convection is likely in the southwest Pacific encompassing the Marquesas, the Tuamotu Archipelago, Eastern Kiribati, Pitcairn sland, and Western Kiribati.

Near or below average rainfall is expected for those island groups, except for Eastern Kiribati where below normal rainfall is forecast. Enhanced convection is likely near the Equator in the far western Southwest Pacific, with Papua New Guinea, the Solomon Islands, Vanuatu, Wallis & Futuna and Samoa expected to receive above normal rainfall. Near or above average rainfall is forecast for the Northern Cook Islands, the Southern Cook Islands, Niue, Fiji and New Caledonia. Near normal rainfall is forecast for Tuvalu and the Austral Islands. No clear precipitation guidance is offered for Tokelau or the Society Islands.

Many global models show a continued strengthening in the near equatorial Pacific sea surface temperature anomalies in the coming months, and propagation of a cold tongue from east to west toward the Date line. Above average SSTs are forecast for the Southern Cook Islands, while average or above average sea surface temperatures are forecast for Papua New Guinea, Vanuatu, New Caledonia, the Northern Cook Islands, the Austral Islands, Tokelau, Tuvalu and Wallis & Futuna. Near normal or below normal SSTs are forecast for the Marquesas, while Eastern Kiribati is expected to experience below normal SSTs. No clear SST guidance is offered for Fiji, Niue, the Society Islands, Tonga, and Western Kiribati. Near normal SSTs are forecast for the remainder of the southwest Pacific.

The multi–model ensemble forecast confidence for this rainfall outlook is moderately high. In the past, the average region-wide hit rate for rainfall forecasts issued in July is 64%, 3% higher than the long–term average for all months combined. The SST forecast confidence is mostly moderately high, but the greatest uncertainty is localised around the International Date line.


Rainfall outlook map for July to September 2010

SST outlook map for July to September 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.