Tropical rainfall and SST outlook: June to August 2011

Signals in the global climate models continue to show easing of the rainfall anomalies associated with La Nina that existed this past season.

Signals in the global climate models continue to show easing of the rainfall anomalies associated with La Nina that existed this past season. During June – August, suppressed convection is expected in the southwest Pacific encompassing Tuvalu, the Tuamotu Archipelago and the Northern Cook Islands. Below average rainfall is expected for those island groups. Average or below average rainfall is expected for the Marquesas, the Society Islands, and Tokealu. Enhanced convection is likely along the Southwest Pacific Convergence Zone, which is expected to return to a position close to normal. Tonga and Vanuatu 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, Fiji, Niue, the Austral Islands, New Caledonia, and the Southern Cook Islands. Near normal rainfall is expected for Pitcairn Island, Samoa, the Solomon Islands and Wallis & Futuna.

The global model ensemble shows weakening of near equatorial Pacific sea surface temperature cold anomalies relative to past months. For austral winter, some remnant cold SST anomalies related to the passing La Nina may persist close to and east of the Dateline. Above average SSTs are forecast for the Austral Islands. Average or above average SSTs are forecast for Papua New Guinea, the Solomon Islands, Niue, Tonga, Fiji, and the Southern Cook Islands. Near normal or below normal SSTs are forecast for Tuvalu, Eastern Kiribati, the Society Islands, Tokelau, and the Tuamotu Archipelago. The Northern Cook Islands and the Marquesas are expected to experience below normal SSTs. Near normal SSTs are forecast for Western Kiribati, Vanuatu, New Caledonia, Samoa, Wallis & Futuna, and Pitcairn Island.

The confidence for the rainfall outlook is moderately high. The average region–wide hit rate for rainfall forecasts issued in June is 60%, 1% lower than all months combined. The SST forecast confidence is mostly high.