Island Climate Update 55 - April 2005

An overview of the present climate in the tropical South Pacific Islands, with an outlook for the coming months, to assist in dissemination of climate information in the Pacific region.
Number 55 – April 2005
March's climate: Suppressed convection over much of the Southwest Pacific. South Pacific Convergence Zone was weaker and further south and west than average. Warmer in many islands, cooler in New Caledonia. Three named tropical cyclones.
ENSO and sea surface temperatures (SST): El Niño/Southern Oscillation conditions should ease to neutral by May.

The evolution of the weak El Niño 2004-2005
Tony Barnston, International Research Institute (IRI)
El Niño and La Niña episodes differ from one another, not only in their relative strengths, but also their seasons of onset, maturity, and demise, as well as the locations of their maximum sea surface temperature (SST) anomaly within the tropical Pacific. The peak SST anomaly at the warmest location in the tropical Pacific during an El Niño may exceed 5°C, as was the case in the great 1997-98 episode, or be only near 2°C, as in the 1994-95 or the 2004-05 episodes.

Forecast validation
Forecast period: January to March 2005
Enhanced convection was expected over Eastern and Western Kiribati and Tuvalu, with average or above average rainfall in Tokelau, the Northern Cook Islands, and the Society Islands of Central French Polynesia. Areas of suppressed convection with below average rainfall were expected over the Marquesas Islands and also New Caledonia, with average or below average rainfall in Vanuatu, Fiji, Tonga, Niue, Southern Cook Islands, and Papua New Guinea.

ENSO and Sea Surface Temperatures
The tropical Pacific Ocean is in a borderline El Niño state, with the monthly Southern Oscillation Index (SOI) back to near zero after its strong negative excursion in February. Surface zonal winds have returned to near normal across the Equatorial Pacific, and outgoing longwave radiation (OLR) anomalies now show suppressed convection across most of the tropical Pacific south of the Equator.

Climate developments in March 2005
The outgoing long-wave radiation (OLR) anomaly pattern was very different from that of February. A large region of suppressed convection prevailed in March, extending from the Solomon Islands east to the Marquesas Islands of Northern French Polynesia, including Tuvalu, Wallis and Futuna, Western Samoa, Tokelau, northern Tonga, and the Northern Cook Islands.

Tropical rainfall outlook: April to June 2005
Continuing incoherence between the atmospheric and oceanic conditions because of the decaying El Niño conditons in the equatorial Pacific, and April to June being transition months from the wet to the dry season, resulted in lack of normal rainfall patterns in the Pacific region.
Enhanced convection is expected over Western Kiribati and the Northern Cook Islands, where the rainfall is forecast to be near or above average.
Dry conditions are expected over the Marquesas Islands of French Polynesia, where the expected rainfall is below average

Tropical Cyclone Update
There were three tropical cyclones in March. Tropical cyclone Percy, which affected Tokelau was, the seventh named cyclone for the season. This was followed by Rae (near the Southern Cook Islands) on 6 March, and then Ingrid, which formed in the Coral Sea south of Papua New Guinea and then tracked west over Queensland and the Gulf of Carpentaria.

Sources of South Pacific rainfall data
This bulletin is a multi-national project with important collaboration from the following Pacific nations:
American Samoa
Australia
Cook Islands
Fiji
French Polynesia
Kiribati
New Caledonia
New Zealand
Niue
Papua New Guinea
Pitcairn Island
Samoa
Solomon Islands
Tokelau
Tonga
Tuvalu
Vanuatu
Requests for Pacific island climate data should be directed to the Meteorological Services concerned.
Acknowledgements
This bulletin is made possible with financial support from the New Zealand Agency for International Development (NZAID), Wellington, New Zealand, wi