Island Climate Update 39 - December 2003

Defining ENSO Events
Dr Brett Mullan, NIWA
Given the very wide interest in the climate impacts of El Niño-Southern Oscillation (ENSO), it is perhaps surprising that there is no universally accepted definition of an ENSO event. There is certainly qualitative agreement on typical characteristics associated with El Niño and La Niña periods.

Forecast validation
Forecast period: September to November 2003
Average or above average rainfall was expected in Papua New Guinea, but suppressed convection in equatorial regions was expected to result in average or below average rainfall in Western and Eastern Kiribati and the Marquesas Islands. Rainfall was also expected to be average or below average in Tuvalu and Fiji.

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 39 – 5 December 2003
November’s climate: Below average rainfall persisted over much of the equatorial Southwest Pacific, with suppressed convection over much of Western and Eastern Kiribati, Tokelau, and also parts of northern French Polynesia. Rainfall was also below average over the southern Coral Sea, and parts of New Caledonia, Fiji, and Niue.

ENSO and Sea Surface Temperatures
Warmer SSTs in western equatorial Pacific
Neutral ENSO state forecast till April 2004
The Equatorial Pacific is in a neutral state, but equatorial SSTs remain higher than normal, especially just west of the Date Line.
The NINO3 SST anomaly for November is about +0.6°C and NINO4 is above +1.1°C (3-month means are +0.6°C and +0.9°C, respectively). Despite the relatively high SST anomalies, zonal winds have been near-normal in the Equatorial Pacific recently, after a period of westerly anomalies in the west.

Climate developments in November 2003
Below average rainfall persists over much of the equatorial Southwest Pacific
Enhanced convection from the Caroline Islands to southern Vanuatu
High rainfall persists in Pitcairn Island
A large region of suppressed convection and generally below average rainfall encompassed much of Western and Eastern Kiribati, Tokelau, and parts of northern French Polynesia.
Rainfall was also below average (75% of average) over the southern Coral Sea, and parts of New Caledonia, Fiji, and Niue.

Rainfall outlook for December 2003 to February 2004
Average or above average from Western Kiribati southeast to the Tuamotu Islands
Trending below average in New Caledonia, Eastern Kiribati and the Marquesas Islands
Enhanced convection is expected in the equatorial region just west of the Date Line, resulting in average or above average rainfall from Western Kiribati across to the Tuamotu Islands, including the Solomon Islands, Tuvalu, Tokelau and Wallis and Futuna. Average or below average rainfall is forecast for New Caledonia and the Marquesas Islands.

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