Three-month outlook

Rainfall outlook for May to July 2002

Above average rainfall in equatorial latitudes from west to east, including Tonga

Average to below average rainfall from Tokelau to the Marquesas Islands

Mainly average rainfall in other areas

The South Pacific Convergence Zone was further south than average about and west of the date line during April, lying across southern Fiji, and extending east to the Southern Cook and Austral Islands. Rainfall is projected to be above average in Western and Eastern Kiribati, as well as Tonga. Rainfall is likely to be average or above average in Vanuatau and in southern and central French Polynesia. Average rainfall is expected in Papua New Guinea, trending southeast to the Southern Cooks. Average to below average rainfall is projected from Tokelau to the Marquesas. As the southern hemisphere dry season has commenced, the model skill tends to be lower than in previous months.

Rainfall outlook map for May to July 2002

Probabilities of rainfall departures from average

Broad-scale rainfall patterns and anomalies in the southern tropical Pacific area are estimated from the state of large-scale regional climate factors, such as La Niña or El Niño, their effect on the South Pacific and Tropical Convergence Zones, surface and sub-surface sea temperatures, and computer models of the global climate.

Rainfall estimates for the next three months for Pacific Islands are given in the adjacent table. The tercile probabilities (e.g. 20:30:50) are derived from the interpretation of several global climate models. They correspond to the odds of the observed rainfall being in the lowest (driest) one third of the rainfall distribution, the middle one third, or the highest (wettest) one third of the distribution. On the long-term average, rainfall is equally likely (33% chance) in any tercile.

The probabilities shown express the expected shift in the distribution from the long-term average, based on predictions of oceanic and atmospheric conditions. The amount of inter-model forecast consistency is indicated by the levels of confidence expressed in the table.

Rainfall outcomes as estimated from models and historical records. The third column indicates the probability of bottom (below), middle (average) or top (above) tercile rainfall, where a percentage is given. The rainfall outlook (second column) is subjectively estimated from the probabilities of bottom, middle and top terciles.