Tropical rainfall and SST outlook: October to December 2015

The dynamical models are all in agreement to forecast continuing El Niño conditions for the October – December  2015 period. As a consequence most regions of the southwest Pacific usually affected by the South Pacific Convergence Zone are forecast to experience a drier than normal October – December season.

Below normal rainfall is forecast for the Austral Islands, New Caledonia, the Society Islands, Wallis & Futuna, the Federated States of Micronesia, the southern Cook Islands, Papua New Guinea, Samoa, the Solomon Islands, Vanuatu, Niue, Tonga and Fiji. Normal or below normal rainfall is forecast for the northern Cook Islands and the Tuamotu archipelago. Near normal rainfall is expected for Pitcairn Island. Normal or above normal rainfall is forecast for Tokelau and the Marquesas. Above normal rainfall is forecast for Eastern Kiribati, Western Kiribati and Tuvalu.

The global model ensemble forecast for SSTs indicates persistence and further intensification of the higher than normal SSTs present in the central and eastern equatorial Pacific, the large region of cooler than normal SSTs in the southwest Pacific is also forecast to intensify and expand. Above normal SSTs are forecast for western Kiribati, eastern Kiribati and the Marquesas. Normal or above normal SSTs are forecast for the Austral Islands, the norther Cook Islands, Tokelau and Tuvalu. Near normal SSTs are forecast for the Federated States of Micronesia, New Caledonia, Pitcairn and the Tuamotu archipelago. Normal or below normal SSTs are forecast for Fiji, Papua New Guinea, the Solomon Islands, the southern Cook Islands, Tonga, Vanuatu and Wallis & Futuna. Below normal SSTs are forecast for Niue.

The confidence for the rainfall outlooks is generally high. The average region-wide hit rate for rainfall forecasts issued for the October – December season is about 66%, three points higher than the average for all months combined. The confidence for the SST forecasts is also generally high.

The figure on the bottom right presents the last six months rainfall anomalies for each Island group alongside the latest ICU rainfall forecast for the October-December 2015 period.

The past 6 months rainfall anomalies are based on the near-real-time TRMM (Tropical Rainfall Measurement Mission) merged satellite product available from http://trmm.gsfc.nasa.gov. The data has been downloaded and available on the link below:

TRMM rainfall anomalies

For each Island group, the monthly value is derived from the average of all grid-points (or "pixels") in the TRMM Dataset that intersect a coastline, to ensure that the values correspond as closely as possible to rainfall on land, and excluding rainfall falling on ocean surfaces.

The climatology used has been established over the 2001 – 2012 period. The categories ("Well-below", "Below", etc) are determined according to the percentage of the normal rainfall for that month. The thresholds are indicated in the colorbar at the bottom: to give an example, "Well-below" normal rainfall means the rainfall for that month was under 40 % of the normal rainfall, "Below" normal rainfall means that between 40 and 80 % of normal rainfall was received, etc.

IMPORTANT NOTE:

Please note that, while we use the same color-scheme for the past rainfall anomalies and the ICU forecast, the type of information presented is different. In the case of the past 6 months, actual rainfall has been estimated by satellite, and the categories are well-defined by monthly estimated rainfall compared to the long-term mean. The ICU forecast, on the other hand, is probabilistic: it indicates the likelihood (percentage chance) of rainfall being at, above, or below normal for the season as a whole. When the percentage chances in two categories are close to each other, we indicate both categories: for example if the forecast is for 35 % chance of receiving below rainfall, and 40 % chance of normal rainfall, the outcome is "Normal or below".

Rainfall anomaly outlook map for October – December 2015
SST anomaly outlook map for October – December 2015
NOTE: Rainfall and sea surface temperature estimates for Pacific Islands for the next three months are given in the tables below. 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.
The last six months rainfall anomalies for each Island group alongside the latest ICU rainfall forecast for the October-December 2015 period.