Tropical rainfall and SST outlook: December 2014 to February 2015
The dynamical models forecasts indicate that the west equatorial as well as some parts of the southeast Pacific are likely to experience normal or above-normal rainfall for the December 2014 - February 2015 season as a whole.
In contrast, large regions in the east and southwest Pacific are expected to experience reduced rainfall. Normal or below normal rainfall is forecast for Fiji, Eastern Kiribati, Niue, Pitcairn Island, Tuvalu, Wallis & Futuna, the Marquesas, New Caledonia, the Tuamotu archipelago and Vanuatu. Normal or above normal rainfall is forecast for the Austral Islands, the Society islands, Western Kiribati and Papua New Guinea.
Near normal rainfall is expected for the Southern Cook Islands and Tonga.
No clear guidance is available for Samoa, the Solomon Islands, the Northern Cook Islands, Tokelau and the Federated States of Micronesia.
The global model ensemble forecast for SSTs indicate higher than normal SSTs over the whole equatorial Pacific, with maximum anomalies just west of the International Dateline as well as east of about 150°W.
Above normal SSTs are forecast for western Kiribati and eastern Kiribati. Normal or above normal SSTs are forecast for Fiji, the Federated States of Micronesia, Samoa, the Solomon Islands, Tokelau, Tuvalu and Wallis & Futuna.
Near normal SSTs are forecast for the Marquesas, Pitcairn Island, the southern Cook Islands, the Tuamotu archipelago and Vanuatu.
No guidance is available (i.e. equal chances are given to each tercile) for the Austral Islands, New Caledonia, Niue, the northern Cook Islands, Papua New Guinea, the Society Islands and Tonga.
The confidence for the rainfall outlooks is moderate to high. The average region–wide hit rate for rainfall forecasts issued for the December – February season is 64 %, two points higher than the average for all months combined. Confidence for the SST forecasts is 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 December 2014-February 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:
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".
