Tropical rainfall and SST outlook: March to May 2016
The dynamical models are in agreement to forecast continuing El Niño conditions for the March – May 2016 period (99% chance), and the SPCZ is forecast to be again displaced north of its climatological position (see page 2), leading to many island groups in the southwest Pacific forecast to experience drier than normal conditions over the forecast period (March – May 2016).
Below normal rainfall is forecast for the southern Cook Islands, New Caledonia, Samoa, Tonga, southern Vanuatu, Wallis & Futuna, Fiji, Niue, northern Vanuatu and the Federated States of Micronesia. Normal or below normal rainfall is forecast for the Austral Islands and the Solomon Islands. Normal or above normal rainfall is forecast for Papua New Guinea and the Society Islands. Above normal rainfall is forecast for Eastern Kiribati, Western Kiribati, the Marquesas, the Northern Cook Islands, Tokelau, the Tuamotu archipelago and Tuvalu. Near normal rainfall is forecast for Pitcairn Island.
As El Niño is forecast to continue over the March – May 2016 period, the large positive Sea Surface Temperature (SST) anomalies currently present in the central and eastern equatorial Pacific are expected to still be present persist over the next three months, although the anomalies are forecast to weaken significantly. The region of cooler than normal SSTs present in the south Pacific is forecast to persist. Above normal SSTs are forecast for western Kiribati, eastern Kiribati and the Marquesas. Normal or above normal SSTs are forecast for the Northern Cook Islands, Tokelau and Fiji. Normal or below normal SSTs are forecast for the Austral Islands, Niue, Pitcarin Island and the Austral Islands. Near normal SSTs are forecast elsewhere. The confidence for the rainfall outlooks is moderate to high. The average region-wide hit rate for rainfall forecasts issued for the March – May season is about 62%, 1% below the average for all months combined. The confidence for the SST forecasts is also moderate to high.
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.
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".