The tropical Pacific remains in a neutral ENSO state. The weak SST anomalies that were present in February along the Equator, with slightly warmer than normal temperatures to the west and cooler than normal SSTs to the east has weakened further in March.
The dynamical models indicate that the SPCZ will be situated slightly south of its climatological position during April – June 2013.
Near or above normal rainfall is forecast for Papua New Guinea, the Solomon Islands and the Federated States of Micronesia.
Normal or below normal rainfall is forecast for the Austral Islands, the Northern Cook Islands, the Southern Cook Islands, Eastern Kiribati, Western Kiribati, Tuvalu, the Tuamotu Islands and Tokelau.
Near normal rainfall is expected for Fiji, New Caledonia, Niue, Pitcairn Island, Samoa, the Society Islands, Tonga, Vanuatu, Wallis & Futuna and the Marquesas.
The SSTs forecasts from the global models generally agree to predict relatively warmer than normal SSTs in the southeast of the Pacific at subtropical and mid-latitude, while anomalies in the tropical domain are forecast to be weak.
Near normal or above normal SSTs are forecast for Fiji, the Northern Cook Islands, the Marquesas, Niue, Papua New Guinea, Samoa, the Solomon Islands, Tonga and the Tuamotu Islands.
Near normal or below normal SSTs are forecast for Eastern Kiribati. Normal sea surface temperatures are expected elsewhere.
The confidence for the rainfall outlook is moderate to high. The average region–wide hit rate for rainfall forecasts issued in March is 57 %, five points lower than the long–term average for all months combined.
The SST forecast confidence is high across the region except for the Marquesas, the Austral Islands and Eastern Kiribati, where uncertainty is greater.
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 April-June 2013 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 from the ftp server at URL ftp://disc2.nascom.nasa.gov/data/TRMM/Gridded/Derived_Products/3B42RT/D…
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 1998 – 2012 period. The categories ("Below", "Normal - Below", etc.) are determined according to the 20th, 40th, 60th and 80th percentiles, so that e.g. when a month's rainfall is below the climatological 20th percentile, it is ranked in the "Below" category, between the 20th and 40th, "Normal - Below", between the 40th and 60th "Normal" and so on.
