Tropical rainfall and SST outlook: January to March 2014
The dynamical models are in relative agreement to indicate drier conditions than normal for the January to March 2014 period in the far eastern Pacific, including parts of French Polynesia.
Slightly wetter than normal conditions are expected in parts of the central Pacific south of the Equator as well as in the equatorial west Pacific.
Near or above normal rainfall is forecast for Fiji, Niue, Tonga and the Federated States of Micronesia.
Near normal rainfall is expected for the Austral Islands, the Southern Cook Islands, Western Kiribati, New Caledonia, Pitcairn Island, Samoa, the Solomon Islands, Tokelau, the Tuamotu archipelago, Tuvalu, Vanuatu and Wallis & Futuna.
Normal or below normal rainfall is forecast for the Northern Cook Islands, the Society Islands, Eastern Kiribati and the Marquesas. No clear guidance is available for Papua New Guinea this month.
The global model ensemble forecast for SST indicates that the region of higher than normal temperatures in the central south Pacific that has been present over the past seven months (see Figure in ENSO section) will persist through the summer (January – March 2014).
Normal SSTs are forecast for the Federated States of Micronesia, Eastern Kiribati, the Northern Cook Islands, the Southern Cook Islands, the Society Islands, the Austral Islands, the Tuamotu archipelago, the Marquesas and Pitcairn Island.
Elsewhere in the Pacific there is poor agreement between the dynamical model forecasts, leading to weak guidance (i.e. climatological probabilities).
The confidence for the rainfall outlook is generally high, except for Papua New Guinea, the Solomon Islands, the Tuamotu archipelago, Tuvalu and the Society Islands, where uncertainty is greater.
The average region–wide hit rate for rainfall forecasts issued in January is 63 %, 3 % lower than the long–term average for all months combined. The confidence is moderate to high for the SSTs forecasts.
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 January - March 2014 period.
The past 6 months rainfall anomalies are based on the near-real-time TRMM (Tropical Rainfall Measurement Mission) merged satellite product.
The data has been downloaded at ftp://disc2.nascom.nasa.gov/data/TRMM/Gridded/Derived_Products/3B42RT/Daily.
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 ("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".
