Tropical rainfall and SST outlook: June to August 2014
The dynamical forecast models are in good agreement this month and predict (June – August 2014 period) a pattern consistent with anomalies generally experienced during El Niño.
The ITCZ is forecast to be shifted and intensified towards the Equator, and large regions of the south Pacific are forecast to be drier than normal.
Near or above normal rainfall is forecast for Eastern Kiribati, Western Kiribati, the Federated States of Micronesia and the Southern Cook Islands.
Near normal rainfall is expected for the Austral Islands, Niue and Tonga.
Normal or below normal rainfall is forecast for the Samoa, Fiji, New Caledonia, Pitcairn Island, the Solomon Islands, the Society Islands, the northern Cook Islands, Papua New Guinea, Tokelau, Tuvalu, Vanuatu, Wallis & Futuna and the Tuamotu archipelago.
Below normal rainfall is forecast for the Marquesas.
The global model ensemble forecast for SSTs indicate a pattern consistent with El Niño, with higher than normal SSTs in the central and eastern Equatorial Pacific and lower than normal temperatures in the southeastern Pacific.
Above normal SSTs are forecast for Western Kiribati and Eastern Kiribati.
Near normal or above normal SSTs are forecast for the Federated States of Micronesia, Samoa, Tokelau and Tuvalu. Near normal SSTs are forecast for Fiji, the Marquesas, New Caledonia, the northern Cook Islands, Papua New Guinea, the Solomon Islands, Tonga, Vanuatu, Wallis & Futuna and Niue.
Normal or below normal SSTs are forecast for the Society Islands, the southern Cook Islands, the Tuamotu archipelago and Pitcairn Island.
Below normal SSTs are forecast for the Austral Islands.
The confidence for the rainfall outlook is generally moderate to high, uncertainty is greater for Eastern Kiribati, Samoa and the Marquesas.
The average region–wide hit rate for rainfall forecasts issued in May is 63 %, equals to the average for all months combined.
Confidence for the SST forecasts is moderate for many Islands (see table below).
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 June-August 2014 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:
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 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".
