Tropical rainfall and SST outlook: August to October 2015
The dynamical models are all in agreement to forecast continuing El Niño conditions over the August-October 2015 period. As a consequence, rainfall totals for the coming seasons are expected to be higher than normal along the Equator in the central and eastern Pacific, while many regions of the southwest Pacific are forecast to experience a drier than normal August-October season.
Below normal rainfall is forecast for Vanuatu, Fiji, New Caledonia, Niue, Tonga and Papua New Guinea. Normal or below normal rainfall is forecast for the Marquesas, Wallis and Futuna, the Northern and Southern Cook Islands, Samoa, the Society Islands, the Solomon Islands and Tuamotu. Near normal rainfall is expected for the Austral Islands. Normal or above normal rainfall is forecast for Pitcairn Island and Tokelau. Above normal rainfall is forecast for Tuvalu, Western Kiribati and Eastern Kiribati. No clear guidance was available for the Federated States of Micronesia.
The global model ensemble forecast for SSTs indicates persistence of the higher than normal SSTs currently present in the central and eastern equatorial Pacific, while the large region of cooler than normal SSTs in the southwest Pacific is forecast to expand. Above normal SSTs are forecast for western Kiribati and eastern Kiribati. Normal or above normal SSTs are forecast for Tuvalu, the Society Islands, the Northern Cook Islands, Tokelau and Samoa. Near normal SSTs are forecast for the Austral Islands, the Marquesas, Pitcairn Island, Papua New Guinea, Tuamotu and Wallis Futuna, Normal or below normal SSTs are forecast for Federated States of Micronesia, New Caledonia, Niue, Solomon Islands, the Southern Cook Islands and Vanuatu. Below normal SSTs are forecast for Fiji and Tonga.
The confidence for the rainfall outlooks is generally high. The average region-wide hit rate for rainfall forecasts issued for the August-October season is about 60%, three points lower than the average for all months combined. The confidence for the SST forecasts is moderate to 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 August-October 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.
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".