The IRI has prepared this experimental Climate Outlook for the Pacific Islands for March - August 2002. Of relevance in the preparation of this outlook is the prediction of warmer than average conditions to develop in the eastern equatorial Pacific during the next 3 to 6 months. Currently the sea surface temperatures (SSTs) in the eastern equatorial Pacific are slightly below their long-term average (SSTs), but have been increasing during the month of February. Above average SSTs continue in the central Pacific near the international date line, extending also across the western part of the basin. During the course of the four forecast seasons March-May 2002, April-June 2002, May-July 2002, June-August 2002, the SST anomalies in the eastern tropical Pacific are expected to increase, becoming above normal by Apr-May-Jun and continuing to warm through Jun-Jul-Aug. A developing weak El Nino is indicated in this scenario. The somewhat warmer than average SSTs that have been present over much of the Indian Ocean are expected to decrease slowly through the forecast period. The area of above-average temperature in the tropical north Atlantic Ocean is predicted to persist but slowly weaken through the period.

This Outlook was prepared using the following procedures and information:

A) Coupled ocean-atmosphere model predictions of tropical Pacific SST covering the forecast period. Particularly heavy weighting has been given to predictions from the coupled model operated by the NOAA National Centers for Environmental Prediction, Climate Modeling Branch. This model suggests a continuation of near-average conditions during the first forecast season. The forecast for near-neutral conditions is consistent with some, but not all, numerical and statistical forecasts of central and eastern Pacific SSTs.

B) Forecasts of the tropical Indian ocean using a statistical model developed by the IRI.

C) Global atmospheric general circulation model (GCM) predictions of the atmospheric response to the present and predicted sea-surface temperature patterns.

D) Other sources of information include NASA's Seasonal to Interannual Prediction Project (GSFC-NASA) and also seasonal prediction research at COLA.

The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by the national meteorological services in the region. Thus, this product may differ from the official forecasts issued in those areas. The Climate Outlook for March - August 2002 is dependent on the accuracy of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is some uncertainty concerning the evolution of SSTs. Spread (variation) in global SST predictions is a source of uncertainty in the Outlook provided here. In particular, the forecasts for the tropical Pacific have been an important influence on the forecasts over Africa. Note that even if perfectly accurate SST forecasts were possible, there would still be uncertainty in the climate forecast due to chaotic internal variability of the atmosphere. These uncertainties are reflected in the probabilities given in the forecast.

It is stressed that the current status of seasonal-to-interannual climate forecasting allows prediction of spatial and temporal averages, and does not fully account for all factors that influence regional and national climate variability. This Outlook is relevant only to seasonal time scales and relatively large areas; local variations should be expected, and variations within the 3-month period should also be expected. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.

This Outlook covers four seasons: March-May 2002, April-June 2002, May-July 2002 and June-August 2002. Maps are given showing tercile probabilities of precipitation and temperature. The maps for precipitation indicate the probabilities that the seasonal precipitation will fall into the wettest third of the years (top number), the middle third of the years (middle number), or the driest third of the years (bottom number). The color shading indicates the probability of the most dominant tercile -- that is, the tercile having the highest forecast probability. The color bar alongside the map defines these dominant tercile probability levels. The upper side of the color bar shows the colors used for increasingly strong probabilities when the dominant tercile is the above-normal tercile, while the lower side shows likewise for the below-normal tercile. The gray color indicates an enhanced probability for the near-normal tercile (nearly always limited to 40%). As before, numbers and their associated histograms show the probabilities of the three terciles. In areas with lots of spatial detail, there may not be sufficient room on the map, to allow histograms for each region. In those cases, some idea of the probabilities may be gained from the color alone. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Areas that are marked by "D" represent regions for which less than 3cm of precipitation typically occurs over the season. Otherwise, for example, in the case of the northwestern Solomon Islands in March-May 2002 (Map A), there is a 40% probability that the precipitation will be in the wettest third of the years, a 35% chance it will be in the near-normal third of the years, and a 25% chance that the precipitation will be in the driest third of the years.

Maps of temperature show expected probabilities that the seasonal temperatures will fall into the warmest third of the years, the middle third of the years, or the coldest third of the years (Map A). The numbers for each region on the temperature maps indicate the probabilities of temperatures to fall in each of the three categories, above-, near-, and below-normal.

An additional precipitation map is provided for the first season indicating probabilities for extreme precipitation anomalies. Extremes are defined as anomalies that fall within the top and bottom 15th percentile of the observed records. A priori, there is a 15% probability of being within the extremely wet category, and a 15% probability of being within the extremely dry category, leaving a 70% probability that the precipitation will not be extreme. The maps indicate areas of increased risk of extreme precipitation totals. Three levels of increased risk are defined: slightly enhanced risk, enhanced risk, and greatly enhanced risk. For slightly enhanced risk, there is a 25-40% probability that precipitation will be within the indicated extreme, i.e. wet or dry. This represents an approximate doubling of the climatological risk. For enhanced risk, there is a 40-50% probability that precipitation will be within the indicated extreme. This represents an approximate tripling of the climatological risk. For greatly enhanced risk, the probability that precipitation will be within the indicated extreme exceeds 50%, i.e. the indicated extreme is the most likely outcome. A similar map is provided in the first season indicating probabilities of extreme temperature anomalies.

Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

March-May 2002 through June-August 2002:

The following discussion briefly describes the probability anomaly forecasts. In this disussion, the first, second, third and fourth forecast periods refer, respectively, to March-April-May, April-May-June, May-June-July, and June-July-August.

Enhancement of the probabilities for categories of precipitation in the Pacific region is often related to expected SST anomalies. Where SST is expected to be above normal, precipitation is often (but not always) expected to be above normal, and vice versa. For the four forecast periods, most of the tropical Pacific SST from the date line westward to Indonesia is expected to stay above normal, and a slightly enhanced to enhanced probability for above normal precipitation is forecast in most of that region. French Polylnesia is generally tending in the opposite direction, with an enhanced probability for below normal rainfall with neutral to below normal SST to the south of the equator. Along the equator from eastern Kiribati eastward to the Galapagos, the climatology forecast is given for the four forecast periods, as slightly below normal SST is expected to become slightly above normal SST from May onwards. This increase in SST, which may be associated with a developing El Nino (see the "El Nino Watch" on the IRI's home page), is limited to a quite narrow band of SST right along the equator. Because of the narrowness, effects on the precipitation forecast at islands such as Kiribati are not strong enough to appear as shifts in the probabilities. Note, however, that if the narrowness in the SST forecast is overdone, and the SST actually warms in a wider band, these islands would be expected to receive above normal rainfall.

Temperature Enhancement of the probabilities for categories of temperature in the Pacific region is often related to expected SST anomalies. Where SST is expected to be above normal, temperature is usually expected to be above normal, and vice versa. During the four forecast periods, most of the tropical Pacific SST from the date line westward to Indonesia is expected to stay above normal, and a slightly enhanced to greatly enhanced probability for above normal temperature is forecast in most of that region. From eastern Kiribati eastward to the Galapagos, the climatology forecast is given for the first period; in this area, slightly below normal SST is expected to give way to near normal to above normal SST by May. During the later two forecast periods the tropical Pacific SST is forecast to shift toward a mild warm phase of ENSO, where the eastern part of the basin will move toward somewhat warm, and the west part of the basin will remain slightly on the warm side. This will modify the circulation pattern and change the temperature forecast pattern assigned for the first forecast period, such that some of the islands south of the equator and east of the date line will have enhanced probabilities of below normal temperature (due to below normal SST presently at the equator to move there), while locations along the equator (the Galapagos, and the eastern islands of Kiribati) are expected to have warming SST and therefore move toward an enhanced probability of above normal temperature. Because the warming of the equatorial SST east of the dateline in the latter periods is expected to be narrow (extending only from about 5 degrees north to 5 degrees south of the equator), the islands in eastern Kiribati are not necessarily expected to become warmer than normal. Note, however, that if the narrowness in the SST forecast is overdone, and the SST actually warms in a wider band, these islands would be expected to have enhanced probabilities of above normal temperature.

OBSERVED CLIMATOLOGY DATA for Mar-Apr-May, Apr-May-Jun, May-Jun-Jul and Jun-Jul-Aug