The IRI has prepared this experimental Climate Outlook for South America 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.

METHODS -

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 in global SST predictions is a source of uncertainty in the Outlook provided here. 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 season should also be expected. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.

OUTLOOK -

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 March-May 2002 (Map A), there is a 25% 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 40% chance that the precipitation will be in the driest third of the years in part of eastern and northeastern Brazil.

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.

Enhanced probabilities for above normal precipitation are forecast in parts of northern South America in all four of the predicted periods, with largest regions of enhancement during the first two periods. Enhanced probabilities for below normal precipitation are forecast for all four forecast periods in significant portions of Brazil. In northeastern Brazil, there is an enhanced probability for near normal is forecast for the first two periods (constituting the rainy season), turning toward dryish in the third and fourth forecast periods. Enhanced probabilities for below normal precipitation are forecast for the third and fourth forecast period in extreme southeastern Brazil and/or Uruguay.

Enhanced probabilities for above normal temperature are forecast for much of South America for all four forecast periods, with strongest enhancement in the northern (tropical) portions. Enhanced probabilities for below normal temperature are forecast for extreme southern South America for the first two forecast periods, and for part of eastern (coastal) Brazil for the first three forecast periods.

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

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