The IRI has prepared this experimental Climate Outlook for South America for April - September 2002. Of relevance in the preparation of this outlook is the prediction of eastern equatorial Pacific sea surface temperatures (SSTs) warming to slightly above normal at the beginning of the first forecast period. In the coming 6 months, the SSTs anomalies are expected to increase somewhat more. Currently the SSTs across much of the eastern and central equatorial are slightly above their long-term average (SSTs). Higher than average SSTs have been in place in the far western Pacific and near the international dateline over the last several months. Higher than average SSTs have also more recently developed immediately off the west coast of South America. This same general pattern is predicted to be maintained in the first season of the forecast April-June 2002, while during the second, third and fourth seasons of the forecast the equatorial SST thoughout the entire eastern and central Pacific is predicted to become progressively warmer, attaining the level of a weak El Nino (SSTs between 0.5 and 1 degree C above normal) by the fourth period May-July 2002, June-August 2002, July-September 2002. Note that this prediction for a weak El Nino is that given by one ENSO prediction model. It is described here because it served as the boundary forcing for the following climate prediction, which is primarily based on dynamical global climate models. This should not be confused with the IRI's ENSO Statement, which while indicating a significant potential for an El Nino to develop also highlights the uncertainty still present in the prediction at this time. In the other tropical oceans, warmer than average SSTs continue to dominate much of the Indian Ocean. These are expected to decrease slowly through the forecast period, becoming slightly below normal near the coast of southern Asia and in parts of western Indonesia in the later forecast periods. The area of above-average temperature in the tropical north Atlantic Ocean is expected to persist but gradually weaken through the four forecast periods.

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 April - September 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: April-June 2002, May-July 2002, June-August 2002 and July-September 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 April-June 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 much of central 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.

April-June 2002 through July-September 2002:

The following discussion briefly describes the probability anomaly forecasts:

Enhanced probabilities for below normal precipitation are forecast in parts of central and southern Brazil in all four of the predicted periods, extending into northern Brazil in the third and especially the fourth forecast period. Enhanced probabilities for near normal or below normal precipitation are forecast for northeastern Brazil during their rainy season this year. During the first three forecast periods enhanced probabilities for toward above normal precipitation is forecast for varying portions of northern and northwestern South America, due to the above normal SST in the northern tropical Atlantic.

Enhanced probabilities for above normal temperature are forecast for most of tropical South America for all four forecast periods, extending as far south as Pagaguay and southern Brazil. An exception is the enhanced probabability for below normal temperature in a small part of eastern Brazil in the first two forecast periods, caused by slightly below normal SST in the adjacent Atlantic at that latitude. During all four forecast periods, an enhanced probabability for below normal temperature is forecast in southern Chile and Argentina, aided by below normal SST in the vicinity.

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