The IRI has prepared this experimental Climate Outlook for South America for January - June 2002. Of relevance in the preparation of this outlook is the prediction of near-average conditions in the central and eastern equatorial Pacific through early March 2002, followed by slightly warmer than normal sea surface temperatures (SSTs) from mid-March through June. Currently the SSTs across much of the eastern and central equatorial are near their long-term average, (SSTs), although slightly lower than average SSTs exist in the eastern portion of the tropical Pacific basin and warmer than average SSTs persist in the central and western parts of the basin. Thus, the Nino 4 region remains slightly above normal, Nino 3.4 is near to very slightly below normal, and Nino 3 and Nino 1+2 are slightly below normal. Overall, this particular "flavor" of near neutral equatorial Pacific SST conditions is expected to persist for the first two overlapping seasons of the forecast, January-March 2002, February-April 2002, while during the later two seasons, March-May 2002, April-June 2002, they are expected to become slightly above average even in the eastern tropical Pacific, suggesting the possibility of a developing mild warm ENSO state. The slightly warmer than average SSTs that continue in parts of the Indian Ocean are expected to decrease slowly toward normal through the forecast period. The area of above-average temperature in the tropical and subtropical north Atlantic Ocean, and the weaker pattern of slightly warm and slightly cold SST in parts of the equatorial and tropical south Atlantic, are expected to continue weakly, although uncertainty is quite high for the portion near and south of the equator.

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 January - June 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: January-March 2002, February-April 2002, March-May 2002 and April-June 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 experiencing their dry season (less than 15% of annual precipitation) and for which less than 5cm of precipitation typically occurs over the season. Otherwise, for example, in the case of the west-central interior of Brazil in January-March 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 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.

January-March 2002 through April-June 2002:

The following discussion briefly describes the probability anomaly forecasts:

Slightly enhanced to enhanced probabilities for below normal precipitation are forecast for part of the northern coast of South America during Jan-Feb-Mar. A slightly enhanced probability for above normal precipitation is forecast for portions of interior northern or western Brasil or its western neighbors (e.g. Colombia, Peru) during Jan-Feb-Mar, Feb-Mar-Apr and Mar-Apr-May. In the Nordeste and surrounding vicinity, slightly enhanced probabilities for above normal precipitation are forecast for Jan-Feb-Mar, shifting to near normal for the three longer lead seasons. With the expected change from the currently below normal SSTs to above normal SSTs off the west coast of Ecuador and northern Peru, the forecast for the Galapagos begins with an enhanced probability for below normal preciptation for Jan-Feb-Mar, becoming above normal for the three longer-lead seasons.

Greatly enhanced probabilities for above normal temperature are forecast for northern Suriname and Guiana for the Jan-Feb-Mar period, surrounded by more weakly enhanced probabilities for warmth in most of the remainder of the continent, with the exception of the Galapagos and extreme southern Chile and Argentina, where a slightly enhanced probability for below normal temperatures is forecast. This slight tilt of the odds toward cold in the southern tip of the South America continues through the Apr-May-Jun period. As the forecast periods progress from Jan-Feb-Mar through Apr-May-Jun, the extensive regions having enhanced probabilities for above normal temperature shrink somewhat to account for the increased uncertainty with lead time. However, two regions that tend to maintain the warmish forecast are central Argentina/Chile and part of northwest Brazil and nearby portions of Peru, Ecuador and Colombia. After a forecast for an enhanced probability for below normal temperature for Jan-Feb-Mar, the Galapagos is given increasing probabilities for above normal temperature as the lead time increases, as the SST surrounding the islands is expected to increase during the coming six months.

OBSERVED CLIMATOLOGY DATA for Jan-Feb-Mar, Feb-Mar-Apr, Mar-Apr-May and Apr-May-Jun

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