The IRI has prepared this experimental Climate Outlook for South America for December 2001 - May 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. Currently the sea surface temperatures (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 normal, and Nino 3 and Nino 1+2 are slightly below normal. Overall, near neutral equatorial Pacific SST conditions are expected for the first two overlapping season s of the forecast, December-February 2002, January-March 2002, while during the two seasons February-April 2002, March-May 2002, they are expected to become near to slightly above average even in the eastern tropical Pacific with slightly below average SST becoming more limited to slightly south of the equator. The warmer than average SSTs that continue in much of the Indian Ocean are expected to decrease slowly toward normal through the forecast period. The area of above-average temperature in the tropical north Atlantic Ocean, and more weakly in part of the tropical south Atlantic, is expected to continue but weaken through the forecast 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 two 3-month forecast seasons, turning toward slightly above normal SST in the latter two seasons. This forecast SST scenario 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 December 2001 - May 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: December-February 2002, January-March 2002, February-April 2002 and March-May 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 December-February 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 Argentina and part of central Chile.

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.

December-February 2002 through March-May 2002:

The following discussion briefly describes the probability anomaly forecasts:

Greatly enhanced probabilities for above normal precipitation (50%) are forecast for much of northeastern Brazil for the December-January-February period, weakening to 45% for the second two 3-month periods and finally to 40% for the final period of March-April-May. This forecast is caused in part by the combination of above normal equatorial north Atlantic SST offshore of this regions, below normal SST in the south tropical Atlantic, and slightly below normal SST in the far eastern tropical Pacific east of 150W longitude. These SST conditions were prominent in the October observations and are expected to continue but weaken throughout the forecast periods. Note that there is an enhanced probability for precipitation in the extreme high 15% of the distribution in part of northeast Brazil for December-January-February. Greatly enhanced probabilities for below normal precipitation (50%) are forecast for northern Columbia and Venezuela for December-January-February, surrounded by more weakly enhanced probabilities for below normal precipitation in the remainder of those countries and other countries in northern and northwestern South America. Note that there is a slightly enhanced probability (of 25% to 40%) for precipitation being in the extreme low 15% of the distribution in part of northern Colombia for December-January-February. This forecast continues, but in weakened and more areally limited form, in January-February-March, and disappears (except for the Windward Islands and a small part of northern Venezuela) in the final two forecast periods. Slightly enhanced probabilities for below normal precipitation are forecast for part of central Argentina and central Chile for December-January-February and January-February-March.

Greatly enhanced probabilities for above normal temperature are forecast for much of South America for the December-January-February period, with maximum probabilities for warm at 50% in northern Bolivia and southeastern Peru and along part of the Colombia/Venezuela border. At least slightly enhanced probabilities for above normal temperature cover approximately two-thirds of the continent in December-January-February, decreasing to less than one-fourth of the continent by the fourth forecast period of March-April-May. Slightly enhanced probabilities for below normal temperature are forecast for a portion of eastern Brazil through February-March-April, and for a small portion of southern Chile and Argentina for January-February-March and February-March-April. Slightly enhanced probabilities for near normal temperature are forecast for parts of northern or northeastern South America during all four forecast periods except for January-February-March.

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

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