The IRI Experimental Forecast Division has prepared this Climate Outlook for South America for October 1998 - March 1999. Of relevance in the preparation of this Outlook have been the continued evolution of cooler than average conditions in the eastern and central equatorial Pacific Ocean (La Niņa), and the persistence of warmer than average conditions in the western equatorial Pacific. The sea-surface temperatures of the central and western tropical Indian Ocean have been cooling from record high temperatures, and this trend is expected to continue. It has been assumed that the northern and tropical Atlantic Ocean will remain warmer than normal, and that sea-surface temperatures in the South Atlantic will increase during 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 the continued development of moderate La Niņa conditions, with the persistence of positive sea-surface temperature anomalies in the region of Indonesia. This sea-surface temperature forecast is consistent with most other numerical and statistical forecasts.

B) Forecasts of the tropical Atlantic ocean using statistical models developed by the CPTEC.

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

D) Statistical analyses of the observed climate over South America.

E) Other sources of information include: ECMWF, and the Queensland Department of Natural Resources

The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by the national meteorological services in South America. Thus, this product may differ from the official forecasts issued in those areas. This Outlook has been prepared using all information that was reasonably accessible. Inclusion of other climate information and guidance requires further arrangements. The IRI is engaged in establishing such collaborative arrangements with the goal of improving its capability to provide the best and most complete global climate guidance. The Climate Outlook for October 1998 - March 1999 is dependent on the quality of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is some spread in coupled model predictions concerning the evolution of SSTs. This spread in predictions is a source of uncertainty in the Outlook provided here, which assumes that tropical Pacific SST's will continue to decline at a modest rate, resulting in moderate La Niņa conditions by the end of the year. Also, it is known that Atlantic Ocean SSTs may play some role in modulating precipitation changes over parts of South America. Thus, the uncertainties in Atlantic Ocean SST values during the forecast period may lead to additional uncertainty over some parts of the region.

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. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.

OUTLOOK -

This Outlook covers two seasons: October - December 1998 and January - March 1999. Maps are given showing expected tercile probabilities of precipitation. The maps 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). An additional 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. 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. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Areas marked "D" experience their dry season during the forecast period, typically receiving less than 15% of their annual precipitation total during the three months. Otherwise, for example, in the case of Colombia in October - December 1998 (Map A), there is a 45% 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 20% chance that the precipitation will be in the driest third of the years. 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. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

October - December 1998:

PRECIPITATION

The following regions of enhanced probabilities of precipitation anomalies are indicated:

P.A) highly enhanced probabilities of above-normal precipitation over north-western half of Venezuela,Guyana, Surinam and French Guiana;

P.B) enhanced probabilities of above-normal precipitation over the part of the continent north of 7S, with higher probabilities of wet conditions over north-western part of Venezuela, Guyana, Surinam and French Guiana (a slightly enhanced risk of extremely wet conditions is indicated);

P.C) enhanced probabilities of near-normal precipitation over central part of Brazil, northern half of Bolivia, most parts of Peru, and northern-most Paraquay;

P.D) enhanced probabilities of below-normal precipitation over the latitudinal belt in between 25S and 45S, with increased chances of dry conditions over the eastern half of Argentina, southern part of Paraguay, south-most parts of Brazil and Uruguay (an enhanced risk of extremely dry conditions is indicated);

TEMPERATURE

The following regions of enhanced probabilities of temperature anomalies are indicated:

T.A) enhanced probabilities of above-normal temperatures over most of the continent excluding the southern-most part and the eastern part of central Brazil;

T.B) enhanced probabilities of near-normal temperatures over the eastern part of central Brazil;

January - March 1999:

PRECIPITATION

The following regions of enhanced probabilities of precipitation anomalies are indicated:

P.i) enhanced probabilities of above-normal precipitation over southern half of Colombia, southern-most Venezuela and north-west Brazil;

P.ii) slightly enhanced probabilities of near-normal precipitation over north-eastern Brazil;

P.iii) slightly enhanced probabilities of below-normal precipitation over southern-most part of Equador and north-western regions of Peru;

P.iv) slightly enhanced probabilities of near-normal rainfall over Uruguay, north-eastern regions of Argentina and south-western parts of Brazil and Paraguay;

TEMPERATURE

The following regions of enhanced probabilities of temperature anomalies are indicated:

T.i) enhanced probabilities of above-normal temperatures over most of the Northern part of the continent excluding eastern half of Brazil and west coasts of Columbia, Equador and Peru;

T.ii) slightly enhanced probabilities of above to near-normal temperatures over the eastern half of Brazil;

T.iii) slightly enhanced probabilities of below to near-normal temperatures over the west coasts of Columbia, Equador and Peru;

T.iv) slightly enhanced probabilities of above to near-normal temperatures over most of Argentina, Chile, southern half of Paraguay, Uruguay, southern-most regions of Brazil.

Cutoffs and Max/Min extremes for OND and JAS.

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