South America Guide for Oct. 1997

Climate Outlook - Rainfall

SOUTH AMERICA October 1997 - March 1998

The IRI Climate Forecast Division has prepared this Climate Forecast Outlook for South America for October 1997 - March 1998. Of special relevance in the preparation of this Outlook has been the exceptionally strong El Niño episode now underway in the tropical Pacific, and the expectation that strong El Niño conditions will persist through much of the forecast period. This present El Niño is comparable, if not stronger than the major event in 1982-83 (at the time the strongest of the century) which was associated with severe climate anomalies around the globe that caused major socio-economic losses in many regions.

METHODS -

This Outlook was prepared using the following procedures and information:

A) Statistical analyses of the observed historical response of the climate system over South America during the stron- gest 12 El Niño events over the period 1890-1989.

B) Coupled ocean-atmosphere model predictions of tropical Pacific sea surface temperatures (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 moderating, but still relatively strong, El Niño conditions will persist in the tropical Pacific through the forecast period.

C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted SST patterns in the tropical Pacific (especially) and in other oceans as well. These models have been shown to reproduce important aspects of the climatic response to El Niño in simulations covering such significant events as the 1982-83 event described above.

The procedures, models, and data used to derive this Climate Outlook are 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 in the time available, using all information that was reasonably accessible. Inclusion of other climate information and guidance requires further arrangements. The IRI is engaged in estab- lishing such collaborative arrangements with the goal of improving its capability to provide the best and most complete global climate guidance. The Climate Outlook for January-March, especially, are dependent of the quality of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information. However, there is considerable spread in coupled model predictions concerning the rate at which tropical Pacific SSTs will decline in early 1998 (assuming the El Niño peaks near the end of 1997). This spread in predictions is a primary source of uncertainty in the Outlook provided here, which assumes that tropical Pacific SSTs will decline at a modest rate during early 1998. Also, it is known that tropical and/or mid-latitude Atlantic SSTs play a role in modulating rainfall changes over parts of eastern South America. The impacts of possible changes in Atlantic SSTs during the forecast period have not been considered in this report. It is also 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 guid- ance products, users are strongly advised to contact their National Meteorological Services.

OUTLOOK -

This Outlook covers two seasons; October-December 1997 and January-March 1998. Maps are given showing expected probabilities that the seasonal rainfall will fall into the driest third of the years, the middle third of the years, or the wettest third of the year (see Map Caption below for further details, cautions and caveats).

October-November-December 1997:

During the first season (October-December), the map is dominated by 3 main regions of enhanced probabilities of major rainfall anomalies. These include:

A) High probabilities for below normal rainfall over much of northern South America, including portions of the Nordeste and parts of Amazonia in Brazil.

B) Enhanced probabilities of above normal rainfall along the tropical Pacific coast of South America extending from the southwest coastal Columbia through Peru. These areas experienced heavy rainfall and major flooding events during 1982-83.

C) Above normal rainfall in parts of southern/southeastern South America. These areas experienced some episodes of severe flooding during 1982-83.

January-February-March 1998:

During the second season, the same basic patterns of anomalous rainfall probabilities remain the most outstanding. The main modifications are:

i) The dominant area of enhanced probability of above normal rainfall along the tropical west coast of South America (see B above) extends farther south.

ii) The area of enhanced probability of above normal rainfall in the southern South America (see C above) is shifted southward.

Note: With respect to the northeastern portion of Brazil (the Nordeste), the forecast suggests enhanced probabilities of below normal rainfall for the beginning of the February-April wet season in this region, which is well known for its recurrent periods of drought.

The probabilities given here are based mainly on the statistical analysis of the historical data. Further confidence is added from the atmospheric GCMs forced with a variety of predicted SST scenarios including persistence, statistical estimates, and dynamical forecasts. These atmospheric GCM climate predictions are in agreement with each other and with the statistical analysis for the regions of enhanced probabilities of major rainfall anomalies over the northern half of South America, and the GCMs perform very well over much of this region. Discrepancy between the GCMs and statistical analysis in the south eastern part of the continent may be the result of inadequate resolution of the Andes in the GCMs.

Map Caption

For each sub-region, during the 1997-98 season, the likelihood associated with this Outlook is represented by the probabilities of that sub-region receiving below-normal, near-normal or above normal rainfall (see keys on Maps), based on model results and expert interpretation. Over many years, rainfall will fall equally frequently into three categories. For example, between 1900 and 1989 30 years will fall into the below-normal category, 30 years will be nor- mal and 30 will fall into the above-normal category.

If all three outcomes were equally likely this season, the chance for each category would be 33%. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Otherwise, for exam- ple, in the case of the Nordeste in October-December 1997 (Map A), there is a 15% probability that the rainfall will be in the wettest third of the years, a 25% chance it will be in the near-normal third of the years, and a 60% chance that the rainfall will be in the driest third of the years. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

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