Climate Outlook - Rainfall & Temperature

SOUTH AMERICA January - June 1998

The IRI Climate Forecast Division has prepared this Climate Forecast Outlook for South America for January - June 1998. Of special relevance in the preparation of this Outlook has been the exceptionally strong El Niņo episode now underway in the tropical Pacific, and the expectation that strong El Niņo conditions will persist at least for the first half of the forecast period. This present El Niņo appears to have peaked in December 1997 with SST anomalies stronger than the major event in 1982-83 (at that 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 Africa during the strongest 12 El Niņo events over the period 1890-1989, and also the strongest eight El Niņo events since 1950.

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 moderate, but still relatively strong, El Niņo 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 Niņo in simulations covering such significant events as the 1982-83 event described above.

D) Consensus guidances for January-March 1998 for western and south-eastern South America, produced in Lima, Peru on 28 October and in Montevideo, Uruguay, on 10 December 1997, form important inputs to Map A. Contributors to the consensus guidances included the Meteorological Services from seven countries (Argentina, Bolivia, Brazil, Chile, Ecuador, Paraguay and Uruguay) and national, regional and international institutes and organizations (University of Buenos Aires; Federal University of Parana; National University of Asuncion; National University of Colombia; University of Chile; University of the Republic, Uruguay; IMARPE; IGP; INPESCA; DHN; CIMA/ CONICET/UBA; INPE/CPTEC; World Meteorological Organization; International Research Institute for climate prediction; US National Oceanic and Atmospheric Administration Office of Global Programs; Inter-American Institute for Global Change Research). The guidance was drawn up by assessing output from coupled ocean-atmosphere models and physically-based statistical models. Additional information available from these sources was used in the April-June Outlook.

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 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 April-June, especially, are dependent on 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 1998. 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 detail 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 guidance products, users are strongly advised to contact their National Meteorological Services.

OUTLOOK -

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

January-February-March 1998:

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

PRECIPITATION

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

P.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.

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

TEMPERATURE:

The map is dominated by the following main regions of enhanced probabilities of temperature anomalies:

T.A) There are enhanced probabilities of above normal temperatures over most of South America north of about 20S.

T.B) Over eastern Argentina, Uruguay and the far southern part of Brazil, probabilities of below average temperatures are enhanced.

T.C) Along most of the west coast south of about 25S, probabilities of near-normal temperatures are enhanced.

T.D) There is an area of enhanced probability of near-normal temperature over north-western Brazil.

April-May-June 1998:

PRECIPITATION

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

P.i) The dominant area of enhanced probability of below-normal rainfall over northern South America (see P.A above) becomes confined to the northern half of Brazil.

P.ii) The probabilities of above-normal rainfall in the south-east (see P.C above) increase.

TEMPERATURE:

During the second season, the same basic patterns of anomalous temperature remain. The main modifications are:

T.i) Over most of Argentina, Uruguay and Paraguay, probabilities of near-normal temperature are enhanced.

T.ii) The area of enhanced probability of near-normal temperature in the north-western Brazil (see T.D above) disappears.

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

The numbers for each region indicate the probabilities of rainfall/temperature in each of the three categories, below-, near- and above-normal. Above-normal rainfall/temperature is defined as within the wettest/warmest third of recorded precipitation totals/temperature in each region; below-normal rainfall/temperature is defined as within the driest/coolest third of rainfall totals; near-normal is the third centered around the climatological median. The top number indicates the probability of rainfall/temperature occurring in the above-normal category, the middle number is for near-normal and the bottom for below-normal. In the case of the west coast of Argentina in January-March 1998 (Map A), there is a 40% probability that the rainfall occurring in the above-normal category (i.e. within the range of the wettest third of recorded precipitation); a 35% chance in the near-normal category; and a 25% chance in the below-normal category. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

Cutoffs and Max/Min extremes for JFM and AMJ.