Climate Outlook - Rainfall

Statement from the South-eastern South America Regional Climate Outlook Forum
December 10, 1997, Montevideo, Uruguay

El Niņo-Related Climate Forecast South-eastern South America Outlook
January 1998 - March 1998

SUMMARY

Wetter than average conditions over the period January-March 1998 are expected over much of the central parts of south-eastern South America, including north-eastern Argentina, southern Paraguay and parts of southern Brazil and western Uruguay. Dry conditions are expected only in areas further north and close to the Andes. Stronger impacts in the year following El Niņo events typically occur only in autumn and early winter, after the period covered in this Outlook.

THE CLIMATE OUTLOOK FORUM

On 10 December 1997 a South-eastern South American Climate Outlook Forum convened to formulate consensus guidance for the early 1998 season in the region. The Forum was attended by Meteorological Services from South- eastern South American countries, and climate scientists from universities and national and international research institutes. These specialists reviewed the state of the global climate system and its implications for South-eastern South America. One of the principal factors taken into account is the major El Niņo event occurring in the tropical Pacific Ocean. Although much stronger impacts in the year following El Niņo events typically occur only in autumn and early winter, after the period covered in this Outlook, recent El Niņo occurrences have had significant impacts on rainfall across much of the region south of 20°S during January-March.

The Forum was co-sponsored by the Association Rural del Uruguay, the Inter-American Institute for Global Change Research (IAI), the National Oceanic and Atmospheric Administration (NOAA), the International Research Institute for Climate Prediction (IRI) and the World Meteorological Organization (WMO). Participants at the Forum included representatives of Meteorological Services from four countries (Argentina, Brazil, Paraguay and Uruguay) and climate scientists and other experts from national, regional and international institutes and organizations (University of Buenos Aires, Department of Atmospheric Sciences; CIMA/CONICET/UBA; Federal University of Parana, Department of Physics; INPE/CPTEC; National University of Asuncion, Faculty of Exact and Natural Sciences; University of the Republic, Uruguay; World Meteorological Organization; International Research Institute for Climate Prediction; U.S. National Oceanic and Atmospheric Administration Office of Global Programs).

METHODOLOGY

The regional climate assessment began with consensus agreement that the current El Niņo will remain over the forecast period (January-March 1998). The sea-surface temperature (SST) forecasts were based on coupled ocean- atmosphere models, physically-based statistical models and expert interpretation. The region considered included continental areas bounded from 20 to 40°S and east of the Andes to 47°W. This area was divided into sub-regions according to previous statistical analyses of the impact of El Niņo events in the region. The outlook was based on dynamical forecasts presented by INPE/CPTEC and IRI models as well as results of detailed studies of El Niņo impacts in this region.

The current status of seasonal to inter-annual forecasting allows prediction of spatial and temporal averages, and may not fully account for all factors that influence regional and national climate variability. This Outlook is relevant only to seasonal timescales and relatively large areas, and local variations may occur.

OUTLOOK

The experts provided probability distributions to indicate the likelihood of below-, near- or above-normal rainfall for each sub-region (see Map). Above-normal rainfall is defined as within the wettest third of historically recorded precipitation totals in each region; below-normal rainfall is defined as within the driest third of precipitation totals; near-normal is the third centered around the climatological median. Users are strongly advised to contact participating institutions and other climate information sources for interpretation of this Outlook and for additional guidance.

Above-normal rainfall is expected in southern Paraguay, the south-western part of southern Brazil, western Uruguay and north-eastern Argentina. Normal to above-normal rainfall is expected in central eastern Argentina. In Brazil, normal rainfall in the south is considered most likely, however, north of about 22°S, normal to below-normal rainfall may occur. Near the Andes, dry conditions are expected in north-western Argentina, and there is a zone immediately to the east where average conditions are anticipated. Further south, in central western Argentina, rainfall prospects are uncertain.

Temperatures during January-March are expected to be cooler than average where above-average rainfall is indicated.

The confidence that can be placed in a three-month outlook is relatively high. Beyond three months the reliability of statements about rainfall prospects for the region decreases because of uncertainty in the evolution of SSTs at longer time-scales. However, there are no signs that widespread dry conditions will occur in April-June, and there are indications of wet conditions in the south-eastern part of southern Brazil, Uruguay and eastern Paraguay.

MAP CAPTION

The numbers indicate the probabilities of rainfall in each of the three categories. The first number indicates the probability of rainfall occurring in the above-normal category, the second number is for near-normal and the third for below-normal. In the case of north-western Argentina near the Andes, for example, there is a 20% probability of rainfall occurring in the above-normal category (i.e., within the range of the driest third of historically-recorded rainfall); a 40% chance in the near-normal category; and a 40% chance in the below-normal category. Boundaries between sub-regions should be considered as transition zones. A "C" stands for "climatology", which indicates that there is no clear scientific basis for favoring a prediction of below-normal or above-normal rainfall.

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