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Climate Outlook - Rainfall
Statement from the Northern South America Regional Climate Outlook Forum

January 20, 1998, Fortaleza, Brazil

THE CLIMATE OUTLOOK FORUM

A Climate Outlook Forum was convened on January 19, 1998 to formulate a consensus precipitation forecast in northern and northeastern South America for the period February - May 1998. The Forum was comprised of climate researchers and representatives of meteorological services from Brazil, Colombia, French Guyana, Suriname, Venezuela, and the USA. Participating institutions included the Instituto Nacional de Pesquisas Espaciais (INPE), the Fundação Cearense de Meteorologia e Recursos Hídricos (FUNCEME), the Instituto Nacional de Meteorologia (INMET), the Universidad Simon Bolivar (USB), the Universidad Nacional de Colombia, the National Oceanic and Atmospheric Administration (NOAA) Office of Global Programs, the International Research Institute for Climate Prediction (IRI), the World Meteorological Organization (WHO) and the Inter-American Institute for Global Change Research (IAI). The participants reviewed the state of the global climate system and its implication for northern and northeastern South America.

METHODOLOGY

Sea surface temperature (SST) anomalies in the tropical Pacific and Atlantic oceans are among the most important predictors of rainfall anomalies in northern South America. The present SST anomalies in the central and eastern Pacific are among the largest ever recorded, with positive anomalies in some locations exceeding 5° C. Predictions call for warm conditions to continue for at least 3 months. Anomalously warm SST's, referred to as El Niño, have been historically associated with dry conditions over most (but not all) of northern and northeastern South America. By contrast, prediction of rainfall anomalies based on present SST anomalies in the Atlantic is more ambiguous, with a weak dipole pattern favorable for enhanced rainfall in parts of northern South America (i.e. opposing the effect of El Niño).

This regional climate outlook began with consensus that the current El Niño event is the strongest recorded in this century. The sea surface temperature (SST) forecast and other factors affecting the climate of northern and northeastern South America were based on atmospheric general circulation models, coupled ocean/atmosphere models, physically-based statistical models and expert interpretation. The Forum endorsed the use of the NOAA National Centers for Environmental Prediction (NCEP) coupled model as the most reliable indicator of the evolution of this El Niño event in terms of sea surface temperature for the next three months.

In the discussions, experimental seasonal forecasts from the CPTEC/COLA, IRI/MPI ECHAM, NCEP, ECMWF, ARPEGE-CLIMAT climate models, and statistical models for north eastern Brazil and Venezuela have been considered. The forecast area in the present exercise is limited from 15°S to the northern coast of South America, excluding Peru, Ecuador, and Bolivia. It is necessary to remember that the end of the period February-May is arbitrary, and cuts through different climatic seasons in different sub-regions.

OUTLOOK

The Climate Outlook addresses the February-April, 1998 period for northern and northeastern South America. The experts provided probability distributions to indicate the likelihood of below-, near-, or above-normal rainfall for each sub-region (see attached map). Users are strongly advised to contact participating institutions and other climate information sources for interpretation of this Outlook and for additional guidance. It is emphasized that the locations of the boundaries between the subregions are only qualitatively correct, and should be considered as transition zones rather than sharp boundaries.

All regions have enhanced probability of drier than normal except the southern parts of Northeast Brazil (Bahia) and the southwestern coastal region of Colombia. In particular, the northern parts of Northeast Brazil (eastern Maranhão, Piauí, Ceará, Rio Grande do Norte, Paraíba and Pernambuco and parts of northern Bahia) have particularly high probabilities of a drier than normal period.

The ocean-atmosphere system is changing at this time, and should the situation in the tropical oceans be altered significantly in the three-to-four weeks to follow, the climate outlook for the upcoming rainy season in northern Northeast Brazil (March-May) may require modification. In particular, the meteorological community should keep watch on the developments in the Atlantic.

Recent studies have indicated an inverse relation in which dryness in northern coastal regions of Northeast Brazil is accompanied by enhanced rainfall along the coast of Guyanas. The present outlook does not reflect this, as in the Guyanas and northern Venezuela, February-April is a dry season.

The area of interest is divided in 11 homogeneous sub-regions with the probabilities of those sub-regions receiving below-normal, near-normal, or above-normal precipitation (see key on map) for the outlook period.

 

By definition, over many years rainfall amounts will be distributed equally into all three categories. For example, if the climatology is determined between 1961 and 1990, ten years this 30-year period will be below-normal, ten will be near-normal, and ten will be above-normal. In other words, the probability of each category will be 33%.