Climate Outlook - Rainfall & Temperature

SOUTH AMERICA April-September 1998

The IRI Experimental Forecast Division has prepared this Climate Outlook for South America for April - September 1998. Of relevance in the preparation of this Outlook has been the strong El Niņo episode now subsiding in the tropical Pacific, and the expectation that warm El Niņo conditions will persist for the next few months. Also relevant to this Outlook are the warm sea surface temperature (SST) anomalies in the Indian Ocean basin and in the tropical Atlantic Ocean.

METHODS -

This Outlook was prepared using the following procedures and information:

A) 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 weakening, El Niņo conditions in the tropical Pacific through the forecast period.

B) Statistical prediction of the tropical Atlantic SST's

C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted SST patterns.

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

E) Additional information available from several sources, involving national, regional and international institutes and organizations from (Argentina, Bolivia, Brazil, Chile, Ecuador, Paraguay and Uruguay, 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; US National Oceanic and Atmospheric Administration Office of Global Programs; Inter-American Institute for Global Change Research) was used in the April-June Outlook.

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 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-September 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 considerable spread in coupled model predictions concerning the evolution of SSTs. This spread in predictions is a primary source of uncertainty in the Outlook provided here, which assumes that tropical Pacific SSTs will continue to decline at a modest rate. Also, it is known that tropical and/or mid-latitude Atlantic SSTs may play some role in modulating rainfall changes over parts of eastern South America.

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; April-June 1998 and July-September 1998. Maps are given showing expected 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. Otherwise, for example, in the case of Uruguay in April-May-June (Map A), there is a 50% probability that the rainfall occurring in the above-normal category (i.e. within the range of the wettest third of recorded precipitation); a 30% chance in the near-normal category; and a 20% chance in the below-normal category. The numbers for each region on temperature maps indicate the probabili- ties 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. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

April-June 1998:

PRECIPITATION:

During this season the map is dominated by 4 main regions of enhanced probabilities of major precipitation anoma- lies. These include:

P.A) Slightly enhanced probabilities for below-normal rainfall to continue over much of northern South America. This includes, in particular, the Nordeste and surrounding areas which, also have a perceived risk for experiencing extreme below-normal precipitation (precipitation values reaching values only seen in the driest 15% of years in the observed record). See map.

P.B) Continuing enhanced probabilities of above-normal rainfall along the tropical Pacific coast of South America extending from Equador through Peru. A highly enhanced risk of extreme above-normal precipitation (precipitation values reaching values only seen in the wettest 15% of years in the observed record) is perceived for this region. See map.

P.C) Continuing enhanced probabilities for above normal rainfall in parts of central eastern South America, this region also carries a highly enhanced risk for extreme above-normal precipitation. See map.

P.D) Enhanced probabilities for below-normal rainfall over the south-most part of the continent.

TEMPERATURE:

The map is dominated by 3 main regions of enhanced probabilities of temperature anomalies:

T.A) Highly elevated probabilities for warm conditions over the northern third of South America

T.B) Slightly enhanced probabilities for near-normal to above-normal temperatures over the central part of the continent.

T.C) Enhanced probabilities for the warmer than normal temperatures over the southern parts of Argentina and Chile.

July-September 1998:

PRECIPITATION:

The map is dominated by the following regions of enhanced probabilities of major precipitation anomalies:

P.i) Elevated probabilities for below-normal rainfall over Venezuela and northern Colombia.

P.ii) Enhanced probabilities for above-normal precipitation over Nordeste and surrounding areas.

P.iii) Highly elevated probabilities for above-normal rainfall over the Uruguay, Paraguay, south-most parts of Brazil, northern Argentina.

P.iv) Slight chances for below-normal rainfall over the southern parts of Argentina and Chile.

TEMPERATURE:

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

T.i) Continuing enhanced probabilities for warmer than normal temperatures over the northern half of the South America, except for the region of the enhanced probabilities for the near-normal precipitation along the east coast.

T.ii) Slightly enhanced probabilities for colder temperatures over the Uruguay, Paraguay, south-most parts of Brazil, northern Argentina.

Cutoffs and Max/Min extremes for AMJ and JAS.

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