The IRI Experimental Forecast Division has prepared this Climate Outlook
for South America
for October 1999 - March 2000. Of
relevance in the preparation of this Outlook has been the current weak
La Niņa
conditions (lower than average sea-surface temperatures (SSTs) in the
central equatorial Pacific Ocean) that are
expected to amplify slightly through the
end of 1999
and then decline slowly in early 2000.
The persistence of slightly
warmer than average conditions in the western Pacific has been assumed also. The SSTs
of the tropical Indian Ocean were assumed to return to near normal
conditions with the period October-December 1999.
The forecast of tropical South Atlantic SSTs suggests a
warming trend through December 1999.
METHODS -
This Outlook was prepared using the following procedures and information:
A) Coupled ocean-atmosphere model predictions of tropical Pacific 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 the resurgence of weak to moderate La
Niņa conditions, with the persistence of slightly positive sea-surface temperature anomalies in the western Pacific.
The return of colder that average conditions in the central and eastern equatorial Pacific is consistent with most other
numerical and statistical forecasts.
B) Forecasts of the tropical Indian and Atlantic oceans using statistical models developed by the IRI and CPTEC
respectively.
C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted
sea-surface temperature patterns.
D) The South-eastern South America Regional Climate Outlook Forum consensus guidance.
E) Other sources of information include (
ECMWF
).
The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by
the national meteorological services in the region. Thus, this product may differ from the official forecasts issued in
those areas. The Climate Outlook for October 1999 - March 2000 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 some uncertainty
concerning the detailed evolution of SSTs. Spread in global SST predictions is a source of uncertainty in the Outlook
provided here. In particular, the predicted warming over the southern tropical Atlantic has been a primary influence
on the forecasts over 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: October - December 1999 and January - March 2000. Maps are given showing tercile probabilities of precipitation and temperature. The maps for precipitation 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). A qualitative outlook of climatology ("C") indicates that
there is no basis for favoring any particular category. Areas that are marked by "D" represent regions experiencing
their dry season (less than 15% of annual precipitation) and for which less than 5cm of precipitation typically occurs
over the season. Otherwise, for example, in the case of Uruguay in October - December 1999
(Map A), there is a 25% probability that the precipitation will be in the
wettest third of the years, a 35% chance it will be in the near-normal third
of the years, and a 40% chance that the precipitation will be in the driest third of the years.
An
additional precipitation 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. A priori, there is a 15% probability of being within the extremely wet category, and a 15% probability of
being within the extremely dry category, leaving a 70% probability that the precipitation will not be extreme. The
maps indicate areas of increased risk of extreme precipitation totals. Three levels of increased risk are defined:
slightly enhanced risk, enhanced risk, and greatly enhanced risk. For slightly enhanced risk, there is a 25-40% probability that precipitation will be within the indicated extreme, i.e. wet or dry. This represents an approximate doubling
of the climatological risk. For enhanced risk, there is a 40-50% probability that precipitation will be within the indicated extreme. This represents an approximate tripling of the climatological risk. For greatly enhanced risk, the probability that precipitation will be within the indicated extreme exceeds 50%, i.e. the indicated extreme is the most
likely outcome.
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(Map A). The numbers for each region on the temperature
maps indicate the probabilities 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.
October - December 1999:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
P.A) slightly enhanced probabilities of above-normal precipitation over Panama,
the northern coast of Colombia and the north-western coast of Venezuela;
P.B) slightly enhanced probabilities of near-normal to above-normal precipitation over
eastern Colombia and western Venezuela;
P.C) enhanced probabilities of above-normal precipitation over much of northern-most
South America, from eastern Venezuela and extending through northern Brazil,
within this region, from about 65W-50W, exists a slightly enhanced risk of
extreme above-normal precipitation (within top 15 percentile of historical record);
P.D) slightly enhanced probabilities of near-normal to above-normal precipitation over
the region encompassing the Brazilian state of Espirito Santo;
P.E) slightly enhanced probabilities of below-normal precipitation over an area covering
southern Brazil, most of Paraguay, Uruguay, and north-western Argentina;
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.A) greatly enhanced probabilities of above-normal temperatures over north-western
South America covering interior Ecuador, northern Peru, Colombia, Venezuela, and
stretching as far east as French Guiana;
T.B) greatly enhanced probabilities of below-normal temperatures over coastal Ecuador
and the northern-most coast of Peru;
T.C) enhanced probabilities of above-normal temperatures over the central and southern Peru
and the northern half of Chile;
T.D) slightly enhanced probabilities of near-normal to above-normal temperatures over
the interior of Brazil, mainly over the Amazon region;
T.E) slightly enhanced probabilities of above-normal temperatures over the northeastern
tip of Brazil;
T.F) slightly enhanced probabilities of near-normal temperatures over the central eastern part
Brazil;
T.G) slightly enhanced probabilities of near-normal to above-normal temperatures over Paraguay and
northern Argentina;
T.H) enhanced probabilities of above-normal to near-normal temperatures over southern Brazil,
Uruguay, and western Argentina;
January - March 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
P.i) enhanced probabilities of above-normal precipitation over the north east sector of Brazil;
P.ii) enhanced probabilities of below-normal precipitation over the northernmost areas of South
America, including eastern Colombia to French Guiana;
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.i) slightly enhanced probabilities of above-normal temperatures over central America, into
northern Colombia and north-western Venezuela;
T.ii) enhanced probabilities of above-normal temperatures over interior Peru, Bolivia and
western Paraguay;
T.iii) slightly enhanced probabilities of below-normal to near-normal temperatures over
the north-eastern sector of Brazil;
![[key]](/climate/forecast/images/key.jpg)
