The IRI has prepared this experimental Climate Outlook for South America
for October 2000 - March 2001. Of relevance in the
preparation of this Outlook has been the near-average sea-surface temperatures
(SSTs)
in the
central equatorial Pacific Ocean.
Near neutral equatorial Pacific SST conditions are in effect for both seasons of the forecast,
October-November-December 2000 and
January-February-March 2001.
The SSTs of the tropical Indian Ocean were assumed to remain normal.
Tropical Atlantic SSTs are above-normal during the first forecast season and are expected to return toward normal during
the second season.
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 a continuation of
near-average conditions during the first forecast season.
The forecast for near-neutral conditions is consistent with some, but not all, numerical and statistical
forecasts of central and eastern Pacific SSTs.
B) Forecasts of the tropical Indian ocean using a statistical model developed by the
IRI.
C) Global atmospheric general circulation model (GCM) predictions of the atmospheric response to the present and predicted
sea-surface temperature patterns.
D) Other sources of information include
CPTEC;
UKMO;
ECMWF;
Goddard Space Flight Center
.
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 - March 2000-01 is dependent on the accuracy of the SST predictions. For
the tropical Pacific, these predictions can be expected to provide useful information, but there is some uncertainty
concerning the evolution of SSTs. Spread in global SST predictions is a source of uncertainty in the Outlook
provided here. Note that even if perfectly accurate SST forecasts were possible, there would
still be uncertainty in the climate forecast due to chaotic internal variability of the atmosphere. These uncertainties
are reflected in the probabilities given in the forecast.
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, and
variations within the 3-month season should also 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 2000 and January - March 2001. 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 region V in October - December 2000
(Map A), there is a 45% 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 20% 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 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region V) enhanced probabilities of above-normal precipitation over portions of northeast Brazil, French Guiana,
Surinam, Guyana, and eastern and central Venezuela;
Region VI) greatly enhanced probabilities of above-normal precipitation in northeast Brazil, from near the mouth of the
Amazon to somewhat east of Fortaleza but not to the easternmost point of the country (A slightly enhanced risk of extreme above normal conditions is indicated);
Region VII) slightly enhanced probabilities of below-normal precipitation in a portion of southern Brazil and
extreme southeastern Paraguay and northeastern Argentina.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region VII) enhanced probabilities of above-normal temperatures in western Colombia, Ecuador, northwestern Peru and the
Galapagos Islands;
Region VIII) greatly enhanced probabilities of above-normal temperatures on the eastern side of the Andes, including southwestern Bolivia and northwestern Argentina;
Region IX) slightly enhanced probabilities of above-normal temperatures in southern Chile and southern Argentina;
Region X) enhanced probabilities of below-normal temperatures over northern Brazil, extending
into French Guiana and part of Surinam;
Region XI) slightly enhanced probabilities of above-normal temperatures over northern Argentina, Paraguay, part of
southern Brazil, and southern Bolivia.
January - March 2001:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region III) slightly enhanced probabilities of below-normal precipitation from Honduras southward into
northwestern Colombia.
Region IV) slightly enhanced probabilities of below-normal precipitation over northern Guyana, northern Venezuela, and
the islands of the Lesser Antilles in the Caribbean;
Region V) slightly enhanced probabilities of above-normal precipitation along the coast of Ecuador and northwestern Peru.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region III) enhanced probabilities of above-normal temperatures over northern Bolivia, eastern Peru, part of western
Brazil, and southern and central Colombia;
Region IV) slightly enhanced probabilities of above-normal temperatures in a large region surrounding region III,
including central, western and northern Brazil; northern Paraguay; northern Chile; Bolivia; western Peru; Ecuador; western, northern and eastern Colombia; and all of the central American countries northward to the border of Mexico.
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