The IRI has prepared this experimental Climate Outlook for South America
for July - December 2000. Of
relevance in the preparation of this Outlook has been the weakening remains of the 1998-2000
La Niņa
conditions (lower than average sea-surface temperatures (SSTs) in the
central equatorial Pacific Ocean).
Thus, near neutral but slightly below-average equatorial Pacific SST conditions are in effect for the first season of the forecast,
July-August-September 2000, and near-normal
conditions are expected for the second forecast season,
October-November-December 2000.
The SSTs of the tropical Indian Ocean were assumed to remain near-normal.
Tropical Atlantic SSTs were not expected to change significantly.
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 final demise of La
Niņa conditions and a return to near-average conditions during the first forecast season.
The forecast for 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 and Atlantic oceans using statistical models developed by the IRI and CPTEC,
respectively.
C) Global atmospheric general circulation model (GCM) predictions of the atmospheric response to the present and predicted
sea-surface temperature patterns.
D) The
Ninth Forum for the Regional Climate Oulook for the Southeast of South America consensus
guidance.
E) Other sources of information include
CPTEC;
UKMO;
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 July - December 2000 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: July - September 2000 and October - December 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 region I in July - September 2000
(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.
July - September 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region I) slightly enhanced probabilities of below-normal precipitation over northern Venezuela and northern-most Colombia;
Region II) greatly enhanced probabilities of above-normal precipitation over the central
northern section of South America, from Suriname to east of the Amazon River mouth
(there are slightly enhanced probabilities of the part of this region nearest the
mouth of the Amazon of receiving extremely above normal seasonal precipitation);
Region III) slightly enhanced probabilities of near-normal to below-normal precipitation over the east-facing tip of northeastern Brazil;
Region IV) enhanced probabilities of near-normal precipitation over much of Paraguay;
Region V) slightly enhanced probabilities of above-normal precipitation the southeastern corner of Brazil, Uruguay, and a portion of western Argentina;
Region VI) enhanced probabilities of below-normal precipitation over western Argentina, between approximately 45S and 37S;
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region I) slightly enhanced probabilities of near-normal temperatures over
the Gulf coasts of Costa Rica and Panama, and over northern Colombia;
Region II) greatly enhanced probabilities of above-normal temperatures along the western coastal sections of Colombia and Ecuador, and extending westward to the Galapagos Islands;
Region III) slightly enhanced probabilities of above-normal temperatures over much of Venezuela and central Colombia;
Region IV) enhanced probabilities of below-normal temperatures over Guyana, Suriname, and French Guiana, extending eastward into Brazil to the mouth of the Amazon;
Region V) slightly enhanced probabilities of normal temperatures in a region encircling Region IV
that extends into northern Brazil from the border of Venezuela to northeastern Brazil;
Region VI) enhanced probabilities of above-normal temperatures over the northeastern corner of Brazil;
Region VII) slightly enhanced probabilities of near-normal to above-normal temperatures over
western Peru, and extending south into Chile to approximately 25S;
Region VIII) enhanced probabilities of above-normal temperatures over much of central and southern Bolivia, northwestern Argentina, and central Chile;
Region IX) slightly enhanced probabilities of above-normal temperatures mainly over Paraguay, but extending slightly into eastern Bolivia, southern Brazil, and northern Argentina;
Region X) enhanced probabilities of above-normal temperatures over part of southern Brazil;
Region XI) slightly enhanced probabilities of near-normal temperatures in a band extending
from central Chile, and extending north of the Pampas region of Argentina across to the
southeastern tip of Brazil, Uruguay and eastern Argentina;
Region XII) slightly enhanced probabilities of below-normal temperatures over parts of southern
Chile, extending through much of central Argentina;
October - December 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region II) slightly enhanced probabilities of below-normal precipitation over much of Venezuela, extending eastward over Guyana, Suriname, and French Guiana;
Region III) slightly enhanced probabilities of above-normal precipitation over parts of the northern
coast of Brazil;
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region IV) enhanced probabilities of above-normal temperatures over Ecuador and much of Colombia and Venezuela extending nothward into central America;
Region V) slightly enhanced probabilities of above-normal temperatures over southeastern
Brazil;
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