Climate Outlook
SOUTH AMERICA July - December 2008
Issued: June 2008
The IRI has prepared this experimental Climate Outlook for South America for July - December 2008.
Of relevance in the preparation of this outlook is a likelihood that the tropical Pacific SSTs will continue to be weakly below average during the first two forecast periods in the central and west-central portions decay of the La Nina, but the eastern portion may be slightly above average. During the later forecast periods this same anomaly configuration is likely to continue but with weakening of the positive SST anomalies in the eastern portion. Such tropical Pacific conditions are indicated in the SST predictions on which these climate forecasts are based (SSTs). See the IRI's ENSO update for a discussion on the ENSO outlook (see IRI Probabilistic ENSO forecast). In the equatorial Indian Ocean, slightly below average SSTs are predicted for the easter portion, with above average near the African coast. This pattern continues but weakens for the longer lead forecast periods. In the tropical Atlantic Ocean, SSTs are predicted to be above average in the eastern portion and near average elsewhere. This pattern continues but weakens for the longer lead forecast periods. (July-September 2008, August-October 2008, September-November 2008, October-December 2008).
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
NASA's
Seasonal to Interannual Prediction Project (GSFC-NASA)
and also seasonal prediction research at COLA.
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 2008
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 four seasons: July-September 2008,
August-October 2008, September-November 2008 and October-December 2008.
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). The color
shading indicates the probability of the most dominant tercile -- that
is, the tercile having the highest forecast probability. The color bar
alongside the map defines these dominant tercile probability levels. The
upper side of the color bar shows the colors used for increasingly
strong probabilities when the dominant tercile is the above-normal
tercile, while the lower side shows likewise for the below-normal
tercile. The gray color indicates an enhanced probability for the
near-normal tercile (nearly always limited to 40%). As before, numbers
and their associated histograms show the probabilities of the three
terciles. In areas with lots of spatial detail, there may not be
sufficient room on the map, to allow histograms for each region. In
those cases, some idea of the probabilities may be gained from the color
alone. 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 for which less than 3cm of
precipitation typically occurs over the season.
Otherwise, for example, in the case of
July-September 2008 (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 in most os southeastern Brazil.
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.
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. A similar map is provided
in the first season indicating probabilities of
extreme temperature
anomalies.
Boundaries between sub-regions should be considered as transition zones,
and their location considered to be only qualitatively correct.
July-September 2008 through October-December 2008
The following tables summarize the precipitation and temperature probability forecasts:
Summary of PRECIPITATION forecast for South America
Leads 1, 2, 3, and 4 refer, respectively, to the upcoming seasons:
Jul-Aug-Sep Aug-Sep-Oct Sep-Oct-Nov Oct-Nov-Dec
A non-enhanced probability for above or below normal is 33%.
(There is a near-normal category whose non-enhanced probability is also 33%.)
The following countries or regions out of the 22 in South America
have at least half of their area under a PRECIPITATION forecast for:
At least At least
Substantially slightly slightly Substantially
enhanced enhanced enhanced enhanced
probability probability probability probability
(>48%) for (>38%) for (>38%) for (>48%) for
below normal below normal above normal above normal
COLOMBIA
leads 1,2,3 and 4
ECUADOR
lead 1
FRENCH GUIANA
leads 1,2,3 and 4
GUYANA
leads 1,2,3 and 4
TRINIDAD TOB
leads 2,3 and 4
SURINAME
leads 1,2,3 and 4
URUGUAY
leads 2 and 3
VENEZUELA
leads 1,2,3 and 4
ARGENTINA-Central
leads 2 and 3
BRAZIL-North
leads 1 and 2
BRAZIL-Southeast
leads 1 and 2
Summary of TEMPERATURE forecast for South America
Leads 1, 2, 3, and 4 refer, respectively, to the upcoming seasons:
Jul-Aug-Sep Aug-Sep-Oct Sep-Oct-Nov Oct-Nov-Dec
A non-enhanced probability for above or below normal is 33%.
(There is a near-normal category whose non-enhanced probability is also 33%.)
The following countries or regions out of the 22 in South America
have at least half of their area under a TEMPERATURE forecast for:
At least At least
Substantially slightly slightly Substantially
enhanced enhanced enhanced enhanced
probability probability probability probability
(>48%) for (>38%) for (>38%) for (>48%) for
below normal below normal above normal above normal
ECUADOR
lead 1
URUGUAY
lead 4
BRAZIL-Southeast
leads 1 and 2
BRAZIL-South
leads 1,2 and 3
OBSERVED CLIMATOLOGY DATA for Jul-Aug-Sep,
Aug-Sep-Oct, Sep-Oct-Nov and Oct-Nov-Dec
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CLIMATOLOGICAL AVERAGE:
TERCILE THRESHOLDS (33%-ile & 67%-ile):
EXTREME THRESHOLDS (15%-ile & 85 %-ile):
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