Climate Outlook
AUSTRALIA May - October 2002
Issued: April 2002
The IRI has prepared this experimental Climate Outlook for Australia for
May - October 2002. Of relevance in the preparation of this
outlook is the prediction of near-average to slightly warmer than
average conditions in the eastern equatorial Pacific for the next 6 to
9 months. Currently the sea surface temperatures (SSTs) across much of
the eastern and central equatorial are near their long-term average
(SSTs),
although slightly lower than average SSTs have developed along the
immediate western coast of South America and warmer than average SSTs
persist in the extreme western part of the basin. Near neutral
equatorial Pacific SST conditions are in effect for the first season of
the forecast,
May-July 2002,
June-August 2002,
July-September 2002,
while during the second season,
August-Octover 2002,
they are expected to be slightly above average. The warmer
than average SSTs that continue to dominate much of the Indian Ocean
are expected to decrease slowly through the forecast period. The area
of above-average temperature in the tropical south Atlantic Ocean is
expected to persist through at least the first half of the forecast
period.
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
May - October 2002
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: May-July 2002,
June-August 2002, July-September 2002 and August-Octover 2002.
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
a small part of northeastern Queensland in May-July 2002
(Map A),
there is a 40% 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 25% chance that the precipitation
will be in the driest third of the years.
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.
May-July 2002 through August-Octover 2002:
The following discussion briefly describes the probability anomaly forecasts:
Precipitation
Slightly enhanced probabilities for below normal precipitation are
forecast for one or two minor portions of Australia during
November-December-January and December-January-February. Part of
New Guinea has a similarly dryish outlook for November-December-January.
A tendency toward enhancement of above normal precipitation is forecast
for tropical Pacific islands near the international date line, and below
normal precipitation in the near-equatorial eastern tropical Pacific
islands (e.g. in Atuona, northern French Polynesia). This forecast pattern
is based on the expectation of a continuation of the presently above normal
SST in the tropical Pacific near and west of the date line, and slightly
below normal SST in the eastern tropical Pacific basin, particularly just
south of the equator so as to affect the northernmost islands of French Polynesia.
Temperature
A greatly enhanced probability for above normal temperature (60%) is
forecast for New Zealand for November-December-January, owing to the
significantly above normal SST presently in the vicinity which is expected
to persist for at least the first two forecast periods. Enhancement of the
probability for above normal temperature weakens to 50% for
December-January-February, 40% for January-February-March, and to 25%-40%-35%
(favoring near normal temperature) for February-March-April in much of New Zealand.
A substantial portion of Australia is forecast to have a slightly enhanced
probability for above normal temperature during November-December-January and
December-January-February, dissipating toward climatology in much of the continent
for the last two forecast periods. Probabilities for above normal temperature
are 45% in Tasmania for November-December-January. A greatly enhanced probability
for above normal temperature (50% or 55%) is forecast for most of the tropical
Pacific islands for November-December-January, with somewhat more weakly
enhanced probabilities for December-January-February and January-February-March.
OBSERVED CLIMATOLOGY DATA for May-Jun-Jul,
Jun-Jul-Aug, Jul-Aug-Sep and Aug-Sep-Oct
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