Climate Outlook
PACIFIC ISLANDS May - October 2007
Issued: April 2007
The IRI has prepared this experimental Climate Outlook for Pacific Islands for May - October 2007.
Of relevance in the preparation of this outlook is a likelihood that the tropical Pacific SSTs, having transitioned from El Nino to neutral conditions between January and early March, will continue to cool during the coming weeks and months, becoming cooler than average and possibly crossing the borderline into weak La Nina conditions by May or June, particularly in the eastern portion of the basin (NINO3). Such tropical Pacific conditions are indicated in the SST predictions on which these climate forecasts are based. See the IRI's ENSO update for a discussion on the ENSO outlook (see IRI Probabilistic ENSO forecast). Slightly warmer than average SSTs are now observed in the west-central tropical Pacific, while below normal SSTs are found in much of the eastern one-third, from about 130W eastward. Much of the equatorial Indian Ocean has slightly above normal SSTs. The northern tropical Atlantic Ocean shows above-average SSTs, and southern tropical Atlantic has near to slightly above normal SSTs (SSTs). The positive SST anomalies in the Indian Ocean are predicted to remain approximately constant or very slowly weaken over the course of the forecast periods, and those in the tropical Atlantic will very slowly weaken north of the equator, while immediately on and south of the equator the weak anomalies will remain approximately constant or very slowly weaken further. (May-July 2007, June-August 2007, July-September 2007, August-October 2007).
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 2007
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 (variation) in global SST predictions is a source of
uncertainty in the Outlook provided here. In particular, the forecasts
for the tropical Indian and Atlantic oceans have been an important
influence on the forecasts over Pacific Islands. 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 period 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 2007,
June-August 2007, July-September 2007 and August-October 2007.
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
the islands of Wallace and Futuna and Samoa in May-July 2007
(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 2007 through August-October 2007
The following tables summarize the precipitation and temperature probability forecasts:
Summary of PRECIPITATION forecast for Pacific Islands
Leads 1, 2, 3, and 4 refer, respectively, to the upcoming seasons:
May-Jun-Jul Jun-Jul-Aug Jul-Aug-Sep Aug-Sep-Oct
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 28 in Pacific Islands
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
PAPUA NEW GUI
leads 3 and 4
Amer Samoa
leads 1,2,3 and 4
Cook Is south
lead 1
Easter Is
lead 4
Fiji
lead 1
Galapagos
leads 1,2,3 and 4
Johnston Is
leads 2,3 and 4
Kiribati-Main
leads 1,2,3 and 4
Kiribati-Main
lead 4
Kiribati-east
leads 1,2,3 and 4
Kiribati-east
lead 2
Marshall Is
lead 1
Nauru
leads 2,3 and 4
New Caledonia
leads 3 and 4
Niue
leads 1,2,3 and 4
Samoa
leads 1,2,3 and 4
Solomon Is
lead 2
Tonga
leads 2 and 4
Tuvalu
leads 2,3 and 4
Vanuatu
leads 1,2,3 and 4
Wake Is
leads 1,2 and 3
Summary of TEMPERATURE forecast for Pacific Islands
Leads 1, 2, 3, and 4 refer, respectively, to the upcoming seasons:
May-Jun-Jul Jun-Jul-Aug Jul-Aug-Sep Aug-Sep-Oct
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 28 in Pacific Islands
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
PAPUA NEW GUI
leads 1,2,3 and 4
Amer Samoa
leads 1,2,3 and 4
Cook Is south
leads 1,2,3 and 4
Cook Is south
leads 2,3 and 4
Easter Is
leads 1,2,3 and 4
Fiji
leads 1,2,3 and 4
Fr Poly-Soc-Aus
leads 1,2,3 and 4
Fr Poly-Tua-Mar
lead 1
Fr Poly-Tua-Mar
lead 4
Galapagos
leads 1,2,3 and 4
Galapagos
leads 1,2 and 4
Guam
lead 4
Hawaii
leads 1,2,3 and 4
Johnston Is
lead 1
Johnston Is
lead 4
Kiribati-Main
lead 1
Marshall Is
leads 1,2,3 and 4
Fed St Micronesia
lead 1
Nauru
leads 1 and 3
New Caledonia
lead 4
Niue
leads 1,2,3 and 4
Niue
leads 1,2 and 3
Palau
leads 3 and 4
Samoa
leads 1,2,3 and 4
Solomon Is
lead 1
Tokelau
leads 1,2,3 and 4
Tokelau
lead 1
Tonga
leads 2,3 and 4
Tuvalu
lead 1
Vanuatu
leads 1,3 and 4
OBSERVED CLIMATOLOGY DATA for May-Jun-Jul,
Jun-Jul-Aug, Jul-Aug-Sep and Aug-Sep-Oct
CLIMATOLOGICAL AVERAGE:
TERCILE THRESHOLDS (33%-ile & 67%-ile):
EXTREME THRESHOLDS (15%-ile & 85 %-ile):
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