Climate Outlook
PACIFIC ISLANDS May - October 2011
Issued: April 2011
The IRI has prepared this experimental Climate Outlook for Pacific Islands for May - October 2011.
Of relevance in the preparation of this outlook is a high likelihood that the central and eastern tropical Pacific SSTs will be somewhat cooler than average during the first forecast period, indicating borderline or weak La Nina conditions, slowly dissipating to ENSO-neutral but still slightly cooler than average conditions during the 2nd, 3rd and 4th periods. 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). Slightly below average SST is predicted in the equatorial Indian Ocean during the first forecast period, weakening to very slightly below average for the second period, toward average conditions by the third period and very slightly above average for the fourth forecast period. SSTs in the tropical Atlantic are predicted to be slightly above average during the first forecast period, weakening toward average during the second and third periods and becoming near average for the fourth period. (May-July 2011, June-August 2011, July-September 2011, August-October 2011).
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 2011
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 2011,
June-August 2011, July-September 2011 and August-October 2011.
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
eastern Kiribati (Christmas and Fanning Islands) in May-July 2011
(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.
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 2011 through August-October 2011
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 1,2,3 and 4
Cook Is south
lead 1
Fiji
leads 1 and 4
Fr Poly-Soc-Aus
lead 1
Galapagos
lead 1
Guam
leads 1 and 2
Kiribati-Main
leads 1,3 and 4
Kiribati-east
lead 1
Fed St Micronesia
lead 1
Nauru
leads 2,3 and 4
New Caledonia
leads 1 and 2
Niue
lead 1
Solomon Is
leads 2,3 and 4
Tuvalu
leads 1,2,3 and 4
Tuvalu
leads 1,2,3 and 4
Vanuatu
leads 1,3 and 4
Wake Is
leads 1 and 2
Wake Is
lead 1
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
Amer Samoa
leads 1,2,3 and 4
Amer Samoa
leads 2,3 and 4
Cook Is south
leads 1,2,3 and 4
Cook Is south
leads 1,2,3 and 4
Easter Is
leads 3 and 4
Fiji
leads 1,2,3 and 4
Fiji
leads 1,2,3 and 4
Fr Poly-Tua-Mar
leads 2,3 and 4
Fr Poly-Tua-Mar
leads 2,3 and 4
Hawaii
leads 3 and 4
Hawaii
lead 4
Johnston Is
leads 3 and 4
Kiribati-Main
leads 2 and 3
Marshall Is
leads 3 and 4
New Caledonia
leads 1,2,3 and 4
New Caledonia
leads 3 and 4
Niue
leads 1,2,3 and 4
Niue
leads 1,2,3 and 4
Samoa
leads 1,2,3 and 4
Samoa
leads 2 and 3
Tokelau
lead 4
Tonga
leads 1,2,3 and 4
Tonga
leads 1,2,3 and 4
Tuvalu
leads 2 and 3
Vanuatu
leads 1,2,3 and 4
Vanuatu
leads 1,2,3 and 4
Wake Is
leads 3 and 4
OBSERVED CLIMATOLOGY DATA for May-Jun-Jul,
Jun-Jul-Aug, Jul-Aug-Sep and Aug-Sep-Oct
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CLIMATOLOGICAL AVERAGE:
TERCILE THRESHOLDS (33%-ile & 67%-ile):
EXTREME THRESHOLDS (15%-ile & 85 %-ile):
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