The IRI Experimental Forecast Division has prepared this Climate Outlook for Europe for October 1999 - March 2000.
Of relevance in the preparation of this Outlook has been the current weak
La Niņa
conditions (lower than average sea-surface temperatures (SSTs) in the
central equatorial Pacific Ocean) that are
expected to amplify slightly through the
end of 1999
and then decline slowly in early 2000.
The persistence of slightly
warmer than average conditions in the western Pacific has been assumed also. The SSTs
of the tropical Indian Ocean were assumed to return to near-normal
conditions with the period October-December 1999.
The forecast of tropical South Atlantic SSTs suggests a
warming trend through December 1999.
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 the resurgence of weak to moderate La
Niņa conditions, with the persistence of slightly positive sea-surface temperature anomalies in the western Pacific.
The return of colder that average conditions in the central and eastern equatorial Pacific is consistent with most other
numerical and statistical forecasts.
B) Forecasts of the tropical Indian and Atlantic oceans using statistical models developed by the IRI and CPTEC
respectively.
C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted
sea-surface temperature patterns.
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 October 1999 - March 2000 is dependent on the quality of the SST predictions. For
the tropical Pacific, these predictions can be expected to provide useful information, but there is some uncertainty
concerning the detailed evolution of SSTs. Spread in global SST predictions is a source of uncertainty in the Outlook
provided here. In particular, the predicted warming over the southern tropical Atlantic has been a primary influence
on the forecasts over South America.
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. 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: October - December 1999 and January - March 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 Portugal in October - December 1999
(Map B), there is a 25% probability that the precipitation will be in the wettest third of the years, a 40% chance it will be in the near-normal third
of the years, and a 35% 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.
October - December 1999:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
P.A) slightly enhanced probabilities of near-normal precipitation Ireland, England and Wales, northern France, Belgium, Luxembourg, Netherlands, western Germany, and much of Denmark;
P.B) slightly enhanced probabilities of near-normal precipitation over Portugal, most of Spain, and the south-easternmost part of France;
P.C) enhanced probabilities of near-normal precipitation over Belarus, northern Ukraine, and part of western Russia;
P.D) slightly enhanced probabilities of near-normal precipitation over Iraq, and north-eastern Iran.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.A) slightly enhanced probabilities of above-normal temperatures over Iceland;
T.B) greatly enhanced probabilities of above-normal temperatures over an area of north-western Europe,
extending from Ireland to southern Sweden, and including the North Sea and Baltic Sea coasts of mainland Europe;
T.C) enhanced probabilities of near- to below-normal temperatures over northern Norway and Sweden, most of Finland, and the Kola Peninsula of Russia;
T.D) enhanced probabilities of near- to above-normal temperatures over an area extending from north-eastern Poland, into western Russia, including some of the Baltic States, Belarus, and northern Ukraine;
T.E) enhanced probabilities of above-normal temperatures over a north-west to south-east band stretching from France through to north-eastern Libya, Egypt, and north-western Saudi Arabia;
T.F) greatly enhanced probabilities of above-normal temperatures over the Iberian peninsula, Sicily, northern Algeria, Tunisia, and north-western Libya;
T.G) enhanced probabilities of above-normal temperatures over northern northern Morocco;
T.H) slightly enhanced probabilities of above-normal temperatures over an area between the Black and Caspian seas, extending southwards into northern Iraq.
January - March 2000:
PRECIPITATION
The following region of enhanced probabilities of precipitation anomalies is indicated:
P.i) slightly enhanced probabilities of above-normal precipitation over north-western Europe.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.i) enhanced probabilities of above-normal temperatures over Iceland;
T.ii) enhanced probabilities of above-normal temperatures over an area of north-western Europe extending from the United Kingdom to the westernmost part of Finland;
T.iii) slightly enhanced probabilities of above-normal temperatures most of southern Europe from Portugal to Romania.
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