The IRI Experimental Forecast Division has prepared this Climate Outlook for Europe
for July - December 1999. Of
relevance in the preparation of this Outlook have been the
weakening of La Niņa
conditions (lower than average sea-surface temperatures (SSTs) in the
central equatorial Pacific Ocean), but
La Niņa is expected to reamplify toward the
end of the year. The persistence of slightly
warmer than average conditions in the western Pacific have been assumed also. The SSTs
of the southern tropical Indian Ocean were assumed to
remain above average. The forecast of tropical South Atlantic SSTs suggests a strong
warming trend through December 1999.
METHODS -
This Outlook was prepared using the following procedures and information:
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.
D) The West African Regional Climate Outlook Forum consensus guidance.
E) Other sources of information include (
CPC ;
ECMWF ;
South African Weather Service ;
UKMO
.
The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by
the national meteorological services in Europe. Thus, this product may differ from the official forecasts issued in
those areas. The Climate Outlook for July - December 1999 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: July - September 1999 and October - December 1999. 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 India in July - September 1999
(Map B), there is a 35% 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 25% 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.
July - September 1999:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
P.A) slightly enhanced probabilities of above-normal precipitation in southern Mauritania and northern Senegal;
P.B) slightly enhanced probabilities of below-normal precipitation over Guinea, Guinea Bissau, Sierra Leone, and
western Liberia;
P.C) greatly enhanced probabilities of above-normal precipitation over an area south of about 10S from Ivory Coast
to central Nigeria (an enhanced risk of extremely wet conditions is indicated);
P.D) enhanced probabilities of near-normal precipitation over south-eastern Nigeria, southern Cameroon, equatorial
Guinea, and Gabon;
P.E) enhanced porobabilities of normal- to above-normal precipitation over an area extending from southern Mali to
southern Chad;
P.F) slightly enhanced probabilities of below-normal precipitation over Central African Republic, southern Sudan,
and south-western Ethiopia
P.G) slightly enhanced probabilities of below-normal precipitation over Yemen, Oman, and southern Saudi Arabia.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.A) greatly enhanced probabilities of above-normal temperatures over north-western Mauritania, and southern
Morocco;
T.B) greatly enhanced probabilities of above-normal temperatures over West Africa, south of about 15N;
T.C) greatly enhanced probabilities of above-normal temperatures over the whole of Africa south of the equator,
including Madagascar (the probabilities are greatest over Mozambique, Zambia, Malawi, Zimbabwe, Swaziland, and
north-eastern South Africa;
T.D) greatly enhanced probabilities of above-normal temperatures over Somalia and eastern Ethiopia;
T.E) enhanced probabilities of above-normal temperatures over most of Ethiopia, southern Sudan, and parts of southern Chad;
T.F) enhanced probabilites of normal to below-normal temperatures over the eastern part of the Arabian peninsula;
T.G) greatly enhanced probabilities of above-normal temperatures over Egypt and Lybia;
T.H) enhanced probabilities of above-normal temperatures over Tunisia and most of Algeria;
T.I) enhanced-probabilities of above-normal temperatures over Turkey and Greece.
October - December 1999:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
P.i) slightly enhanced probabilities of above-normal precipitation over the south coast of West Africa;
P.ii) enhanced probabilities of below-normal precipitation over part of East Africa.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.i) slightly enhanced probabilities of above-normal temperatures over West Africa south of about 10N;
T.ii) slightly enhanced probabilities of above-normal temperatures over Zambia, western Tanzania, Rwanda, Burundi,
and southern-eastern Democratic Republic of Congo.
![[key]](/climate/forecast/images/key.jpg)
