Geoscience Reference
In-Depth Information
Atlantic tropical cyclone prediction model
NEW DEVELOPMENTS
Tropical cyclones can cause major damage because of the strength of their winds, and the associated flooding caused
by rainfall or oceanic storm surges. Weather forecasters use atmospheric models to predict the tracks of these storms
and provide storm warnings or even initiate evacuation of areas likely to be affected.
More recently attempts have been made by a research group at Colorado State University in the United States to
predict the degree of activity of a hurricane season in the North Atlantic up to eleven months in advance. For the
tracks of individual storms it is possible to produce reliable estimates for only a few days ahead. The forecasts of
future activity are based on the values of a series of indices which have been shown to be related to subsequent
seasonal variations in Atlantic activity. These indices involve oceanic and atmospheric properties which show
statistical relationships with hurricane activity. The detailed physical processes which would be required to develop
the storms are not involved in the calculations.
The first index is the state of El Niño-Southern Oscillation in the tropical South Pacific. Strong El Niños in this area
favour upper-level westerly winds over the Atlantic which typically reduce hurricane activity. The La Niña state favours
hurricane activity, as in 2000. The state of the Quasi-biennial Oscillation in the stratosphere is considered, as there
are usually more hurricanes when the equatorial stratospheric winds are from a westerly direction than when they
are from the east. The state of surface pressure in the north-east Atlantic is included, as when this ridge is anomalously
weak during the prior autumn and spring periods the trade winds in the area are weaker. This means there is reduced
upwelling of cold water in the Canary Islands area and warmer sea surface temperatures - which favour hurricane
formation. Similarly the sea surface temperature anomalies in three areas of the north, tropical and south Atlantic
are included to give a measure of the warmth of the ocean; warmer seas favour more hurricanes. Spring and early
summer sea-level pressure anomalies and zonal wind anomalies over the eastern Caribbean have an impact through
low pressure and easterly anomalies, indicating enhanced seasonal activity, whilst positive values imply suppressed
hurricane activity. Surprisingly, rainfall in west Africa shows some relationship with hurricanes. When rainfall in the
western Sahel in the previous August-September period is above average, and when August-November Gulf of
Guinea rainfall of the previous year is also above average, there are more strong hurricanes. Finally note is taken of
the west-to-east surface pressure and temperature gradients across West Africa between February and May, as
strong gradients are associated with greater hurricane frequency.
Using these indices as predictors based on the period 1950-97, statistically based forecasts are made. Analogues
are also examined for those years with similar precursor climatic signals to the forecast year. If the analogue year
was followed by a year of increased hurricane activity it is assumed that the forecast year will also experience more
storms than average. So far the predictions have worked reasonably well, though like many statistical models
the significance of the variables can vary. For example, the significance of the Sahel rainfall factor has declined
since 1995. Further work is continuing to improve the performance of the model. It can be found on
warm or cold conditions and be either showery or dry.
Embedded within the major flows, we can also find
disturbances that appear in a variety of forms around the
globe. In temperate latitudes, the disturbances of the
westerly zone generate large-scale spiralling cloud systems
which produce much of the rainfall in these zones. They
tend to follow favoured tracks, so that some areas, such
as western Canada, Iceland and north-western Europe
receive a regular supply of precipitation. In tropical areas
there is even more variety in the forms of disturbances.
CONCLUSION
Within the major circulation systems of the atmosphere,
there are many features that give rise to the majority of
weather events which make up our climate. Flowing away
from the main centres of high pressure we have the air
streams or air masses which transport energy and
moisture polewards and equatorwards. Depending upon
their direction of flow and their degree of cyclonic
curvature of the isobars, such air flows can give rise to
