ATLANTIC TROPICAL CLIMATE AND HURRICANE CYCLES
Lloyd J. Shapiro
Stanley L. Goldenberg
To investigate the physical mechanisms responsible for the interannual
variability in tropical cyclone activity for the North Atlantic basin.
Previous studies have established contemporaneous and
predictive relationships between hurricane climatic fluctuations and the
El Nino/Southern Oscillation (ENSO), Atlantic sea surface temperatures
(SSTs), west African monsoon rainfall, and other parameters related to
the large-scale circulation in the Atlantic region. The physical
mechanisms responsible for some of these relationships have, however,
not been established.
Correlation techniques are used to isolate the physical
mechanisms responsible for the observed relationships between hurricane
activity in the North Atlantic basin and parameters related to the
The interannual variability of major hurricane activity in the
North Atlantic explained by west African Sahel rainfall is almost
three times that explained by ENSO. It was demonstrated that a
likely reason for this result is
that the changes in vertical shear in the main tropical cyclone
development region (10 to 20 degrees North) are much more influenced by associations with
changes in rainfall than by changes related to eastern Pacific
equatorial SST anomalies (Goldenberg and Shapiro 1995).
The research also suggests that
there is a dichotomous relationship between the boreal
summertime west African rainfall and eastern Pacific SSTs as well as
tropical cyclone activity. When the eastern Pacific SSTs are above
the long-term mean, very strong relationships exist, while for
average or below-average SSTs there appear to be little or no
Also, due to the highly unfavorable vertical shear in October, a
necessary condition for major hurricane development in that month
is the highly favorable anomalies that are only associated with a
combination of anomalously cold eastern equatorial Pacific SSTs and wet western Sahel conditions.
the years in the study have been stratified into FIT and
not-FIT years, where FIT years (as classified by other
investigators) contain a tropic-wide mode linking regions of
Central America, India and West Africa with the tropical central Pacific SST
anomalies including those associated with El Nino/Southern
Oscillation (ENSO). Results of the study indicate that
while a very strong statistically significant relationship
between the tropical Pacific SSTs and west African Sahel rainfall
(FIGURE 1) as well as Atlantic basin hurricane
activity was present during the FIT years, during the non-FIT years
the relationsip was nonexistent. This result clearly demonstrates
that grouping all of the years together gives an overly-simplistic
picture of the connection between Atlantic hurricane activity and
ENSO-associated tropical Pacific SSTs.
Also (FIGURE 2), the strongest relationships during FIT years are with SSTs
in the equatorial central Pacific one to two months prior to the peak
months of the hurricane season. This result shows promise for
increasing the predictability of the impact of Pacific SSTs on the
Atlantic hurricane season.
Goldenberg, S. B., and L. J. Shapiro, 1995: A new look at the
relationships between El Nino, West African rainfall, and North Atlantic
tropical cyclone activity. Preprints, 21st Conference on Hurricanes and
Tropical Meteorology, AMS, Miami, Florida, 585-587.
Goldenberg, S. B., and L. J. Shapiro, 1995: Physical mechanisms for the
association of El Nino and west African rainfall with Atlantic major
hurricanes. J. Climate, submitted.
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