ATLANTIC SEA SURFACE TEMPERATURES AND HURRICANE FORMATION
Lloyd J. Shapiro
To investigate the physical mechanisms responsible for the observed
relationship between interannual fluctuations in Atlantic sea surface
temperatures and tropical cyclone activity for the North Atlantic basin.
Climatic fluctuations on interannual and interdecadal time scales are
associated with changes in sea surface temperature (SST).
Hurricanes rarely form when the underlying
SST is below a threshold value of about
26 degrees Centigrade. During the summer months climatological SSTs are
above this threshold over the substantial part of the tropical and
subtropical North Atlantic basin.
Nevertheless, variations in SST in the Atlantic basin could
influence both the overall frequency, as well as strength, of tropical
storms and hurricanes in that region.
Increases in SST in the Atlantic have been associated, historically,
with increased hurricane frequency (figure).
The physical mechanism for this correlation has not, however, been
A recent study suggests that the observed correlation is not a direct
result of higher Atlantic SSTs themselves influencing
hurricane formation, but rather an indication of an indirect
relationship due to large-scale circulation features that influence
both SSTs and hurricanes.
Previous studies have established that fluctuations in the magnitude of
vertical wind shear are one of the most important factors associated with
changes in seasonal hurricane frequency, particularly major hurricanes
(with maximum sustained surface winds > 50 m/sec).
Relationships between Atlantic major hurricane frequency and equatorial eastern
Pacific SSTs (El Nino)
as well as West African Sahel rainfall are explained primarily
by their associations with vertical shear over the main
tropical cyclone development region.
SST and wind data over the Atlantic
basin from 1968-1992 are being used to isolate the
physical mechanism responsible for the
association between SSTs and tropical cyclone (storm, hurricane, or major
during August-October, the most active months of the hurricane season.
As part of the analysis,
the covarying modes of Atlantic SST and wind variability are derived.
A few significantly correlated patterns of vertical wind shear
and horizontal gradients of SST in the Atlantic
are found to be significantly correlated with tropical cyclone frequency.
The dominant mode of coupled horizontal SST gradient and vertical wind
shear variability is highly correlated with El Nino and West African
Sahel rainfall. Lag correlations support the conclusion that
the Atlantic SST fluctuations associated with this mode,
which explain the predominant part of the relationship between the
SSTs and major hurricane frequency,
are a response to large-scale circulations
including those associated with El Nino.
The second dominant coupled mode, which is
highly correlated with tropical storm and hurricane frequency but is
unrelated to El Nino or West African Sahel rainfall,
evidences anticyclonic vertical shear
downstream (to the north) of warm SST anomalies in the main tropical cyclone
development region between 10 and 20 degrees North.
Correlations support the conclusion that
the SST fluctuations associated with this mode have a direct influence on
Atlantic tropical storm and hurricane frequency.
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