Atlantic Sea Surface Temperatures and Hurricane Formation
Principal Investigator:
Lloyd J. Shapiro
Objective:
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.
Rationale:
Interannual fluctuations in sea surface temperature (SST) in the Atlantic are associated with fluctuations in Atlantic basin hurricane frequency. Hurricanes rarely form when the underlying SST is below a threshold value of about 26.5 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 have been correlated, historically,
with fluctuations in hurricane frequency. These correlations imply that underlying SSTs may have a direct influence on the development of the hurricanes, presumably by providing a more or less favorable thermodynamic environment for intensification of we
aker systems.
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 the tropospheric 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.
Method:
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 hurricane) frequency during August-October, the most
active months of the hurricane season.
Accomplishment:
A singular value decomposition (SVD) has been used to establish the dominant covarying modes of tropospheric wind shear and SST as well as horizontal SST gradients. The dominant SVD mode of covarying vertical shear and SST gradients, which
comprises equatorially-confined near-zonal vertical wind shear fluctuations across the Atlantic basin, is highly correlated with both equatorial eastern Pacific SST anomalies (associated with El Nino) and West African Sahel rainfall. While this mode is s
trongly
related to tropical storm, hurricane and major hurricane frequency in the Atlantic, it is not associated with any appreciable Atlantic SST signal.
By contrast, the second SVD mode of covarying vertical shear and horizontal
SST gradient variability is associated with SST fluctuations concentrated
in the main tropical cyclone development region between 10 and 20 degrees
North (figure). This mode is significantly
correlated with hurricane and
tropical storm frequency, but not with major hurricane frequency. Statistical tests confirm the robustness of the mode, and lag correlations and physical reasoning demonstrate that the SST anomalies are not due to the developing tropical cyclones
themselves. Anomalies of SST and vertical shear during years where the mode has substantial amplitude confirm the resemblance of the individual fields to the modal structure, as well as the association of hurricane development with the warmer SSTs. Altho
ugh
SSTs are of secondary importance to vertical shear in modulating hurricane formation, the results support the conclusion that warmer SSTs directly enhance development. The lack of correlation with major hurricanes implies that the underlying SSTs are not
a
significant factor in the development of these stronger systems. The results of the present analyses have been submitted for formal publication (Shapiro and Goldenberg 1996).
Key references:
Shapiro, L. J., and S. B. Goldenberg, 1996: Atlantic sea surface temperature and hurricane formation, J. Climate, submitted.
Last modified: 8/9/96