Principal Investigator: Lloyd J. Shapiro
Collaborating scientist(s):

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: 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 established. 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: 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.
Key reference:

Click here to return to the AOML project overview page.
Click here to return to the Implement Seasonal to Interannual Climate Forcasts page.