Principal Investigator: Lloyd J. Shapiro
Collaborating scientist(s):
Stanley L. Goldenberg
Objective: To investigate the physical mechanisms responsible for the interannual variability in tropical cyclone activity for the North Atlantic basin.
Rationale: 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.
Method: 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 large-scale circulation.
Accomplishment: 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 relationships. 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.

Most recently, 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.

Key reference:
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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