The WMO's recent publication on "Global Perspectives on Tropical Cyclones" (1995 WMO TD No. 693) summarize the recent advances in this challenging field of science and offers an assessment that there is a "lack of any strong relationship between cyclone numbers and simultaneous SSTs". In this investigation, we provide observational evidence showing that, in fact, there is a strong association, especially for the interdecadal variations. The physical links between large-scale environmental conditions and Atlantic tropical storminess are investigated in detail, resulting in some refinements in our understanding of the tropical cyclone activity. This is an outgrowth of the "Threats Assessments Project" of CPC.
Five decades of TC and SST data (1950-97) and four decades of NCEP/NCAR reanalyses (1961-97) were used. The large-scale controlling influences of the atmosphere, in terms of streamfunction, vorticity and vertical shear of the horizontal winds were examined, in addition to the lower boundary impact. Both in situ relationship and far-field influences were studied. The vertical shear of the horizontal winds as a discriminator of the tropical cyclone frequency has been widely accepted, but, without much firm evidence as cautioned by the WMO text book mentioned above as well as Zehr's 1992 report. In this study, a special effort was spent in providing the needed evidence that a negative vertical shear anomaly is indeed an effective discriminator.
The Atlantic tropical cyclone activity shows prominent interannual as well as interdecadal variations. The local SSTs and vertical wind shear are found to be influential. The Pacific ENSO events also exert a significant amount of influence on the Atlantic tropical storminess. The physical link between the two activities is emphasized and found to be consistent with what was suggested by Gray (1984) and Shapiro (1987). During El Nino (La Nina) seasons, the increased (decreased) east Pacific convection and the associated eastward extension (retraction) of the upper-lever westerly winds are likely to result in an enhanced (reduced) vertical shear over the western north Atlantic
region. These far field influenced flow anomalies can contribute to the subdued (enhanced) Atlantic tropical cyclone activity on the interannual time-scales.
The interdecadal variations of the Atlantic tropical cyclone activity are found to be in sync with the local variations of SSTs and vertical wind shears. The Pacific far-field influence is not consistently observed, showing opposite association between the early decades and the recent decades