SST and Vortex Structure Observed Prior to the Formation of Atlantic Tropical Cyclones

Accurate predictions of tropical cyclone (TC) formation remain a challenge for the forecasting community. Two environmental conditions considered necessary but insufficient for TC formation are sea surface temperature (SST) exceeding 26°C and a pre-existing cyclonic vortex. The role of SST in TC development has been widely studied, and the 26°C threshold appears valid for systems of tropical origin. However, TCs have formed in the Atlantic basin over SSTs less than 26°C. In such cases, the TC almost always originates from a non-tropical precursor (e.g., a frontal system or surface gale center).

Less is known about the structure of the cyclonic vortex that is also required to produce a TC. Previous research has either assumed the presence of a low-level circulation or used a minimum threshold. Formation of TCs in recent Atlantic hurricane seasons has demonstrated that incipient vortex strength varies greatly among systems. TCs originating from tropical precursors tend to be initially weaker at both low- and mid-levels, while TCs originating from non-tropical precursors often have stronger circulations at one or both levels.

This study examines the relationship between SST and vortex structure prior to the formation of TCs. A statistical analysis is performed on Atlantic TCs detected during the years 1982-2010 to determine the minimum vortex strength necessary for TC formation for a given SST.