What is the sea-surface temperature threshold for tropical cyclone formation?

Over the past few decades, it has been widely believed that a minimum sea surface temperature (SST) of 26 or 26.5° C is necessary for TC formation. Recently Dare and McBride (2011) compared the NOAA/NCDC SST dataset (Reynolds et al. 2007) with TC formation data from the IBTrACS TC dataset for the years 1981 to 2008 to test if indeed a robust SST threshold existed. They found that more than 93% (98%) of TCs were located over water of 26.5° (25.5°) or greater at the time they first were recorded to exceed 17 ms-1 (hereafter TC formation time). These percentages increased further when the SST within 48 hours of formation was considered.

The robustness of any threshold depends on the proportion of events that fail to satisfy the threshold. Testing TC formation threshold robustness is confounded by subjectivity in (i) the determination of TC wind speeds (and hence the formation time and location) and (ii) discriminating between tropical and subtropical storms. Dare and McBride found little sensitivity to the former, but did not test the sensitivity to their latitudinal definition of tropical systems: storms that formed equatorward of 35°. While this geographical definition is likely to include nearly all tropical systems it is likely to also include subtropical storms that gain a significant proportion of their energy from the release of baroclinic instability. In this paper we choose to define tropical systems as storms that gain the majority of their kinetic energy from the release of thermodynamic instability, for which an SST threshold is likely to be more critical. Thus, we consider a dynamic definition of the tropics following Tory et al. (2013) as the region equatorward of the subtropical jet (STJ). The STJ definition of the poleward edge of the tropics is not new, having appeared in a number of papers over the last few decades (e.g., Fueglistaler et al., 2009). When it is well defined the STJ is associated with an abrupt change in tropopause height and strong baroclinicity.

In this paper we will reassess the conclusions of Dare and McBride using the dynamic definition of the tropics. Preliminary results have shown that all TCs identified by Dare and McBride to have formed over water less than 25.5° C (for the full 48 hour period prior to formation) were located poleward or coincident with the STJ. Furthermore, of the remaining storms that were located over water less than 25.5° C at the time of formation, twelve were short-lived, borderline TCs, four had spent considerable time over significantly warmer water in the previous 48 hours, and only one intensified significantly over cooler water. Interestingly, that one case occurred downstream of an intensifying upper-troposphere PV anomaly in an equatorward intrusion of the STJ.

References: Dare, R. A. and J. L. McBride, 2011: The threshold sea surface temperature condition for tropical cyclogenesis. J. Clim., 24, 4570—4576. Fueglistaler, S., A. E. Dessler, T. J. Dunkerton, I. Folkins, Q. Fu, and P. W. Mote (2009), Tropical tropopause layer, Rev. Geophys., 47, RG1004, doi:10.1029/2008RG000267

Reynolds, R. W., T. M. Smith, C.Liu, D. B. Chelton, K. S. Casey and M.G. Schlax, 2007: Daily high-resolution blended analyses for sea surface temperature. J. Clim., 20, 5473—5496.

Tory, K. J., S. S. Chand, J. L. McBride, H. Ye and R. A. Dare, 2013: Projected Changes in Late-twenty-First-Century Tropical Cyclone Frequency in 13 Coupled Climate Models from Phase 5 of the Coupled Model Intercomparison Project. J. Clim., 26, 9946—9959.