Revisiting the 26.5°C Sea Surface Temperature Threshold for Tropical Cyclone Development

Abstract

A high sea surface temperature is generally accepted to be one of the necessary ingredients for tropical cyclone development, indicative of the potential for surface heat and moisture fluxes capable of fueling a self-sustaining circulation. Although the minimum 26.5°C threshold for tropical cyclogenesis has become a mainstay in research and education, the fact that a nonnegligible fraction of storm formation events (about 5%) occur over cooler waters casts some doubt on the robustness of this estimate. Tropical cyclogenesis over subthreshold sea surface temperatures is associated with low tropopause heights, indicative of the presence of a cold trough aloft. To focus on this type of development environment, the applicability of the 26.5°C threshold is investigated for tropical transitions from baroclinic precursor disturbances in all basins between 1989 and 2013. Although the threshold performs well in the majority of cases without appreciable environmental baroclinicity, the potential for development is underestimated by up to 27% for systems undergoing tropical transition. An alternative criterion of a maximum 22.5°C difference between the tropopause-level and 850-hPa equivalent potential temperatures (defined as the coupling index) is proposed for this class of development. When combined with the standard 26.5°C sea surface temperature threshold for precursor-free environments, error rates are reduced to 3%–6% for all development types. The addition of this physically relevant representation of the deep-tropospheric state to the ingredients-based conceptual model for tropical cyclogenesis improves the representation of the important tropical transition-based subset of development events.

Development mechanisms for Mediterranean tropical‐like cyclones (medicanes)

“…The sea‐surface temperatures (SST) over which they form are below the threshold of 26.5 °C observed for most tropical cyclones (Miglietta et al, ), since cold‐air intrusions in the extratropics may increase the conversion efficiency of thermal energy into mechanical energy (Palmén, ), making possible the development of a TLC even in January. This mechanism is similar to that responsible for the formation of tropical cyclones farther from the Equator, close to the Tropics (McTaggart‐Cowan et al, ).…”

On the Seasonal Cycles of Tropical Cyclone Potential Intensity

“…1a) and are above the canonical 26.58C threshold for TC development year-round (e.g., Palmén 1948; although this requisite has been recently reexamined, and its specific value is expected to increase with climate change; e.g., Vecchi and Soden 2007;Johnson and Xie 2010;McTaggart-Cowan et al 2015). The deep mixed layer in the Pacific warm pool region results in a weak SST seasonal cycle (mean amplitude of 1.5 K) at the lower tropical latitudes (Fig.…”

Influence ofENSO,ENSOModoki, and theIPOon tropical cyclogenesis: a spatial analysis of the southwest Pacific region

“…Anomalous conditions conducive to TC genesis include large-scale thermodynamic environments, SSTs exceeding 26.5 ∘ C (McTaggart-Cowan et al, 2015), and high values of RH (positive anomalies). Cyclonic VORT (negative anomalies; Gray, 1968Gray, , 1975 and anomalously negative EVWS (low zonal shear; DeMaria, 1996;Paterson et al, 2005) are also conducive for TC genesis.…”