A Simple Model of Sea-Surface Cooling under a Tropical Cyclone

Abstract

A major ocean response to tropical cyclone (TC) wind is the mixing of warm sea-surface water with cool subsurface water, which decreases the sea-surface temperature (SST). The decreased SST (δT) under the TC (rather than the cooled water in the wake after the storm has passed) modifies the storm’s intensity and is of interest to TC intensity studies. Here, the author shows that δT (non-dimensionalized by some reference temperature) is linearly related to Ψ, a dimensionless (nonlinear) function of TC and ocean parameters: the TC maximum wind, radius, and translation speed, as well as the ocean’s 26 °C and 20 °C isothermal depths (Z26 and Z20). The Ψ can be estimated from observations. The modelled δT is validated against sea-surface cooling observed by satellites, δTo, for typhoons during the May–December 2015 period in the western North Pacific. The result yields a best-fit, linear relation between δTo and Ψ that explains ~60% of the observed variance: r2 ≈ 0.6 (99% confidence). Tests show that the cube of the TC maximum wind and the ocean’s Z26 account for 46% and 7%, respectively, of the observed variance, indicating their predominant influence on TC-induced cooling. Contributions from other parameters are less but not negligible.

Keywords

• tropical cyclones
• wind power
• upper-ocean warm layer Z₂₆
• sea-surface cooling
• ocean mixing
• SST