Abstract
Abstract Tropical cyclones drive intense ocean vertical mixing that explains most of the surface cooling observed in their wake (the “cold wake”). In this paper, the authors investigate the influence of cyclonic rainfall on the cold wake at a global scale over the 2002–09 period. For each cyclone, the cold wake intensity and accumulated rainfall are obtained from satellite data and precyclone oceanic stratification from the Global Eddy-Permitting Ocean Reanalysis (GLORYS2). The impact of precipitation on the cold wake is estimated by assuming that cooling is entirely due to vertical mixing and that an extra amount of energy (corresponding to the energy used to mix the rain layer into the ocean) would be available for mixing the ocean column in the hypothetical case with no rain. The positive buoyancy flux of rainfall reduces the mixed layer depth after the cyclone passage, hence reducing cold water entrainment. The resulting reduction in cold wake amplitude is generally small (median of 0.07 K for a median 1 K cold wake) but not negligible (>19% for 10% of the cases). Despite similar cyclonic rainfall, the effect of rain on the cold wake is strongest in the Arabian Sea and weak in the Bay of Bengal. An analytical approach with a linearly stratified ocean allows attributing this difference to the presence of barrier layers in the Bay of Bengal. The authors also show that the cold wake is generally a “salty wake” because entrainment of subsurface saltier water overwhelms the dilution effect of rainfall. Finally, rainfall temperature has a negligible influence on the cold wake.
Highlights
While Tropical Cyclone (TC) track forecasts have steadily improved over the last 20 years, there has been little improvement of TC intensity forecasts (DeMaria et al 2007)
The ocean surface can influence the storm intensity in mainly two ways: first, high ambient sea surface temperature (SST) ahead of the storm allows for potentially stronger storms; second, the ocean surface cooling induced by the storm may reduce enthalpy fluxes toward the atmosphere and, provide a negative feedback to the storm development (Chang and Anthes 1978; Schade and Emanuel 1999; Schade 2000; Bender and Ginis 2000; Cione and Uhlhorn 2003; Kaplan and De Maria 2003)
Ocean surface cooling induced by tropical cyclones can inhibit their intensification
Summary
While Tropical Cyclone (TC) track forecasts have steadily improved over the last 20 years, there has been little improvement of TC intensity forecasts (DeMaria et al 2007). Only Jacob and Koblinsky (2007) have investigated these effects in a case study, using a highresolution ocean model forced by observed winds from hurricane Gilbert in the Gulf of Mexico They have found that the stabilizing effect of rain can weaken the cold wake by 0.2 to 0.5 K locally but that the associated sensible flux only marginally influences the amplitude of the TC-induced cooling. This gives an insight into the various physical processes that modulate the effect of rainfall on the cold wake, depending on the region.
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