Observation-Based Estimates of Surface Cooling Inhibition by Heavy Rainfall under Tropical Cyclones NICOLAS C. JOURDAIN,* ,1 MATTHIEU LENGAIGNE, #,@ JE ´ ROME VIALARD, # GURVAN MADEC, #,& CHRISTOPHE E. MENKES, #, ** EMMANUEL M. VINCENT, # SWEN JULLIEN, 11 AND BERNARD BARNIER 1 1 Laboratoire des Ecoulements Ge´ophysiques et Industriels, Centre National de la Recherche Scientifique, Grenoble, France # Laboratoired’Oce´anographie et du Climat:Expe´rimentationet Approches Nume´riques, CNRS/IRDN/UPMC/MNHN, Paris, France @ National Institute of Oceanography, Goa, India & National Oceanographic Center, Southampton, United Kingdom ** Institut de Recherche pour le De´veloppement, Noume´a, New Caledonia 11 Laboratoire d’Etudes en Ge´ophysique et Oce´anographie Spatiale, Toulouse, France (Manuscript received 8 May 2012, in final form 17 September 2012) 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 accu- mulated 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. 1. Introduction 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). Past studies have pointed to several im- portant physical processes that may induce uncertainties in intensity forecasts: storm inner core dynamics, struc- ture of the atmospheric synoptic-scale environment, and air–sea interactions (Marks and Shay 1998; Emanuel 2000). The ocean surface can influence the storm intensity in mainly two ways: first, high ambient sea surface tem- perature (SST) ahead of the storm allows for potentially stronger storms; second, the ocean surface cooling in- duced by the storm may reduce enthalpy fluxes toward the atmosphere and, hence, 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). The results of Schade (2000) suggest that cyclone intensity is most sensitive to cooling under the storm, hence providing a strong incentive to better un- derstand this cooling (referred to as the ‘‘cold wake’’ in many studies). In situ observations of individual TCs (e.g., Jacob et al. 2000; D’Asaro et al. 2007) or numerical case studies (e.g., Price 1981; Greatbatch 1983) suggest that vertical mixing associated with the intense TC wind forcing is responsible * Current affiliation: University of New South Wales, Sydney, New South Wales, Australia. Corresponding author address: Nicolas C. Jourdain, CCRC, Mathews Building, Level 4, University of New South Wales, Sydney, NSW 2052, Australia. E-mail: nicolas_jourdain@yahoo.fr JANUARY 2013 JOURDAIN ET AL. 205 DOI: 10.1175/JPO-D-12-085.1 Ó 2013 American Meteorological Society