In this paper, we investigate processes responsible for the smaller amplitude TC-induced surface cooling in the BoB during the postmonsoon compared to the premonsoon season. Because detailed observations underneath TCs are scarce, we analyze a global ocean model simulation forced by realistic TC winds derived from an analytic shape adjusted to observed TC tracks and magnitude over the 1978–2007 period. Our approach samples the ocean response to 135 TCs in the BoB over this 30-year period. The model exhibits TC-induced SST cooling that is about 3 times larger during the premonsoon than during postmonsoon season, in agreement with observations.
 As discussed by V12b, the amplitude of TC cooling is to a large extent explained by two parameters: the wind power input of the TC atmospheric forcing and the cooling inhibition from background oceanic conditions. TC wind power input does not significantly change between the premonsoon and postmonsoon seasons, suggesting that seasonal changes in oceanic structure are responsible for larger TC-induced cooling amplitude during premonsoon season. The heavy precipitation and river discharge during and following the monsoon result in a very intense upper-ocean freshening and the formation of a thick BL. Thermal structure also undergoes marked changes between premonsoon and postmonsoon seasons, with a cooler mixed layer over most of the BoB following the monsoon, resulting in a deeper upper thermal stratification. These thermal and haline stratification changes reduce the entrainment of cooler thermocline waters into the mixed layer and consequently reduce TC-induced cooling during the postmonsoon season. Our analysis indeed reveals that stronger cooling inhibition by oceanic stratification is responsible for a cooling amplitude reduction by a factor of three during the postmonsoon season.
 We then assess the respective contributions of seasonal changes in thermal and haline stratification to the reduction of TC-induced cooling. To that end, we use a simple bivariate statistical model that allows accurately predicting the amplitude of TC-induced cooling from wind power (WPi) and CI indices. This allows demonstrating that the strong near-surface salinity stratification during the postmonsoon season is responsible for ∼40% of cooling decrease, with SST changes explaining the remaining 60%. The respective contributions of thermal and haline stratification however strongly vary spatially within the Bay: haline stratification explains most of the TC-induced cooling inhibition offshore of the eastern coast of India (∼80%), where salinity seasonal changes are strongest while thermal stratification explains all the TC-induced cooling inhibition in the southwestern part of the BoB.
 Our modeling study confirms previous case studies. Observations indeed suggest that TC-induced surface cooling is larger during the premonsoon [Gopalakrishna et al., 1993; Rao, 1987; Sengupta et al., 2008] than during the postmonsoon season in the BoB [Chinthalu et al., 2001; Sengupta et al., 2008; Subrahmanyam et al., 2005]. Sengupta et al. suggest that weaker surface cooling during the postmonsoon season largely results from the presence of salinity and BL changes through individual case studies analysis. Our results, however, suggest that thermal changes are a major contributor to the difference in TC-induced cooling amplitude between the two seasons, although the effects of the changes in haline stratification also significantly contribute the seasonal TC-induced cooling changes, especially around the rim of the northern BoB.
 Satellite observations show that the amplitude of TC-induced surface cooling is also larger during the premonsoon (∼1.2°C) than during postmonsoon (∼0.6°C) season in the Arabian Sea (Figures 14a and 14b). Our model simulation (38 Arabian Sea TCs over the 1978–2007 period) reproduces these premonsoon/postmonsoon TC-induced surface cooling contrasts in the Arabian Sea (Figures 14a and 14b). Figures 14c and 14dshows the histograms of TC-induced cooling as a function of WPi for premonsoon and postmonsoon seasons for the Arabian Sea. Unlike the BoB, there is a stronger wind power input before the monsoon than after (average WPi of ∼2.5 against ∼1.6). Compared to the Bay of Bengal, the regression slope of the TC-induced cooling to WPi does not considerably change between the premonsoon and postmonsoon seasons in the Arabian Sea (0.87°C/0.74°C, against 0.70°C/0.28°C in the BoB) and is close to the 0.70°C slope during premonsoon season in the BoB. The stronger cooling during premonsoon season hence largely results from changes in TC wind power input. Owing to a less salinity-stratified upper ocean [e.g.,Shenoi et al., 2002], the CI is lower in the Arabian Sea than in the BoB (Figure 6). During the postmonsoon season, salinity stratification acts to decrease the CI in the central part of the Arabian Sea (Figures 11b and 11e) due to lower salinity at depth (not shown). This hence partly compensates the CI increase due to changes in the thermal structure west of 70°E (Figures 11a and 11d), resulting in a relatively small influence of the oceanic stratification on the cooling in the Arabian Sea, compared to the BoB.
 The rather coarse resolution model used in the present study simulates rather accurately the contrasted TC-induced cooling amplitude between premonsoon and postmonsoon seasons, suggesting that the 1/2° resolution is sufficient to capture the TC-induced mixing, a dominant process in the cold wake formation. Refined resolution in the BoB within our global model [Biastoch et al., 2008b] or the use of high-resolution regional models [Diansky et al., 2006; R. Benshila et al., The upper Bay of Bengal salinity structure in a high-resolution model, submitted toOcean Modelling, 2012] may however be required to better represent the TC-induced Ekman suction that shoals the thermocline near the eye, increasing the cooling efficiency of vertical mixing [Yablonsky and Ginis, 2009; Jullien et al., 2012]. Although presumably of secondary importance [Jacob and Koblinsky, 2007; Jourdain et al., 2012], a realistic parametrization of the TCs-related precipitation may as well improve the TC-induced ocean response, by accounting for their stabilizing effect on the water column. As shown byJourdain et al. , this stabilizing effect may reduce the cooling amplitude by 5 to 10% in the Bay, hence reducing the model TC-induced cooling overestimation (Figure 7).
 A significant increase in the horizontal and vertical model resolutions together with an improved representation of river discharge and precipitation patterns within the Bay [Papa et al., 2010] may also allow a better representation of the mixed layer and of the offshore export of coastal freshwaters. The model underestimation of the freshening in the northern BoB during the postmonsoon season indeed probably results in an underestimation of the salinity influence on the TC-induced surface cooling. Estimates derived from the observed climatology however suggest that at least 50% of the reduction in TC-induced cooling amplitude may be related to thermal changes (Figure 11). The exact quantification of the influence of salinity may require a more exhaustive and in-depth analysis of oceanic controls on TC-induced cooling within the Bay using observations. The Argo program [Gould et al., 2004] provides a unique opportunity to investigate this issue: started in 2002, this program has reached its targeted density in late 2006. The availability of both temperature and salinity profiles in the upper ocean with reasonable temporal and spatial coverage may allow quantifying the respective contributions of salinity and temperature stratifications on TC-induced cooling inhibition from in situ observations over the recent period.
 This influence of salinity on TC-induced cooling calls for a better description and understanding of salinity variations within the Bay. Previous studies have already shown that the seasonal salinity evolution is largely determined by the fresh water sources/sinks and the redistribution of the resulting low/high-salinity water by ocean currents [e.g.,Rao and Sivakumar, 2003; Vinayachandran et al., 2005; Sengupta et al., 2006]. However, the paucity of observations in coastal regions does not yet allow providing a robust and precise estimate of the intensity and extent of the BoB freshening. In addition, little is known about the interannual variability of SSS in the BoB. Although a limited amount of repeated observations along shipping lanes suggest that salinity variability is high in the tropical Indian Ocean [Delcroix et al., 2005; Rao and Sivakumar, 2003], details of basin-wide spatiotemporal structure of salinity interannual variations in the BoB and their mechanisms are still lacking.
 This major influence of salinity also advocates for the use of an adequate oceanic index in statistical TC intensity prediction schemes. As shown by Yu and McPhaden , buoyancy content in the BoB upper layer has a higher correlation with salinity content than with heat content. A commonly used metric of TC sensitivity to the ocean is the Tropical Cyclone Heat Potential (TCHP), a measure of the heat content between the sea surface and the depth of the 26°C isotherm, computed from altimeter-derived vertical temperature profile estimates [Shay et al., 2000; Goni and Trinanes, 2003]. This index is useful for identifying warm anticyclonic features, where hurricanes often undergo sudden intensification in the western Atlantic [e.g., Shay et al., 2000] and Northwestern Pacific [e.g., Lin et al., 2005]. Using TCHP allows improving statistical intensity forecasts in these regions [DeMaria et al., 2005; Mainelli et al., 2008], where sea level variability is closely related to changes in the depth of the main thermocline, and salinity plays a lesser role. The present study advocates for the use of a different TC oceanic metric that accounts for the effect of salinity, as already suggested by V12b and Yu and McPhaden . The CI proposed by V12bis a relevant option, since it accounts for the effect of salinity stratification on TC-induced cooling inhibition. This metric can be derived from currently available operational oceanography products constrained by oceanic observations [e.g.,Drévillon et al., 2008] or directly from Argo data and tested in cyclone intensity forecast schemes in place of the currently used TCHP, in particular in the BoB.
 The present study brings further evidence of the haline stratification impact on TC-induced surface cooling. Neglecting haline stratification indeed results in a 50% overestimation of the TC-induced cooling during the postmonsoon season in the BoB (Figures 13b and 13d). Figure 15shows the spatial distribution of the long-term average impact of salinity stratification on the precyclone cooling inhibition.Figure 15hence provides a global view of regions where salinity stratification may significantly influence TC-induced SST cooling. Since TCs mostly develop in deep atmospheric convection regions, the associated climatological rainfall results in a stable haline stratification that inhibits TC-induced cooling, explaining the dominance of negative values inFigure 15. While the BoB, studied in this paper, is associated with a rather strong influence of haline stratification on TC-induced cooling, there is also a moderate influence of haline stratification in the western Pacific and South Indian Ocean TC basins. But there is a very clear influence of haline stratification on TC-induced cooling in the low-salinity region of the tropical western North Atlantic due to the discharge from the Amazon and Orinoco rivers whose waters are advected northwestward by the North Brazilian and Guyana Currents [Muller-Karger et al., 1995; Hellweger and Gordon, 2002]. Given that TC that transit over this area can hit densely populated regions of Mexico and southeastern United States [Weinkle et al., 2012], this region probably deserves a dedicated study.
Figure 15. Spatial distribution of the difference between average standard CI underneath TC tracks (in (J.m−2)−1/3) minus CIS0calculated over the 1978–2007 period. Both quantities are estimated from the weekly averaged stratification model outputs, within 200 km and between 10 days and 3 days before each cyclone eye passage location. This plot indicates where salinity stratification inhibits (blue shades) or enhances (red shades) TC-induced cooling.
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 The haline stratification during the postmonsoon season induces an average 40% reduction of TC-induced cooling and potentially much more locally in the northern and eastern rim of the Bay. Given the potentially strong negative feedback of TC-induced cooling on TC intensity, an assessment of the exact impact of haline stratification on TC characteristics is however lacking. Further studies using high-resolution regional coupled models in this region as well as statistical analysis based on observations are therefore required to further address this issue, in the BoB and in the region influenced by the Amazon River plume.