[1] A 1/2°resolution global ocean general circulation model is used to investigate the processes controlling sea surface cooling in the wake of tropical cyclones (TCs). Wind forcing related to more than 3000 TCs occurring during the 1978-2007 period is blended with the CORE II interannual forcing, using an idealized TC wind pattern with observed magnitude and track. The amplitude and spatial characteristics of the TC-induced cooling are consistent with satellite observations, with an average cooling of $1°C that typically extends over 5 radii of maximum wind. A Wind power index (WPi) is used to discriminate cooling processes under TCs with high-energy transfer to the upper ocean (strong and/or slow cyclones) from the others (weak and/or fast cyclones). Surface heat fluxes contribute to $50 to 80% of the cooling for weak WPi as well as away from the cyclone track. Within 200 km of the track, mixing-induced cooling increases linearly with WPi, explaining $30% of the cooling for weak WPis and up to $80% for large ones. Mixing-induced cooling is strongly modulated by pre-storm oceanic conditions. For a given WPi, vertical processes can induce up to 8 times more cooling for shallow mixed layer and steep temperature stratification than for a deep mixed layer. Vertical mixing is the main source of rightward bias of the cold wake for weak and moderate WPi, but along-track advection becomes the main contributor to the asymmetry for the largest WPis.

In the decades, the use of scatterometer data allowed to demonstrate the global ubiquity of the ocean mesoscale thermal feedback (TFB) and current feedback (CFB) effects on surface winds and stress. Understanding these air‐sea interactions is of uttermost importance as the induced atmospheric anomalies partly control the ocean circulation and thus can influence the Earth climate. Whether the TFB and CFB effects can be disentangled, and whether satellite scatterometers can properly reveal them, remain rather unclear. Here, using satellite observations and ocean‐atmosphere coupled mesoscale simulations over 45°S to 45°N, we show that the CFB effect can be properly characterized and unraveled from that due to the TFB. We demonstrate that the TFB can be unambiguously characterized by its effect on the stress (and wind) divergence and magnitude. However, its effect on the wind and stress curl is contaminated by the CFB and thus cannot be estimated from scatterometer data. Finally, because scatterometers provide equivalent neutral stability winds relative to the oceanic currents, they cannot characterize adequately the CFB wind response and overestimate the TFB wind response by ≈25%. Surface stress appears to be the more appropriate variable to consider from scatterometer data.

The present study investigates the integrated ocean response to tropical cyclones (TCs) in the South Pacific convergence zone through a complete ocean heat budget. The TC impact analysis is based on the comparison between two long-term (1979–2003) oceanic simulations forced by a mesoscale atmospheric model solution in which extreme winds associated with cyclones are either maintained or filtered. The simulations provide a statistically robust experiment that fills a gap in the current modeling literature between coarse-resolution and short-term studies. The authors’ results show a significant thermal response of the ocean to at least 500-m depth, driven by competing mixing and upwelling mechanisms. As suggested in previous studies, vertical mixing largely explains surface cooling induced by TCs. However, TC-induced upwelling of deeper waters plays an unexpected role as it partly balances the warming of subsurface waters induced by vertical mixing. Below 100 m, vertical advection results in cooling that persists long after the storm passes and has a signature in the ocean climatology. The heat lost through TC-induced vertical advection is exported outside the cyclogenesis area with strong interannual variability. In addition, 60% of the heat input below the surface during the cyclone season is released back to the oceanic mixed layer through winter entrainment and then to the atmosphere. Therefore, seasonal modulation reduces the mean surface heat flux due to TCs to about 3 × 10−3 PW in this region exposed to 10%–15% of the world’s cyclones. The resulting climatological anomaly is a warming of about 0.1°C in the subsurface layer and cooling below the thermocline (less than 0.1°C).

International audienceIn this paper, we explore the global responses of surface temperature, chlorophyll, and primary production to tropical cyclones (TCs). Those ocean responses are first characterized from the statistical analysis of satellite data under ~1000 TCs over the 1998–2007 period. Besides the cold wake, the vast majority of TCs induce a weak chlorophyll response, with only ~10% of induced blooms exceeding 0.1 mg m. The largest chlorophyll responses mostly occur within coastal regions, in contrast to the strongest cold wakes that generally occur farther offshore. To understand this decoupling, we analyze a coupled dynamical-biogeochemical oceanic simulation forced by realistic wind vortices applied along observed TC tracks. The simulation displays a realistic spatial structure of TC-induced blooms and its observed decoupling with TC cold wakes. In regions of strong TC energy input, the strongest cold wakes occur in regions of shallow thermocline (<60 m) and the strongest blooms in regions of shallow nitracline and/or subsurface chlorophyll maximum (<60 m). Shallow thermoclines are found over many open ocean regions, while regions of shallow nitracline and/or subsurface chlorophyll maximum are most prominent in near-coastal areas, explaining the spatial decoupling between the cold and bloom wakes. The overall TC contribution to annual primary production is weak and amounts to ~1%, except in a few limited areas (east Eurasian coast, South tropical Indian Ocean, Northern Australian coast, and Eastern Pacific Ocean in the TC-prone region) where it can locally reach up to 20–30%. Nearly 80% of this TC-induced annual primary production is the result of the biogeochemical response to the 30% strongest TCs

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 EddyPermitting 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.

The South Pacific Convergence Zone (SPCZ) is poorly represented in global coupled simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), with trademark biases such as the tendency to form a "double Intertropical convergence zone" and an equatorial cold tongue that extends too far westward. Such biases limit our confidence in projections of the future climate change for this region. In this study, we use a downscaling strategy based on a regional atmospheric general circulation model that accurately captures the SPCZ present-day climatology and interannual variability. More specifically, we investigate the sensitivity of the projected rainfall response to either just correcting present-day CMIP5 Sea Surface Temperature (SST) biases or correcting projected SST changes using an emergent constraint approach. While the equatorial western Pacific projected rainfall increase is robust in our experiments and CMIP5, correcting the projected CMIP5 SST changes yields a considerably larger reduction (~ 25%) than in CMIP5 simulations (~ + 3%) in the southwestern Pacific. Indeed, correcting the projected CMIP5 warming pattern yields stronger projected SST gradients, and more humidity convergence reduction under the SPCZ. Finally, our bias-corrected set of experiments yields an increase in equatorial rainfall and SPCZ variability in the future, but does not support the future increase in the frequency of zonal SPCZ events simulated by CMIP5 models. This study hence suggests that atmospheric downscaling studies should not only correct CMIP5 present-day SST biases but also projected SST changes to improve the reliability of their projections. Additional simulations with different physical parameterizations yield robust results.

International audienceThis study presents the first multidecadal and coupled regional simulation of cyclonic activity in the South Pacific. The long-term integration of state-of the art models provides reliable statistics, missing in usual event studies, of air-sea coupling processes controlling tropical cyclone (TC) intensity. The coupling effect is analyzed through comparison of the coupled model with a companion forced experiment. Cyclogenesis patterns in the coupled model are closer to observations with reduced cyclogenesis in the Coral Sea. This provides novel evidence of air-sea coupling impacting not only intensity but also spatial cyclogenesis distribution. Storm-induced cooling and consequent negative feedback is stronger for regions of shallow mixed layers and thin or absent barrier layers as in the Coral Sea. The statistical effect of oceanic mesoscale eddies on TC intensity (crossing over them 20 % of the time) is also evidenced. Anticyclonic eddies provide an insulating effect against storm-induced upwelling and mixing and appear to reduce sea surface temperature (SST) cooling. Cyclonic eddies on the contrary tend to promote strong cooling, particularly through storm-induced upwelling. Air-sea coupling is shown to have a significant role on the intensification process but the sensitivity of TCs to SST cooling is nonlinear and generally lower than predicted by thermodynamic theories: about 15 rather than over 30 hPa °C−1 and only for strong cooling. The reason is that the cooling effect is not instantaneous but accumulated over time within the TC inner-core. These results thus contradict the classical evaporation-wind feedback process as being essential to intensification and rather emphasize the role of macro-scale dynamics

2Vera Oerder et al.Abstract Satellite observations and a high-resolution regional ocean-atmosphere coupled model are used to 7 study the air/sea interactions at the oceanic mesoscale in the Peru-Chile upwelling current system. Coupling This mechanism is robust as it does not depend on the choice of planetary boundary layer parameterization.