Impact of Tropical Cyclones on the Heat Budget of the South Pacific Ocean

Abstract

he 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).