Impact of Tropical Cyclones on the South Pacific Ocean

Tropical cyclones are among the most extreme events of the tropical atmosphere. It is known they induce an intense cooling along their track, but their role on the climatological ocean response remains the subject of debate. 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. Our results show a significant thermal response of the ocean to at least 500-m depth, resulting from 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 out of the cyclogenesis area. This effect displays strong interannual variability. In addition, 60% of the heat input below the surface during the cyclone season is released back into the atmosphere through winter entrainment. 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. This amounts for 10% of previous estimates.