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

Abstract

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).

Observation-Based Estimates of Surface Cooling Inhibition by Heavy Rainfall under Tropical Cyclones

“…Little evaporation occurs within a disk of ;18 (Cione et al 2000) because air is near saturation and most of the moisture feeding atmospheric convection is advected from as far as 1600 km from the eye (Trenberth et al 2007). To estimate evaporation within our disks of 28 radius, we have analyzed outputs from a 1 /38 interannual ocean simulation from the regional ocean modeling system (ROMS) model (Jullien et al 2012), two-way coupled to an equivalent atmospheric configuration from the Weather Research and Forecasting (WRF) model (Jourdain et al 2011). The total evaporation e during the TC passage is well described by the linear relationship e 5 0.23p.…”

“…Such microwave SST estimates are relatively insensitive to cloud coverage, but do not provide reliable estimates under rainfall (Wentz et al 2000). They do, however, provide an estimate of the cold wake, which reaches its full amplitude two or three inertial periods (2-4 days) after the TC passage (e.g., Shay et al 1992;Jacob et al 2000;Jullien et al 2012;Vincent et al 2012a;Levy et al 2012), when satellite observations are much less affected by rain. The TC-induced SST cooling DSST is extracted from TMI-AMSRE after removal of the seasonal cycle.…”

“…The SST signature in the wake of tropical cyclones is reasonably well captured by the 0.258 blended Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI)-Advanced Microwave Scanning Radiometer (AMSR-E) daily satellite product (see Lloyd and Vecchi 2011;Vincent et al 2012a;Jullien et al 2012; http:// www.ssmi.com/sst/microwave_oi_sst_data_description. html).…”

“…This vertical mixing largely results from the intense vertical shear related to the strong inertial currents driven by TCs (Price 1983;Greatbatch 1984;D'Asaro 1985;Shay et al 1989;Price et al 1994;D'Asaro et al 1995;Crawford and Large 1996;Tsai et al 2008;Samson et al 2009;Jullien et al 2012). Recently, Vincent et al (2012a) have confirmed that vertical mixing is the dominant cooling process close to cyclone tracks and for strong cyclones by using an ocean general circulation model to study the cold wake of more than 3000 TCs over the last 30 years.…”

Western boundary upwelling dynamics off Oman

“…Mixed-layer thickness is diagnosed by comparing a bulk Richardson number to a critical value. Below the mixed layer, the interior mixing scheme takes into account three processes: static instability (convection), vertical shear, and internal wave breaking (for details on interior and boundary layer mixing, see the appendix in Jullien et al 2012). Bottom stress is linear with a drag coefficient C d = 3 × 10 −4 m s −1 .…”