Restratification of the Upper Ocean after the Passage of a Tropical Cyclone: A Numerical Study

The role of baroclinic instability in the restratification of the upper ocean after the passage of a tropical cyclone (TC) is determined by means of numerical simulations. Using a regional ocean model, the Regional Ocean Modeling System (ROMS), a high-resolution three-dimensional simulation that includes the process of baroclinic instability and is initialized with moderate-amplitude eddy structures reproduces the satellite-observed decay rate of the TC-induced sea surface temperature (SST) anomaly and is also in qualitative agreement with published observations after the passage of Hurricane Fabian in 2003 that showed decaying cold and warm anomalies located in the climatological mixed layer (CML) and upper thermocline, respectively. The model ocean is restratified after approximately one month with a net heat gain in the water column due to anomalous air sea heat fluxes. The model shows, however, that vertical heat fluxes associated with baroclinic instability dominate over air sea heat fluxes in restoring the CML heat content during the first month. A comparison with two-dimensional simulations that exclude baroclinic adjustment further highlights the importance of baroclinic instability: it can not only input a considerable amount of heat into the CML, but also establish strong stratification there, inhibiting the downward penetration of heat contributed by diabatic heating at the surface; both effects hasten the recovery of the SST. Additional experiments were performed to examine the sensitivity of the model results to changes in Newtonian cooling rate, changes in the magnitude of the eddy structures used to initialize the simulation, and changes in poststorm wind strength; the results indicate that, although some of them may have a significant effect on the recovery time of the SST, their influence on the contribution of baroclinic instability to the recovery of the CML heat content is modest. However, the contribution of baroclinic instability exhibits pronounced positive dependence on the depth of the mixing layer relative to the CML depth and the relative size of the area with unperturbed water. Its dependence on the shape of the spatial variation of the mixing depth is relatively weak but in a more complicated manner. These dependencies are consistent with those predicted by a simple front adjustment model, whereas the latter also suggest that the contribution of baroclinic instability is independent of the prestorm stratification below the CML. Overall, the idealized simulations in this study suggest that, for a typical situation in the real ocean, barclinic instability can account for approximately 50% of the full recovery of the CML heat content, whereas under specific conditions the contribution can be significantly smaller. Those estimates provide a limit to the maximum net warming of the water column after the initial mixing event and thus have important implications regarding estimating the long-term effect of TCs on the upper-ocean heat budget.