Simulations of the Ocean Response to a Hurricane: Nonlinear Processes

Abstract

Superinertial internal waves generated by a tropical cyclone can propagate vertically and laterally away from their local generation site and break, contributing to turbulent vertical mixing in the deep ocean and maintenance of the stratification of the main thermocline. In this paper, the results of a modeling study are reported to investigate the mechanism by which superinertial fluctuations are generated in the deep ocean. The general properties of the superinertial wave wake were also characterized as a function of storm speed and central latitude. The Massachusetts Institute of Technology (MIT) Ocean General Circulation Model (OGCM) was used to simulate the open ocean response to realistic westward-tracking hurricane-type surface wind stress and heat and net freshwater buoyancy forcing for regions representative of midlatitudes in the Atlantic, the Caribbean, and low latitudes in the eastern Pacific. The model had high horizontal [?(x, y) = ?°] and vertical (?z = 5 m in top 100 m) resolution and employed a parameterization for vertical mixing induced by shear instability. In the horizontal momentum equation, the relative size of the nonlinear advection terms, which had a dominant frequency near twice the inertial, was large only in the upper 200 m of water. Below 200 m, the linear momentum equations obeyed a linear balance to 2%. Fluctuations at nearly twice the inertial frequency (2f?) were prevalent throughout the depth of the water column, indicating that these nonlinear advection terms in the upper 200 m forced a linear mode below at nearly twice the inertial frequency via vorticity conservation. Maximum variance at 2f in horizontal velocity occurred on the south side of the track. This was in response to vertical advection of northward momentum, which in the north momentum equation is an oscillatory positive definite term that constituted a net force to the south at a frequency near 2f. The ratio of this term to the Coriolis force was larger on the south side of the storm than the north side. The effect was to shift the center of near-inertial circles of particle paths to the south side of the track. Slow storms had more symmetrical wakes for horizontal velocity in the cross-track direction than fast storms, and they generated the strongest vertical velocities. Maximum depth-integrated kinetic energy and vertical velocities were larger for the low-latitude environments (in the Pacific and Caribbean) than at higher latitudes in the Atlantic, because the storm speed for forcing at resonance decreases with latitude. Slower storms exhibited stronger nonlinear superinertial vertical velocities than fast storms and a larger shift of the maximum in vertical velocity at depth to the south side of the storm track. The results suggest that slow storms at low latitudes produce the largest response for kinetic energy and vertical velocity, whereas slow storms at high latitudes produce the largest variance in the vertical velocity at superinertial frequencies. Overall, the findings present a new interpretation of the generation mechanism for fluctuations at 2f and higher harmonics in the velocity field.