Ocean Response to Hurricanes in Presence of the Loop Current
Authors: Frolov Sergey A. 
Published: July 2010

Ocean response to hurricanes in presence of the Loop Current (LC) or a Loop Current Eddy (LCE) was investigated using a high resolution numerical 3-dimensional ocean model. The ability of the model to simulate hurricane-generated currents was validated against two independent data sets collected in hurricanes Katrina (2005) and Georges (1998). Model-simulated currents during these two events were compared with continuous current measurements throughout the upper water column at fixed locations in the deep ocean. It was found that nonlinear interaction between the wake of near-inertial oscillations generated by the storm and the eddy front leads to accelerated vertical and downstream propagation of near-inertial oscillations. This process produces increased amounts of near-inertial energy at depth and, consequently, stronger currents. Due to its nature, the effect is proportional to the amount of near-inertial energy generated by the storm. Maximum current at depth due to this type of nonlinear interaction was simulated in a large, fast moving storm resulting in 2.55 m/s maximum response at 200 meters. In a medium size category five storm moving with typical speed this type of nonlinearity produces ~1.8 m/s maximum response at 200 meters. A simple parametric model was designed to estimate the expected amplitude of hurricane-driven currents at depth in presence of the LC or LCE.

Introduction

Hurricanes are known to generate strong wind driven currents that can create significant loads on offshore structures and cause disruptions to offshore operations. High amplitude currents (over 3 kts) persist in the form of near-inertial waves (oscillations) for 1-2 weeks after the storm passage. The rate of decay of hurricane-generated currents is determined, among other factors, by the rate of vertical (downward) propagation of near-inertial waves. Vertical propagation of near-inertial waves carries near-inertial energy downward leading to decay of hurricane-generated currents near the surface and their amplification at depth. Thus, vertical propagation of near-inertial energy is a very important factor controlling the impact that hurricane-generated currents have on offshore operations.

In the Gulf of Mexico the vertical propagation of near-inertial energy is expected to be strongly affected by the Loop Current (LC) and Loop Current Eddies (LCE). A number of investigators came to the conclusion that the background geostrophic currents have a significant effect on vertical propagation of near-inertial waves. For instance, Murray and Zervakis [22] attempted to simulate the observed features of near-inertial wave propagation in the North Atlantic using numerical models. A comparison of the model with the observed currents revealed some significant differences in the pattern of vertical wave propagation. The observed near-inertial response penetrates deeper and appears as a more beam-like structure than in the model, where the maximum currents concentrate at the top of the thermocline. The authors suggest that the interaction of near-inertial waves with the background ocean geostrophic currents, which were not included in the model, may be responsible for the disagreements. The same conclusion was reached by D'Asaro [5] who used a different numerical model to simulate the observed after storm structure of near-inertial oscillations.

In this study we used a high resolution numerical 3-dimensional ocean model to investigate the effect of the LC and a LCE on hurricane-driven near-inertial currents. The investigation was focused around two principal questions:Does presence of the LC/LCE indeed leads to accelerated downward propagation of hurricane-generated near-inertial currents as predicted by previous studies of weaker mesoscale features in the North Atlantic?What are the spatial extent and the amplitude of resulting combined eddy and wind-driven currents at depth? The latter question was addressed in three steps.