Near-inertial wave generation and dissipation in realistically forced HYCOM simulations

Keshav Raja, Universiy of Southern Mississippi, School of Ocean Science and Engineering, John C. Stennis Space Center, MS, United States, Maarten C Buijsman, University of Southern Mississippi, Stennis Space Center, MS, United States, Samuel Maurice Kelly, University of Minnesota Duluth, Duluth, MN, United States, Jay F Shriver, Naval Research Laboratory, Stennis Space Center, Stennis Space Center, MS, United States, SAND-Brian K Arbic, University of Michigan, Earth and Environmental Sciences, Ann Arbor, MI, United States and Jim G. Richman, Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, FL, United States
Abstract:
The generation, propagation and dissipation of wind generated near-inertial wave (NIW) energy are studied in the presence of mesoscale flows in Hybrid Coordinate Ocean Model (HYCOM) simulations with realistic tidal and atmospheric forcing. NIWs are a substantial source of high-frequency variability in the upper ocean. The background vorticity significantly modulates NIW variability in the thermocline. Although HYCOM has a high frequency wind forcing and associated NIW motions, few studies have been carried out on NIW energetics and their interactions with the background flow in HYCOM. The generation and dissipation of NIWs in HYCOM are yet to be validated against observations.

We have performed 1/25o HYCOM simulations with and without data assimilation. The model has 41 layers with uniform vertical coordinates in the mixed layer and isopycnal coordinates in the ocean interior. The analysis is carried out for Near-Inertial Shear and Kinetic Energy in the North Atlantic experiment (NISKINe) study area, which ranges from 52oN to 61oN and 16oW to 28oW. The energy budget of mixed layer is calculated assuming a uniform slab model. We quantify the wind input and the NIW power radiating below the mixed layer, and estimate the power lost to dissipation in the mixed layer. The isopycnal coordinates in the stratified interior is interpolated into uniform z-coordinates to study NIW propagation. Mode-1 and 2 NIWs are found to propagate away from the region of generation while higher modes are trapped in regions of anticyclones following a wind event. We also find that the map of the most energetic near-inertial frequency identified from rotary spectra at each grid cell corresponds with the map of feff = f + ΞΆ/2 in terms of large scale features but not for small scales. The results from our analyses are compared with field observations from the NISKINe cruises.