Wind-Driven Near-Inertial Waves and Their Impact in the Gulf of Mexico

The continental shelf in the Northern Gulf of Mexico is located along the critical latitude (30° N) where local Coriolis frequency is equal to diurnal frequency, and where existence of near-inertial resonance by land-sea breeze (LSB) circulation has been observed. The enhanced near-inertial waves (NIWs) have been argued to energize vertical mixing in the water column of the Northern Gulf. This study first attempts to investigate the impact of wind-driven near-inertial ocean response using both a simple slab ocean model and a high-resolution primitive-equation ocean model forced by different atmospheric wind products. The results show that the near-inertial wind work during boreal summer is closely linked to the structure of land-sea breeze and varies significantly among different wind products, which further affect vertical mixing in the water column. In addition, since the diurnal winds in all of the reanalyzed wind products are weaker than the in situ buoy-observed winds, and there is a clear linear relationship between near-inertial wind strength and near-inertial wind power input, as well as near-inertial surface current kinetic energy, it suggests that NIW activity is likely to be underestimated in the Northern Gulf when these reanalysis winds are used as atmospheric forcing for an ocean model. The second part of this study proposes a new modeling approach to quantify the impact of near-inertial motions on vertical mixing in the Northern Gulf of Mexico during summer and winter season using a full three-dimensional primitive equation model. In this new approach, we introduce a novel filtering technique that simply changes the rotation direction of the clockwise rotating winds within the near-inertial band at every grid point. Since the clockwise rotating near-inertial winds are primarily responsible for near-inertial wave generation in the ocean, this filtering technique effectively suppresses near-inertial waves while keeping total wind variance approximately intact. Comparing to the previous filtering methods that are based on temporal filtering of near-inertial winds, and thus are not variance preserving, the new technique isolates more effectively the impact of near-inertial waves on ocean mixing