Numerical modeling of internal tides and submesoscale turbulence in the US Caribbean regional ocean

Abstract

The US Caribbean ocean circulation is governed by an influx of Atlantic water through the passages between Puerto Rico, Hispaniola and the Virgin Islands, and an interplay of the Caribbean Sea water with the local topography of the region. We present an analysis of the US Caribbean ocean flow simulated by the USCROMS; which is the ROMS AGRIF model configured for the US Caribbean regional ocean at a horizontal resolution of 2 km. Outputs from the USCROMS show a seasonal variability in the strength of submesoscale turbulence within a mixed layer whose depth varies from −70 to −20 m from winter to summer, and internal tides originating from the passages between the islands. Energy spectra of the simulated baroclinic velocity show diurnal and semi-diurnal maxima and several higher-order harmonic frequency maxima associated with non-linear internal waves forming over steep slopes with super-critical topography in the continental shelf. The strongest conversion rates of the depth-averaged barotropic to baroclinic tidal energy occur at localized regions in the continental shelf with super-critical topography. These regions also exhibit enhanced transport and dissipation of the depth-averaged barotropic and baroclinic tidal kinetic energy. The dissipation in these regions is nearly 3 orders of magnitude stronger than the open ocean dissipation. The energy transport terms show a seasonal pattern characterized by stronger variance during summer and reduced variance during the winter. At the benthic regions, the dissipation levels depend on the topographic depth and the tidal steepness parameter. If the benthic region lies within the upper-ocean mixed-layer, the benthic dissipation is enhanced by surface-forced processes like wind forcing, convective mixing, submesoscale turbulence and bottom friction. If the benthic region lies below the mixed-layer, the benthic dissipation is enhanced by the friction between the super-critical topographic slopes and the periodically oscillating baroclinic tidal currents. Due to bottom friction, the tidal oscillation in the lateral currents adjacent to the sloping topography generates cyclonic and anti-cyclonic vortices with O(1) Rossby number depending on the orientation of the flow. While the cyclonic vortices form positive potential vorticity (q) leading to barotropic shear instability, anti-cyclonic vortices form negative q which leads to periodically occurring inertial instability. The lateral and inertial instabilities caused by the baroclinic tidal oscillations act as routes to submesoscale turbulence at the benthic depths of −100 m to −400 m near the super-critical topography of the continental shelf, forming O(10 km) long streaks of turbulent water with dissipation levels that are 3 orders of magnitude stronger than the dissipation in the open ocean at the same depth. The magnitudes of the dissipation and q at the benthic regions over super-critical continental-shelf topography are also modulated by the spring-neap tidal signals.