CoAuthors

Maarten Buijsman (The University at Southern Mississippi, United States); Brian Arbic (University of Michigan, United States); Christopher Jackson (NOAA, United States); Jorge Magalhaes (University of Porto, Portugal); Jose daSilva (University of Porto, Portugal)

This study aims to quantify the energetics of internal tides in 4-km global Hybrid Coordinate Ocean Model (HYCOM) simulations, including the energy cascade from the primary tidal to higher frequencies and their evolution along internal tide “beams”. The ocean model is run in a forward (non-data-assimilative) mode, forced by realistic atmospheric fields and tidal forcing (M2, S2, K1, O1, N2). Velocity and density fields are band-passed into tidal (D1+D2) and supertidal frequency bands before computing the baroclinic energy budget. Globally, supertidal motions are confined to low latitudes (±30^o), where on average they account for roughly 10% of the total tidal energy. The geographic distribution of supertidal kinetic energy shows a high correlation with the advective flux, a measure of nonlinear wave energy, and good qualitative agreement with observed instances of solitary nonlinear internal waves (Jackson et al. 2012), which are characterized by higher frequencies and wavenumbers. To understand how internal tides evolve along their propagation path and interact with ocean currents, the energetics along the main internal tide beams at the Amazon Shelf and Mascarene Ridge are analyzed in detail. In the Amazon, energy is converted to baroclinic tides at the shelf break at the primary tidal frequencies. The associated pressure fluxes decay over a 1000km distance due to dissipation and energy transfers to higher harmonic pressure and advective fluxes. The supertidal fluxes peak 400km away from the generation site and then remain steady. A high-resolution simulation using the nonhydrostatic Massachusetts Institute of Technology general circulation model (MITgcm) at the Mascarane ridge shows that the horizontal grid resolution in HYCOM is insufficient to resolve subgrid scale solitons impeding an accurate energy cascade to smaller wavelengths, essentially “trapping” energy at frequencies lower than those observed in the MITgcm energy spectrum, which show a series of bulges at frequencies higher than 24cpd. However, in agreement with HYCOM, the energy in the supertidal band increases in proportion to the total tidal energy and is found to be about 50% in MITgcm and about 20% in HYCOM, suggesting that energy at higher frequencies (due to solitons) is underestimated in HYCOM.