Internal tidal sloshing and a non-linear wave source away from topography

Abstract

The ocean-interior shows omnipresent internal wave motions whose importance becomes more and more acknowledged for deep mixing across the stable vertical density stratification for heat and suspended matter redistribution. To observationally study vertical propagation variations of internal tides away from a steep topographic slope, a >1-km long mooring with 760 high-resolution temperature sensors was deployed at 2380 m depth halfway Mount Josephine NE-Atlantic Ocean for 4 months. In the lower 150 m above the seafloor, internal waves slosh, become highly non-linear and break vigorously, every tidal cycle. After signal separation using semidiurnal band-pass and harmonic filters, the vertical phase propagation shows: 1. Phase differences of 90° are observed in isotherms and cross-slope currents over typically 600 m in the vertical, 2. Non-linearity is already found in harmonic records that describe 300-m large-scale internal tides, 3. Non-uniform thin-layer stratification causes about 50% of the semidiurnal variance in small-scales, 4. Upward internal tide propagation starts at the top of non-linear wave breaking, 5. Downward propagation from above leading to energy-focusing at 700 m above the ocean-floor and about 700 m below the nearest seamount-ridge generates interior hydraulic jumps during spring tide.