Saba Bank slopes; turbulence; quasi mode-1 and mode-2 internal tides; inertial subrange internal wave band; high-resolution temperature observations; steep slope with mid-water turbulence below coral reef
The deep sloping sides of Saba Bank, the largest submarine atoll in the Atlantic Ocean, show quite different internal wave characteristics. To measure these characteristics, two 350 m long arrays consisting of primary a high-resolution temperature T-sensor string and secondary an acoustic Doppler current profiler were moored around 500 m water depth at the northern and southern flanks of Saba Bank for 23 days. We observed that the surrounding density stratified waters supported large internal tides and episodically large turbulent exchange in up to 50 m tall overturns. However, an inertial subrange was observed at frequencies/wavenumbers smaller than the mean buoyancy scales but not at larger than buoyancy scales, while near-bottom non-linear turbulent bores were absent. The latter reflect more open-ocean than steep sloping topography internal wave turbulence. Both the Banks’ north-side and south-side slopes are locally steeper ‘super-critical’ than internal tide slope angles. However, the three times weaker north-side slope showed quasi-mode-2 semidiurnal internal tides, not high-frequency solitary waves occurring every 12 h, over the range of observations, centered with dominant near-inertial shear around 150 m above the bottom. They generated the largest turbulence when touching the bottom and providing off-bank flowing turbid waters. In contrast, the steeper south-side slope showed quasi-mode-1 internal tides occasionally having excursions > 100 m crest-trough, with weak inertial shear and smallest buoyancy scale turbulence periodicity occurring near the bottom and about half-way the water column, below abundant coral reefs in shallow <20 m deep waters.
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