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Published January 10, 2019 | Version v1
Journal article Open

Internal Wave Observations Off Saba Bank

  • 1. Royal Netherlands Institute for Sea Research (NIOZ) and Utrecht University, Den Burg, Netherlands

Description

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.

Notes

FUNDING. The NIOZ temperature sensors are partially financed by the Netherlands Organisation for Scientific Research NWO. Part of this research received funding from the SponGES project (European Union's Horizon 2020 research and innovation programme under grant agreement No. 679849) and Atlas project (European Union's Horizon 2020 research and innovation programme under grant agreement No. 678760). FM was supported financially by the Innovational Research Incentives Scheme of NWO (NWO-VIDI 016.161.360). AUTHORS CONTRIBUTIONS. GD and FM conducted the sea expedition and edited, corrected, and contributed the manuscript. HH, GD, and FM analyzed the data. HH wrote the manuscript. ACKNOWLEDGEMENTS. We thank captain and crew of the R/V Pelagia during cruise 64PE432 of NICO (Netherlands Initiative Changing Oceans). We thank M. Laan for his ever-lasting thermistor efforts. We would also like to thank S. Ossebaar for analyzing the inorganic nutrients.

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Funding

SponGES – Deep-sea Sponge Grounds Ecosystems of the North Atlantic: an integrated approach towards their preservation and sustainable exploitation 679849
European Commission
ATLAS – A Trans-AtLantic Assessment and deep-water ecosystem-based Spatial management plan for Europe 678760
European Commission