Bottom-pressure observations of deep-sea internal hydrostatic and non-hydrostatic motions

Abstract

In the ocean, sloping bottom topography is important for the generation and dissipation of internal waves. Here, the transition of such waves to turbulence is demonstrated using an accurate bottom-pressure sensor that was moored with an acoustic Doppler current profiler and high-resolution thermistor string on the sloping side of the ocean guyot 'Great Meteor Seamount' (water depth 549 m). The site is dominated by the passage of strong frontal bores, moving upslope once or twice every tidal period, with a trail of high-frequency internal waves. The bore amplitude and precise timing of bore passage vary every tide. A bore induces mainly non-hydrostatic pressure, while the trailing waves induce mainly internal hydrostatic pressure. These separate (internal wave) pressure terms are independently estimated using current and temperature data, respectively. In the bottom-pressure time series, the passage of a bore is barely visible, but the trailing high-frequency internal waves are. A bore is obscured by higher-frequency pressure variations up to ∼ 4 × 10 3 cpd ≈ 80N (cpd, cycles per day; N, the large-scale buoyancy frequency). These motions dominate the turbulent state of internal tides above a sloping bottom. In contrast with previous bottom-pressure observations in other areas, infra-gravity surface waves contribute little to these pressure variations in the same frequency range. Here, such waves do not incur observed pressure. This is verified in a consistency test for large-Reynoldsnumber turbulence using high-resolution temperature data. The high-frequency quasiturbulent internal motions are visible in detailed temperature and acoustic echo images, revealing a nearly permanently wave-turbulent tide going up and down the bottom slope. Over the entire observational period, the spectral slope and variance of bottom pressure are equivalent to internal hydrostatic pressure due to internal waves in the lower 100 m above the bottom, by non-hydrostatic pressure due to high-frequency internal waves and large-scale overturning. The observations suggest a transition between large-scale internal waves, small-scale internal tidal waves residing on thin (∼1 m) stratified layers and turbulence.

Bed failure induced by internal solitary waves

“…The depth considered for the baseline case (H = 50 m), in which ISWs propagate, is not unique. Nonlinear, ISW‐like, internal waves of depression propagate in water depths in excess of 50 m [ Reeder et al ., ; van Haren , ; Droghei et al ., ]. The results shown so far are limited to a small range of depths in which ISWs have been recorded.…”

Sensing of upslope passages of frontal bores across the trench slope break of the Japan Trench

“…The upslope current and the cooling of water imply passage of the leading edge of the bores that are formed when internal waves encounter a sloping boundary and advance up the slope with nonlinearly growing amplitudes [Thorpe, 2005;Lamb, 2014]. Their trailing waves generate decaying turbulence up to several hours after their passage [Walter et al, 2012;van Haren and Greinert, 2013]. Note that the observational site is unique not only geomorphologically but oceanographically, as the trench slope on the deeper side is sloping at 7.18, while the fore-arc basin on the shallower side is sloping at 1.58 (Figure 1b).…”

“…Although there have been many reports of bore detections, they have been mostly from shallow continental slopes [ Rosenfeld et al ., ; Colosi et al ., ; Klymak and Moum , ; Hosegood et al , ; van Haren , ; Nam and Send , ; Martini et al ., ; van Haren , ; van Haren and Greinert , ] or bay slopes [ Walter et al ., ; Masunaga et al ., ]. Reports from deep trench slopes are rare.…”

Simulation and verification of measurement system for deep-sea pressure and its small fluctuation pressure

“…In the process of ocean exploration and research with an average depth of 3800 m, accurate pressure measurements, such as wave tide measurement, 1,2 tsunami warning system, 3–5 levelness adjustment of oil and gas exploitation platform, 6,7 and precise positioning of underwater robots in the deep sea, 8 are required. Therefore, the pressure sensor should not only measure the larger pressure corresponding to thousands of meters level water depth, but also measure the small fluctuation in pressure corresponding to centimeter level water depth.…”