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

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 similar to 4 x 10(3) cpd approximate to 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-Reynolds-number turbulence using high-resolution temperature data. The high-frequency quasi-turbulent 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 (similar to 1 m) stratified layers and turbulence

Do deep-ocean kinetic energy spectra represent deterministic or stochastic signals?

In analogy with historic analyses of shallow-water tide-gauge records, in which tides and theirhigher harmonics are modified by sea level changes induced by atmospheric disturbances, it is shown thatdeep-sea currents can be interpreted as motions at predominantly inertial-tidal harmonic frequencies modifiedby slowly varying background conditions. In this interpretation, their kinetic energy spectra may not besmoothed into a quasi-stochastic continuum for (random-)statistic confidence. Instead, they are consideredas quasi-deterministic line-spectra. Thus, the climatology of the internal wave field and its slowly varyingbackground can be inferred from line spectra filling the cusps around nonlinear tidal-inertial harmonics, assuggested previously

Energy Release Through Internal Wave Breaking

The sun inputs huge amounts of heat to the ocean, heat that would stay near the ocean's surface if it were not mechanically mixed into the deep. Warm water is less dense than cold water, so that heated surface waters "float" on top of the cold deep waters. Only active mechanical turbulent mixing can pump the heat downward. Such mixing requires remarkably little energy, about one-thousandth of the heat stored, but it is crucial for ocean life and for nutrient and sediment transport. Several mechanisms for ocean mixing have been studied in the past. The dominant mixing mechanism seems to be breaking of internal waves above underwater topography. Here, we quantify the details of how internal waves transition to strong turbulent mixing by using high-sampling-rate temperature sensors. The sensors were moored above the sloping bottom of a large guyot (flat-topped submarine volcano) in the Canary Basin, North Atlantic Ocean. Over a tidal period, most mixing occurs in two periods of less than half an hour each. This "boundary mixing" dominates sediment resuspension and is 100 times more turbulent than open ocean mixing. Extrapolating, the mixing may be sufficiently effective to maintain the ocean's density stratification

Diel Vertical Migration in Deep Sea Plankton Is Finely Tuned to Latitudinal and Seasonal Day Length

Diel vertical migration (DVM) is a ubiquitous phenomenon in marine and freshwater plankton communities. Most commonly, plankton migrate to surface waters at dusk and return to deeper waters at dawn. Up until recently, it was thought that DVM was triggered by a relative change in visible light intensity. However, evidence has shown that DVM also occurs in the deep sea where no direct and background sunlight penetrates. To identify whether such DVM is associated with latitudinal and seasonal day light variation, one and a half years of recorded acoustic data, a measure of zooplankton abundance and movement, were examined. Acoustic Doppler current profilers, moored at eight different sub-tropical latitudes in the North-Atlantic Ocean, measured in the vertical range of 500-1600 m. DVM was observed to follow day length variation with a change in season and latitude at all depths. DVM followed the rhythm of local sunrise and sunset precisely between 500 and 650 m. It continued below 650 m, where the deepest penetrable irradiance level are <10(-7) times their near-surface values, but plankton shortened their time at depth by up to about 63% at 1600 m. This suggests light was no longer a cue for DVM. This trend stayed consistent both across latitudes and between the different seasons. It is hypothesized that another mechanism, rather than light, viz. a precise biochemical clock could maintain the solar diurnal and seasonal rhythms in deep sea plankton motions. In accordance with this hypothesis, the deepest plankton were consistently the first to migrate upwards

Internal tides and energy fluxes over Great Meteor Seamount

International audienceInternal-tide energy fluxes are determined halfway over the southern slope of Great Meteor Seamount (Canary Basin), using data from combined CTD/LADCP yoyoing, covering the whole water column. The strongest signal is semi-diurnal and is concentrated in the upper few hundred meters of the water column. An indeterminacy in energy flux profiles is discussed; it is argued that a commonly applied condition used to uniquely determine these profiles does in fact not apply over sloping bottoms. However, the vertically integrated flux can be established unambiguously. The observed results are compared to the outcome of a numerical internal-tide generation model. For the semi-diurnal internal tide, the vertically integrated flux found in the model corresponds well to the observed one. For the diurnal tide, however, the former is much smaller; this points to non-tidal origins of the diurnal signal, which is indeed to be expected at this latitude (30°), where near-inertial and diurnal periods coincide

Large internal waves advection in very weakly stratified deep Mediterranean waters

The Eastern Mediterranean Sea contains relatively small trenches O(1-10 km) horizontal width that go deeper than 4000 m. At a first glance, these deep waters are homogeneous, with weak currents <0.1 m s(-1). This viewpoint is modified after evaluation of new detailed yearlong temperature observations using 103 high-precision sensors that reveal intense variability of internal waves. Even though temperature variations are within the range of a few mK only, requiring precise correction for the adiabatic lapse rate during post-processing, the images are permanently dynamic. The weak density stratification of buoyancy close to the inertial frequency supports large turbulent overturns indirectly governed or advected by large internal waves. In strongly stratified (near-surface) waters low-frequency inertial internal motions are horizontal, but here they attain a vertical current amplitude sometimes comparable to horizontal currents. This results in occasionally very large internal wave amplitudes (250 m peak-trough), which are generated via geostrophic adjustment presumably from local collapse of fronts. Citation: van Haren, H., and L. Gostiaux (2011), Large internal waves advection in very weakly stratified deep Mediterranean waters, Geophys. Res. Lett., 38, L22603, doi:10.1029/2011GL049707

Fine-structure contamination by internal waves in the Canary Basin

Over a range of 132.5 m, 54 temperature sensors (1 mK relative accuracy) were moored yearlong while sampling at 1 Hz around 1455 m in the open Canary Basin. Coherence between individual records shows a weak but significant peak above the local buoyancy frequency N for all vertical separations Delta z < 100 m, including at sensor interval Delta z = 2.5 m. Instead of a dominant zero-phase difference over the range of sensors, as observed for internal waves at frequencies f < sigma < N, with f denoting the inertial frequency, this superbuoyant coherence shows pi-phase difference. The transition from zero-phase difference, for internal waves, to pi-phase difference is abrupt and increases in frequency for decreasing Delta z < 10 m. For Delta z > 10 m, the transition is fixed at N-t similar or equal to 1.6N, which is also the maximum value of the small-scale buoyancy frequency, and limits the internal wave band on its high-frequency side. In the time domain it is observed that this high-frequency coherence mainly occurs when nonlinearities in the temperature gradient, such as steps in the temperature profile, are advected past the sensors. A simple kinematic model of fine-structure contamination is proposed to reproduce this observation. The canonical -2 slope of the temperature spectrum above N is not observed in the in situ data, which rather slope as -8/3. The -8/3 slope can, however, be reproduced in our model, provided the jumps in the temperature profile are not infinitely thin

A Note on the Role of Mean Flows in Doppler-Shifted Frequencies

The purpose of this paper is to resolve a confusion that may arise from two quite distinct definitions of "Doppler shifts": both are used in the oceanographic literature but they are sometimes conflated. One refers to the difference in frequencies measured by two observers, one at a fixed position and one moving with the mean flow-here referred to as "quasi-Doppler shifts." The other definition is the one used in physics, where the frequency measured by an observer is compared to that of the source. In the latter sense, Doppler shifts occur only if the source and observer move with respect to each other; a steady mean flow alone cannot create a Doppler shift. This paper rehashes the classical theory to straighten out some misconceptions. It is also discussed how wave dispersion affects the classical relations and their application

Vierzig Jahre Sechstagekrieg : Strukturelle Prägekraft für den Nahen Osten

Circulation patterns and sediment dynamics were studied over the Gulf of Valencia (GoV) continental slope during spring and winter 2011–2012. Two moorings were deployed at two locations; at 450 m depth from February to May 2011, and at 572 m depth from October 2011 to February 2012. At both mooring sites, observations were made of currents, temperature and near-bottom turbidity within the lowermost 80 m above the seafloor. The temperature measurements allowed distinction of the different water masses and their temporal evolution. The fluctuations of the boundary between the Western Mediterranean Deep Water (WMDW) and the Levantine Intermediate Water (LIW) masses were monitored, and several intrusions of Western Mediterranean Intermediate Water (WIW) were observed, generally coinciding with changes in current direction. At both mooring sites, the currents generally maintained low velocities <10 cm s-1, with several pulses of magnitude increases >20 cm s-1, and few reaching up to 35 cm s-1, associated with mesoscale eddies and topographic waves. The current direction was mainly towards the SSE on the first deployment and to the ESE on the second deployment. This second location was affected by a strong bottom offshore veering presumably generated by local topographic effects. Increases in suspended sediment concentrations (SSC) were observed repeatedly throughout the records, reaching values >3 mg l-1. However, these SSC variations were uncorrelated with changes in velocity magnitude and direction and/or with temperature oscillations. Results presented in this paper highlight the complex relation between the hydrodynamics and sediment transport over the GoV continental slope, and suggest that other potential sediment resuspension mechanism not linked with current fluctuations, might play a key role in the present-day sedimentary dynamics. Resuspension due to bottom trawling appears to be the most plausible mechanis

Internal Wave Turbulence Near a Texel Beach

A summer bather entering a calm sea from the beach may sense alternating warm and cold water. This can be felt when moving forward into the sea (‘vertically homogeneous’ and ‘horizontally different’), but also when standing still between one’s feet and body (‘vertically different’). On a calm summer-day, an array of high-precision sensors has measured fast temperature-changes up to 1°C near a Texel-island (NL) beach. The measurements show that sensed variations are in fact internal waves, fronts and turbulence, supported in part by vertical stable stratification in density (temperature). Such motions are common in the deep ocean, but generally not in shallow seas where turbulent mixing is expected strong enough to homogenize. The internal beach-waves have amplitudes ten-times larger than those of the small surface wind waves. Quantifying their turbulent mixing gives diffusivity estimates of 10−4–10−3 m2 s−1, which are larger than found in open-ocean but smaller than wave breaking above deep sloping topography