Open-ocean interior moored sensor turbulence estimates, below a Meddy

A one-year time series of moored high-resolution temperature T-sensor data from 1455 m depth on a 3900 m long line in about 5300 m of water in the NE-Atlantic Canary Basin are dominated by salinity (over-)compensated intrusions arising from the effects of Mediterranean outflow waters, which are commonly organized as Meddies. During the passage of a Meddy-core above the T-sensors, no intrusions were observed, thereby making it possible to use the temperature records to quantify turbulence parameters. The present data show that these ocean-interior turbulence estimates are from short-lived (less than 0.5 h) rather intense overturning cells with vertical scales of <5 m. Because the turbulence inertial subrange is found to extend into the internal wave band, the overturns are predominantly driven by shear associated with inertial currents. Kinetic energy, current shear and temperature variance peak at sub-inertial frequencies during the Meddy passage, suggesting wave trapping in the warm anti-cyclonic eddy and/or weakly stratified layers. The observations further show that internal wave displacements are coherent over vertical scales of up to 40 m during the presence of the Meddy compared with vertical coherence scales of less than 25 m during the more common no-Meddy conditions of double diffusion intrusions.Comment: 32 pages, 9 figure

Temperature statistics above a deep-ocean sloping boundary

We present a detailed analysis of the temperature statistics in an oceanographic observational dataset. The data are collected using a moored array of thermistors, 100 m tall and starting 5 m above the bottom, deployed during four months above the slopes of a Seamount in the north-eastern Atlantic Ocean. Turbulence at this location is strongly affected by the semidiurnal tidal wave. Mean stratification is stable in the entire dataset. We compute structure functions, of order up to 10, of the distributions of temperature increments. Strong intermittency is observed, in particular, during the downslope phase of the tide, and farther from the solid bottom. In the lower half of the mooring during the upslope phase, the temperature statistics are consistent with those of a passive scalar. In the upper half of the mooring, the temperature statistics deviate from those of a passive scalar, and evidence of turbulent convective activity is found. The downslope phase is generally thought to be more shear-dominated, but our results suggest on the other hand that convective activity is present. High-order moments also show that the turbulence scaling behaviour breaks at a well-defined scale (of the order of the buoyancy length scale), which is however dependent on the flow state (tidal phase, height above the bottom). At larger scales, wave motions are dominant. We suggest that our results could provide an important reference for laboratory and numerical studies of mixing in geophysical flows.Comment: 22 pages, 10 figures, 3 tables. Accepted versio

AABW-transport variation and its effect on internal wave motions between top and bottom of the Puerto Rico Trench

Slow subinertial variations in Antarctic Bottom Water (AABW) are investigated interacting with internal waves and associated turbulent mixing in the Puerto Rico Trench (PRT), northwest Atlantic. Just below the PRT\u27s top at 5,500 m, a deep-sea mooring was deployed for 14 months. Around 6,100 m, the line held a 200 m long string of 101 high-resolution temperature sensors and a current meter. Around 8,250 m, a similar string held 102 sensors, of which the lowest was 9 m above the bottom. As was measured with shipborne conductivity-temperature-depth profiling down to 7,150 m, PRT waters are very weakly stratified, with local mean buoyancy frequency equaling 1–1.7 times the semidiurnal tidal frequency. The observations show alternating quiescent and relatively turbulent periods that correspond with relatively cooler and warmer AABW, respectively. Over the 2,100 m distance between the two temperature strings, only semidiurnal tidal variations significantly correlate with an average phase difference of 90° (3 h) for the entire recording period. This suggests a dominant baroclinic rather than a barotropic coupling. During quiescent periods, the vertical internal wave scale is rather small with out-of-phase tidal motions between the two data sets and vertical excursions of 40 m. During turbulent periods, internal wave motions at all frequencies are close to in phase between the two data sets, suggesting fast vertical propagation with excursions exceeding 200 m. Increased subinertial energy levels, probably reflecting trapped internal waves, are found near the bottom during such periods. It is suggested that internal wave turbulence dominates the deep-sea transport of heat and suspended materials over the entire PRT height and possibly beyond

Autumnal deep scattering layer from moored acoustic sensing in the subtropical Canary Basin

An enhanced acoustic scatterer reflectance layer was observed in the bathypelagic zone around 1650 m in the subtropical NE-Atlantic Ocean for about two months during autumn. It resembles a classic pattern of diapause-resting, possibly of large zooplankton, shrimp and/or Cyclothone, at great depths well below any sunlight penetration, which is more commonly found at higher latitudes. The observed slow sink and rise of about 2-5 m per day into and out of this deep layer is considerably slower than the more than 1000 m per day of diel vertical migration (DVM). During the two-month period of deep scattering, DVM is observed to be greatly reduced.Comment: 15 pages, 3 figure

Turbulence observations using moored temperature sensors in weakly stratified deep West-Mediterranean waters

In the stably stratified ocean, small-scale turbulence is important for vertical exchange and hence for the mixing of water masses and suspended matter. To observationally study turbulent motions and the buoyancy- and shear-generators behind them, a 100-m tall array of high-resolution temperature (T) sensors was moored at a 2480-m deep seafloor near the steep continental slope of the Western Mediterranean Sea. The area is dominated by boundary flow, (sub-)mesoscale eddies, internal waves, wintertime dense water formation, and very weak vertical density stratification. Various physical oceanographic processes are observed in detail through a seasonal cycle, including: Low shear-driven turbulence in weak stratification in late-summer and autumn, large buoyancy-driven turbulence by geothermal heating from below alternated with inertial internal wave motions in winter, and moderate buoyancy-like turbulence pushed with stratified waters from above in late-winter and spring.Comment: 27 pages, 7 figure

Direct observations of general geothermal convection in deep Mediterranean waters

Like elsewhere in the deep-sea, life in the deep Mediterranean depends on turbulent exchange across the stable vertical density stratification for supply of nutrients and oxygen. Commonly modelled, turbulent exchange is inversely proportional to the stratification rate. However, this proportionality depends on the particular turbulence type, whether it is driven by vertical current differences (shear) or by buoyancy (convection). While shear-turbulence is well observed in stratified seas, direct observations of convection-turbulence are limited. In this paper, high-resolution moored temperature observations show that Mediterranean Sea waters are not stagnant in the lower 109 m above the seafloor at 2480 m, although variations are in the range of only 0.0001-0.001 degrC. In winter, convection-turbulence is regularly observed. Fortnightly averaged spectra show a collapse to the inertial-subrange scaling of dominant shear-turbulence for data from about 100 m above the seafloor, and to the buoyancy-subrange scaling of dominant convection-turbulence at about 10 m above the seafloor. Time-depth images reveal details of convection-turbulence driven from below, which is considered primarily due to general geothermal heating through the Earth crust not related to volcanic vents. When its observation is not masked by (sub-)mesoscale eddies that advect warmer waters from above, the geothermal heat flux matches the deep-sea turbulence dissipation rate, if in the calculations a mixing efficiency of 0.5 is taken typical for natural convection, integration is over 250 m above the seafloor as confirmed from shipborne CTD, and if maximum 2-m-scale buoyancy frequency replaces its 100-m-scale mean equivalent.Comment: 47 pages, 10 figure

Why the Atlantic meridional overturning circulation may not collapse

The extent of anthropogenic influence on the Earths climate warrants studies of the ocean as a major player. The ocean circulation is important for transporting properties like heat, carbon and nutrients. A supposed major conduit is the Atlantic Meridional Overturning Circulation (AMOC). Schematically, it transports heat from the equator to the poles near the surface and carbon in the abyssal return. As the AMOC is a complex nonlinear dynamical system, it is challenging to predict its potential to collapse from a statistical viewpoint using a particular estimator of sea-surface temperature. Whilst this might be robust mathematically, it lacks physical insight of the drivers of the AMOC. As is argued below, physical processes will alter the estimators, and thereby statistical analyses.Comment: 5 pages, 0 figure