arXiv subjects

Hans van Haren

Publications and source records attributed to Hans van Haren.

At least 19 recordsLinked to original sources

arXiv2026

Short spatial mooring-tilt variations from deep Mediterranean observations

Interaction between energy-abundant mesoscale eddies and internal waves can lead to convection-turbulence generation and may prove important for deep-sea life and circulation. However, the size of scales of interacting flows is not well known. In this paper, a diagnostic tool of tilt is tested near the single top-buoyancy of 40 mooring lines 9.5 m apart horizontally and compared with 50-m scale relative vorticity and waterflow above a 2500-m deep flat Northwestern-Mediterranean seafloor. Whilst tilt relates to first order with flow-speed squared induced by mooring-line drag, considerable deviations from this relationship and larger tilt occur when the amplitude of relative vorticity attains values O(f), f the inertial frequency of planetary vorticity. The sign of relative vorticity is of no importance. Such larger-tilt events occur during most intense convection turbulence via warm-water slanting from above. During these events, variations in tilt-angle magnitude are as large as the average tilt O(0.1)degree, thereby reducing variational scales from 50 to 9.5 m. Thus, in a deep-sea environment where flow speeds are <0.07 m s^-1, O(0.01) m s^-1 flow-speed variations provide important turbulent mixing, without deep dense-water formation.

physics.flu-dyn↗

arXiv2026

Small-scale polarization variations in near-inertial deep Mediterranean horizontal waterflow

Deep-sea observations are reported of horizontal waterflow differences with typical amplitudes of 0.02 m s-1 over 50-m small scales so that relative vorticity reaches values of the inertial frequency f. The timeseries observations are made using a complex mooring system deployed in the 2500-m deep Mediterranean Sea, where vertical density stratification is extremely weak, with buoyancy frequency O(f), and the slow waterflow with total speeds <0.07 m s-1 is dominated by inertial internal waves and sub-mesoscale eddies. Horizontal waterflow differences increase when polarization, i.e. direction of traversal of elliptic oscillatory motion, switches sign. Common anticyclonic polarization of inertial motions is predominantly found under near-homogeneous conditions. It alternates with uncommon cyclonic polarization under stratified-water conditions, varyingly over 50-m distances. The alternation is in line with predictions from non-traditional inertio-gravity wave theory, but only when relatively strong turbulent convection causes local reduced stratification, as observed.

physics.ao-ph↗

arXiv2026

Coupling between sub-mesoscale eddies, internal waves, and turbulence in the deep Mediterranean: A spectral investigation

Interaction between energy-abundant (sub-)mesoscale eddies and internal waves can lead to turbulence generation and may prove important for replenishment of nutrients for deep-sea life and circulation. However, observational evidence of such interaction is scarce and precise energy transfer is unknown. In this paper, an extensive spectral study is reported using mooring data from nearly 3000 high-resolution temperature sensors in about half-a-cubic hectometer of seawater above a deep flat Northwestern-Mediterranean seafloor. The number of independent data records partially improves statistics for better determination of spectral slopes, which however do not show a roll-off to the viscous dissipation range of turbulence. The spectra hardly show power-laws omega^p having exponent p = -5/3 representing an inertial subrange that evidences shear-induced isotropic turbulence. Instead, they are dominated by p = -7/5 representing a buoyancy subrange, which evidences convection-induced anisotropic turbulence. In contrast with p=-5/3 that indicates a downgradient cascade of energy, p=-7/5 characterizes by an ambiguous cascade direction. At height h<50 m above seafloor, p=-7/5 is found adjacent to instrumental noise. The p=-7/5 is also found in the sub-mesoscale/internal wave band that is elevated in variance by one order of magnitude. It is reasoned that this sub-inertial range cannot represent isotropic motions, hence p .ne. -5/3 at all heights, and a new deep-sea energy cascade is proposed between mesoscales and turbulence dissipation. Only higher up in more stratified waters an inertial subrange is formed. The transition from internal waves into large-scale turbulence follows p = -2, while a higher-frequency transition from 0 to pi phase change reflects overturns of slanted convection or standing-wave breaking leading to isotropic turbulence.

physics.ao-ph↗

arXiv2026

Whipped and mixed warm clouds in the deep sea

Turbulence is indispensable to redistribute nutrients for all life forms larger than microbial, on land and in the ocean. Yet, the development of deep-sea turbulence was not studied in three dimensions to date. As a disproportionate laboratory, an array of nearly 3000 high-resolution temperature sensors had been installed for three years on the flat 2500-m deep bottom of the Mediterranean Sea. The time series from the half-cubic hectometer mooring-array allows for the creation of unique movies of deep-sea water motions. Although temperature differences are typically 0.001degrC, variable convection-turbulence is observed as expected from geothermal heating through the flat seafloor. During about 40% of the time, an additional turbulence, 3 times stronger in magnitude, is observed from slantwise advected warmer waters to pass in turbulent clouds. Besides turbulent clouds and seafloor heating, movies also reveal weakly turbulent interfacial-wave breakdown that commonly occurs in the open ocean far away from boundaries.

physics.flu-dyn↗

arXiv2026

Deep Mediterranean turbulence motions under stratified-water conditions

Vertically stable in density, stratified-water conditions 'SW' exist in the deep Mediterranean Sea that are characterized by temperature differences of 0.0002-0.01degrC over 125 m above a flat seafloor. These result in a mean buoyancy frequency of N = (1.5-2)f, where f denotes the inertial frequency. Although the stability values are one order of magnitude smaller than found in the ocean, they govern a dynamical deep sea as demonstrated using observations from a 3D mooring-array equipped with nearly 3000 high-resolution temperature sensors. SW-conditions can last up to a fortnight, before waters become near-homogeneous, and occur about 40% of the time, slightly more often in winter than in summer. Under SW, up to 60 m above seafloor is dominated by convection turbulence that is partially driven by geothermal heating 'GH' suppressed by stratification above. The upper-half of the array shows dominant shear turbulence driven by two sources. Interfacial internal waves generate weakly-nonlinear, resonant parametric instabilities that, upon breaking, provide mean turbulence dissipation rates of about one-third of that via general GH. It is about equal to open-ocean values away from boundaries and may represent the dominant source of turbulence there. Like GH, the observed turbulence is local up- and down-going. Tenfold larger mean dissipation rates are observed when slanted convection drives turbulent overturns >10 m and unstable clouds are advected with the mean flow. It confirms theoretical marginal stability analyses, previous vertical waterflow observations, and suggests a relationship between turbulence and sub-mesoscale eddies across the internal wave band. Movies support the findings.

physics.ao-ph↗

arXiv2026

Deep Mediterranean turbulence motions under near-homogeneous conditions

Very weakly density-stratified, near-homogeneous 'NH' conditions are found in the deep Western Mediterranean Sea. Under these conditions, over vertical ranges of several hundreds of meters water temperature varies only a few 0.0001degrC and the buoyancy frequency is smaller than the local inertial frequency. While such waters are characterized as 'quiescent', they are not stagnant and demonstrate regular bursts of turbulent overturns across scales larger than 10 m that are relevant for deep-sea life. As will be shown from a 3D mooring-array with nearly 3000 high-resolution temperature 'T-'sensors, consecutive NH conditions can last up to a fortnight, before stratified waters are advected over the array. At the site, NH conditions occur about 60% of the time. The majority of NH periods is governed by convection turbulence that is driven by geothermal heating from below. The associated turbulence dissipation rate, which is calculated from Ellison scales after precise band-pass filtering, compares with historic geophysical heat-flux measurements. Convection turbulence leads to buoyancy-driven scaling of spectra, not only of temperature in the turbulence range, but also suggesting extensions across the internal-wave band into sub-mesoscales, and (limited observations of) kinetic energy and waterflow differences. Such spectra are found to be uniform over the 124-m vertical T-sensor range above the flat seafloor. Small spectral deviations are observed when very weakly stratified waters are advected sideways or from above, whereby turbulence levels increase by about 30%. Movies show the alternation between calm periods, turbulent clouds passing, and geothermal-heat flares of various sizes.

physics.ao-ph↗

arXiv2026

Heat-flash travel just above a deep Mediterranean seafloor

The deep sea is weakly stratified in density but shows considerable variations in turbulent motions in all three directions. When registered by moored high-resolution temperature 'T'-sensors, the motions cause variations of 0.01degrC or less and in time of minutes or less, which is much faster than hours or longer of internal waves. Occasionally, T-sensors close to the seafloor register minute-long flashes of 0.0005-0.001degrC warmer than the environment. When singular, such flashes may be artefacts. However, in a large mooring-array with 45 vertical lines at 9.5-m horizontal distances, near-seafloor heat flashes are seen to travel, most likely with internal-wave instabilities in overlying stratified waters. The instabilities seem to release the flashes from a geothermally heated seafloor of which turbulence convection is suppressed by warmer waters from above. The forms and turbulence intensity of these rare signals are compared with those induced by a Remotely Operated Vehicle working near the array. Other causes like unidentified marine mammal passing are hypothesized.

physics.ao-ph↗

arXiv2026

Tidal motions in the deep Mediterranean

The Mediterranean Sea is known for its limited tidal motions. For example, surface barotropic tidal elevations have an amplitude of 0.1 m in the Northwestern Mediterranean. Nevertheless, these small tides are noticeable in temperature records at the 2500-m deep seafloor, but only under near-homogeneous conditions when buoyancy frequency N < f, the inertial frequency. After transfer of pressure to temperature units via the local adiabatic lapse rate, the observed internal-wave temperature signals may thus be corrected for 1.5x10-5-degrC amplitude semidiurnal barotropic tides. The remaining baroclinic tides are embedded in the broad and featureless inertio-gravity wave band, with some energy enhancement near its boundaries, also under tenfold-larger energetic stratified water conditions.

physics.ao-ph↗

arXiv2026

Turbulence demonstrates height variations in closely spaced deep-sea mooring lines

It may be important to precisely know heights of moored oceanographic instrumentation. For example, moorings can be closely spaced or accidentally be located on small rocks or in small gullies. Height variations O(1 m) will yield registration of different values when conditions such as small-scale density stratification vary strongly. Such little height variations may prove difficult to measure in the deep sea, requiring high-accuracy pressure sensors preferably on all instruments in a mooring-array. In this paper, an alternative method for relative height determination is presented using high-resolution temperature sensors moored on multiple densely-spaced lines in the deep Western Mediterranean. While it was anticipated that height variations between lines could be detected under near-homogeneous conditions via adiabatic lapse rate O(0.0001degrC m-1) by the 0.00003degrC-noise-level sensors, such was prevented by the impossibility of properly correcting for short-term bias due to electronic drift. Instead, a satisfactory height determination was found during a period of relatively strong stratification and large turbulence activity. By band-pass filtering data of the highest-resolved turbulent motions across the strongest temperature gradient, significant height variations were detectable to within +/-0.2 m.

physics.ao-ph↗

arXiv2026

Stratified-turbulence observations in the deep Mediterranean

A nearly half-cubic hectometer of deep Mediterranean-Sea waters is yearlong sampled with about 3000 high-resolution temperature sensors to study different sources of turbulent waterflows, which are vital for life. Although temperature differences are never larger than 0.01degrC, daily, weekly, and seasonal variations are observed. About half the time, relatively warm stratified waters are moved from 100's of meters higher levels to near the seafloor. These internal-wave and sub-mesoscale eddy-induced motions are half an order of magnitude more turbulent than those induced via general geothermal heating from below, and about one order of magnitude more turbulent than those from open-ocean processes. A rough estimate shows that eddy-induced stratified turbulence is likely more important for deep-sea life than rare, not observed, deep dense-water formation at the abyssal-plain mooring site. With a delay of about a week, the stratified turbulence tracks atmospheric disturbances, which are found 35% more energetic in winter than in summer. From comparison of turbulence-calculation methods, of band-pass filtering with vertical-displacement reordering, for data over one-four days, a generalization is proposed for the filter cut-offs under weakly stratified and near-homogeneous conditions in the deep Mediterranean.

physics.ao-ph↗

arXiv2025

Corrected values of turbulence generated by general geothermal convection in deep Mediterranean waters

A correction by a reduction factor O(100) is proposed for previously calculated turbulence values on unresolved convection-overturns induced by general geothermal heating in the deep Western Mediterranean. The correction includes modified application of reordering method for calculating turbulence values in convection turbulence with and without stratification above or below. The result is improved correspondence between geophysical determined heat flow through the seafloor and turbulence kinetic energy dissipation rate determined from high-resolution temperature sensors moored over 109 m in the overlying waters, with an average mixing coefficient of 0.5.

physics.ao-ph↗

arXiv2024

Intrusions and turbulent mixing above a small Eastern Mediterranean seafloor-slope

Growing evidence is found in observations and numerical modelling of the importance of steep seafloor topography for turbulent diapycnal mixing leading to redistribution of suspended matter and nutrients, especially in waters with abundant internal tides. One of the remaining questions is the extent of turbulent mixing away from and above nearly flat topography, which is addressed in this paper. Evaluated are observations from an opportunistic, week-long mooring of high-resolution temperature sensors above a small seafloor slope in about 1200 m water depth of the Eastern Mediterranean. The environment has weak tides, so that near-inertial motions and -shear dominate internal waves. Vertical displacement shapes suggest instabilities to represent locally generated turbulent overturns, rather than partial salinity-compensated intrusions dispersed isopycnally from turbulence near the slope. This conclusion is supported by the duration of instabilities, as all individual overturns last shorter than the mean buoyancy period and sequences of overturns last shorter than the local inertial period. The displacement shapes are more erratic than observed in stronger stratified waters in which shear drives turbulence, and better correspond with predominantly buoyancy-driven convection-turbulence. This convection-turbulence is confirmed from spectral information, generally occurring dominant close to the seafloor and only in weakly stratified layers well above it. Mean turbulence values are 10-100 times smaller than found above steep ocean topography, but 10 times larger than found in the open-ocean interior.

physics.ao-ph↗

arXiv2024

A global summary of seafloor topography influenced by turbulent water mixing

Turbulent water motions are important for the exchange of momentum, heat, nutrients, and suspended matter including sediments in the deep-sea that is generally stably stratified in density. To maintain ocean-density stratification, an irreversible diapycnal turbulent transport is needed. The geological shape and texture of marine topography is important for water mixing as most of deep-sea turbulence is generated via breaking internal waves at sloping seafloors. For example, slopes of semidiurnal internal tidal characteristics can critically match the mean seafloor slope. In this paper, the concept of critical slopes are revisited from a global internal wave-turbulence viewpoint seafloor topography -- and using moored high-resolution temperature sensor data. Observations suggest that turbulence generation via internal wave breaking at 5+/-1.5% of all seafloors is sufficient to maintain ocean-density stratification. However most, >90%, turbulence contribution is found at supercritical, rather than the more limited critical, slopes measured at 1-minute-scales that cover about 50% of seafloors at water depths < 2000 m. Internal tides (about 60%) dominate over near-inertial waves (about 40%), which is confirmed from comparison of NE-Atlantic data with East-Mediterranean data (no tides). Seafloor-elevation spectra show a wavenumber (k) fall-off rate of k^-3, which is steeper than previously found. The fall-off rate is even steeper, resulting in less elevation-variance, in a one-order-of-magnitude bandwidth around k_T=0.5 cycle-per-km. The corresponding length is equivalent to the internal tidal excursion. The reduction in seafloor-elevation variance seems associated with erosion by internal wave breaking. Potential robustness of the seafloor-internal wave interaction is discussed.

physics.ao-ph↗

arXiv2024

Internal-wave convection and shear near the top of a deep equatorial seamount

The near-equatorial ocean experiences particular dynamics because the Coriolis force is weak. One modelled effect of these dynamics is strong reduction of turbulent mixing in the ocean interior. Unknowns are effects on internal wave breaking and associated turbulent mixing above steeply sloping topography. In this paper, high-resolution temperature observations are analyzed from sensors that were moored near the top of a deep Ceara Basin seamount for one week. A vertical string held sensors between 0.4 and 56.4 meters above the seafloor. The observations show common semidiurnal-periodic internal wave breaking, with tidal- and 56-m mean turbulence values that are not significantly different from those observed near the top of 1000-m shallower mid-latitude Great Meteor Seamount, despite the twice lower vertical density stratification. Profiles of 6-day mean turbulence values yield vertically uniform values except for a small decrease in the lower 2 m above the seafloor. The lower 2-m show a distinct departure from turbulent inertial subrange in temperature variance spectra. In 10-m higher-up, spectral slopes indicate dominant turbulent convection with reduced flow and turbulence, except when a primary tidal bore is present. Further-up than 15 m, shear dominates (stratified) turbulence. The lack of Coriolis force is not found to be important for internal wave-induced turbulence above steeply sloping topography, except that Kelvin-Helmholtz instabilities seem somewhat less chaotic and more organized roll-up than at mid-latitudes.

physics.ao-ph↗

arXiv2023

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.

physics.ao-ph↗

arXiv2023

Detailing secondary frontal bore of internal tides breaking above deep-ocean topography

Above steep deep-sea topography internal tidal waves may break vigorously. The associated turbulent mixing is important for resuspending matter, bringing it tens of meters away from the seafloor for redistribution. While intense turbulence-generation occurs around a primary (frontal) bore during each transition from warming downslope to cooling upslope phase of the internal (tidal) carrier wave, a secondary bore can appear about half a wave-period later before the turn to the warming phase. As will be demonstrated from a 100-day mooring array consisting of 200 high-resolution temperature sensors between h = 6-404 m above a steep slope of a large North-Atlantic seamount and a low-resolution acoustic Doppler current profiler sampling between 50 and 450 m, secondary bores show about the same turbulence intensity as around primary bores but they generally show larger overturns that always reach the seafloor. The secondary bores associate with a sudden drop in along-isobath flow speed, a (renewed) increase in upslope flow of up to |0.2| m s-1, and with first-harmonic quarter-diurnal periodicity which provides a spectral peak for turbulence dissipation rate. While each bore is different in appearance, varying from curved like a primary bore to almost straight upward with a ragged bore, secondary bores are in a first approximation forward breaking in contrast with backward breaking primary bores.

physics.ao-ph↗

arXiv2023

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.

physics.ao-ph↗

arXiv2023

Near-inertial wave propagation between stratified and homogeneous layers

The propagation of inertio-gravity waves (IGW) into the deep-sea is relevant for energy transfer to turbulence where waves break, and thus for redistribution of nutrients, oxygen and suspended matter. In constant stratification, vertical IGW-propagation is readily modelled. In varying stratification, where homogeneous layers alternate with stratified layers, transmission and reflection cause complex patterns. Half-year long moored acoustic Doppler current profiler (ADCP) observations midway between the Balearic Islands and Sardinia in the 2800-m deep Western-Mediterranean Sea occasionally demonstrate a distinct transition, between weakly stratified (N>=2f) and homogeneous (N=<f) layers, of IGW at near-inertial frequencies. Here, N denotes the buoyancy frequency and f the local inertial frequency (vertical Coriolis parameter). The transition in stratification is rather abrupt, within Deltaz=25 m and provides an amplitude-reduction of 1.3 for super-inertial motions. Simulations with non-traditional momentum equations involving the horizontal Coriolis parameter fh qualitatively confirm observed IGW-refraction. The observational area is marked by variations in hydrographic characteristics, with abundant mesoscale eddies to the south and dense-water formation to the north of the site during the previous winter. Thus, also transitions occur from deep homogeneous layers into deeper, recently formed stratified ones. Polarization spectra of shear are bound by IGW-limits related to N=f, while current-polarization to 2fh. These frequencies coincide with large-scale buoyancy frequencies independently observed in various layers using shipborne CTD-profiling.

physics.ao-ph↗