Ann. E. Gargett

This study was motivated by the desire of plankton biologists to understand the turbulent environment experienced by a planktonic community at the time biological variables are measured by a surfacing Bio-Argo float. We use density profiles from a Bio-Argo float deployed over a two year period in the Southern Ocean and ERA5 reanalyses of surface wind speed, waves and surface heat flux in the region of float surfacing to estimate the rate and type of vertical mixing at the times of float profiles. The methodology was developed from the analysis of turbulent vertical velocities measured acoustically from the vertical beam of bottom-mounted VADCPs in coastal waters (Gargett, 2022, 2023) in terms of a “forcing space” defined by surface buoyancy flux B0 and a growth rate for wind/wave (Langmuir) turbulence. When B0 >0 (destabilizing surface heat loss), a mixing layer average of rms vertical velocity wrms can be predicted using scale velocities characteristic of convective and wind/wave turbulence, calculated with the ERA forcing fields and mixing layer depth from the float density profile. VADCP observations of the vertical profiles of vertical turbulent velocity indicate that convective-dominated profiles have little vertical structure, while wind/wave-dominated profiles exhibit a near-surface maximum followed by a decrease to the base of the mixing layer. Estimates of mixing timescales for times short and long relative to a Lagrangian auto-correlation timescale are presented.

Turbulence in the ocean surface layer is forced by a mixture of buoyancy, wind, and wave processes that evolves over time scales from the diurnal scale of buoyancy forcing, through storm time scales, to the annual cycle. This study seeks a predictor for root-mean-square w (rmsw), a time and surface layer average of turbulent vertical velocity w measured by bottom-mounted vertical-beam acoustic Doppler current profilers, in terms of concurrently measured surface forcing fields. Data used are from two coastal sites, one shallow (LEO, 15-m depth) and one deeper (R2, 26-m depth). The analysis demonstrates that it is possible to predict observed rmsw with a simple linear combination of two scale velocities, one the convective scale velocity familiar from the atmospheric literature, the other a scale velocity w_S representing combined wind and wave effects. Three variants are considered for this latter scale velocity, the wind stress velocity alone and two forms using both and U_S, a Stokes velocity characteristic of the surface wave field. At both sites, the two-parameter fit using alone is least accurate, while fits using the other two variants are essentially indistinguishable. At both sites, the coefficient multiplying is the same, within error bounds, and within the range of previous observations. At the deeper site, the coefficient multiplying the wind/wave scale velocity w_S is approximately half that at the shallow site, a difference here attributed to difference in wave character.

Various forms of regime diagrams have become an accepted means of identifying the dominant type of forcing of turbulence in the ocean surface layer. However, all of the proposed forms share a number of issues, demonstrated here, that make them an imperfect tool for this purpose. Instead, I suggest a forcing space consisting of surface buoyancy flux (usually dominated by surface heat flux) and a growth rate defined as the inverse of a theoretical time scale for growth of Langmuir circulations in an unstratified water column. Using coastal data, it is demonstrated that, provided forcing conditions are roughly constant for several hours, location in the upper half-plane of this forcing space predicts organizational characteristics of observed turbulence that range in a systematic way between those of “pure” convection and those of full depth Langmuir circulations. In this upper half-plane, where a convective scale velocity exists and the surface Stokes drift velocity can be computed, allowing calculation of a Stokes scale velocity, a linear combination of the two scale velocities provides a consistent estimate of observed rms turbulent vertical velocity. Time dependence is nevertheless a frequent characteristic of ocean surface layer forcing, if only because of the (usually large) diurnal variation in surface heat flux. It is shown that the time scale of response of surface layer turbulence to time variable forcing depends on whether the major change is due to wind/wave or buoyancy forcing. Relevant modeling studies are suggested.

Measurements of collocated fields of atmospheric forcing, surface waves, and mean and turbulent velocities associated with passage of Tropical Storm (TS) Barry over the U.S. Navy Tower R2 on the Georgia continental shelf are presented. A vertical-beam ADCP enables computation of directional surface wave spectra and hence of directional Stokes functions of depth and time, as well as mean (including tidal) and turbulent velocities throughout the water column. Full-depth turbulent velocity and backscatter structures observed during TS Barry are determined to be Langmuir supercells (LS). The LS appear in the present observations and in similar observations from a shallower site only when a surface growth rate g_* exceeds a critical value, providing a means of predicting how deep an unstratified water column must be before LS will not be expected. When observed, LS structures at Tower R2 are less organized than archetypical LS structures: we suggest that this result is due primarily to smaller near-bottom growth rate in the deeper water column. Despite g_* values above the critical value, and appropriate values of Langmuir and Rayleigh numbers, full-depth velocity/backscatter structures disappear completely for a time between the two wind maxima associated with the TS, as wind veers rapidly clockwise with eye passage to the west of Tower R2. From the observations, the most likely explanation for this hiatus is decreased wave breaking during the period of wind veering, reducing surface supply of “effective” vertical vorticity that dominates generation of Langmuir circulation (LC). This result has significant implications for LES modeling of LC.

This study examines a simple 6-box model of a single pole-to-pole ocean basin. Each of a northern "polar gyre," a southern "polar gyre," and an "equatorial gyre," consisting of north and south subtropical gyres plus the equatorial region, is represented by two boxes: a surface box receiving constant fluxes of both temperature (heat) and salt (freshwater) and a deep box. The model includes four dominant processes: surface flux forcing, horizontal meridional advection driven by Southern Ocean winds, horizontal eddy diffusion at gyre boundaries, and convection, as well as the process of vertical diffusion by small-scale processes. Provided that heat loss from the northern polar gyre is sufficiently larger than that from the southern polar gyre, a steady-state Atlantic Meridional Overturning Circulation (AMOC)-like system, i. e., one with sinking in the north polar gyre and upwelling in a weakly stratified southern polar gyre, is obtained at present values of RF ≡ βFS / αFT, the ratio of surface forcing by fluxes of temperature (T ) and salinity (S ) in the equatorial gyre. Despite the fact that vertical diffusive fluxes are much smaller than those associated with all the other processes, it is shown that implementation in this model of a simple water mass–based representation of different vertical diffusivities for T and S, the two water properties that, with pressure, determine the density of seawater, can lead to profound change in the steady-state modes of the system. With equal diffusivities, the AMOC-like mode with north polar convection shifts abruptly to a mode with equatorial convection at sufficiently large values of RF. With unequal diffusivities, this mode boundary is replaced by an intermediate region of RF values in which all three gyres are stratified. The existence and extent of this stratified regime is shown to result predominantly from the differences between vertical turbulent diffusivities of T and S in the "salt fingering" equatorial gyre. Existence of a stratified regime at values of RF somewhat larger that present implies a tendency towards stable stratification throughout the oceans if, under climate change, the equatorial diffusivity difference were to increase as a result of water mass changes in the subtropical gyres and/or an increase in RF as a result of increased atmospheric freshwater fluxes and/or decreased heat fluxes. This tendency towards an everywhere-stratified ocean is independent of that expected from increased freshwater addition to surface polar oceans due to ice melt.

Greene et al. revisit the suggestion that the turbulent kinetic energy dissipation rate could be estimated through a “large-eddy estimate,” employing acoustic measurements of velocity fields associated with the largest energy-containing scales of ocean turbulence. While the large-eddy estimate as originally proposed used vertical velocity and a vertical eddy length scale, Greene et al. chose instead to substitute a horizontal length scale for the latter. This comment argues that combining a horizontal scale for length with a vertical velocity scale produces a large-eddy estimate of the dissipation rate that is accurate only if the energy-containing eddies are isotropic, and that this condition is highly unlikely in naturally occurring ocean turbulence, subject as it is to influences of stratification, vertical shear, and/or the presence of horizontal boundaries. The problem is documented using data from a large-eddy simulation of Langmuir supercells.

Langmuir circulation cells extending through entire water columns exert a profound influence on sediment transport on shallow shelves, hence their designation as "Langmuir Supercells" (LSs) upon discovery in 2004 in water of 15 m depth at the Long-Term Ecosystem Observatory (LEO) off New Jersey (United States). Until now, similar high-frequency, full water column measurements of turbulent velocities and density fields have not been reported from significantly deeper continental shelf environments. Such deeper measurements are needed to determine whether LSs exist to influence sediment transport outside the inner shelf. In this article, that deficiency is addressed, using measurements from a midshelf location in the South Atlantic Bight. These data indicate that LSs form during high wind and wave forcing in water of 26 m depth and are thus capable of affecting sediment transport over more than about half of the area of this wide, shallow shelf. Relative to those at LEO, the LSs reported here are less organized and more temporally variable despite similar magnitude forcing. Possible causes of cell weakness and variability are considered.

Existing large-eddy simulations (LES) of Langmuir supercells (LS) do not include rotational terms. Despite the fact that the actual coastal ocean is certainly affected by rotation, such simulations are found to provide excellent agreement with a wide range of features of LS observed in the shallow coastal ocean. This note explains why it is indeed acceptable to compare results of a nonrotational LES of LS in a laterally unbounded domain with observations of LS made in a rotating fluid with a lateral boundary.

With standard low-frequency velocity data from current meter moorings, pressure gradient–driven mean flow is determined by low-pass filtering, while tides are estimated by fitting tidal constituents, with accuracy and numbers of constituents determined by record length. With the advent of higher-frequency measurements from cabled coastal ocean observatories, current data also include supertidal variability (fluctuation) associated with a variety of turbulent and internal wave processes. To examine the relationships of such fluctuations to variability of the tides and/or potentially time-variable mean flows within which they are embedded, it is highly desirable to find a method whereby these flow components can be separated over relatively short periods of intensified event-scale forcing. A method is presented that first isolates “fluctuations” and then separates the remaining longer time-scale variability into “tides” and a remaining “mean,” without recourse to extraction of a fitted tide with the error inherent in such a fit over short data records.