Miles A. Sundermeyer

Lateral dispersion over the continental shelf: Analysis of dye release experiments

Lateral dispersion over the continental shelf was examined using dye studies performed as a part of the Coastal Mixing and Optics experiment. Four experiments performed at intermediate depths, each lasting 2.5-5 days, were examined. In some cases the dye patches remained fairly homogeneous both vertically and horizontally throughout an experiment. In other cases, significant patchiness was observed on scales ranging from 2 to 10 m vertically and a few hundred meters to a few kilometers horizontally. The observations showed that the dye distributions were significantly influenced by shearing and straining on scales of 5-10 m in the vertical and 1-10 km in the horizontal. Superimposed on these larger-scale distortions were simultaneous increases in the horizontal second moments of the dye patches, with corresponding horizontal diffusivities based on a Fickian diffusion model of 0.3-4.9 m 2 s -1 . Analysis of the dye data in concert with shear estimates from shipboard acoustic Doppler current profiler observations showed that the existing paradigms of shear dispersion and dispersion by interleaving water masses cannot account for the observed diffusive spreading of the dye patches. This result suggests that some other mechanisms provided an additional diffusivity of order 0.2-4.6 m 2 s -1 . An alternative mechanism, dispersion by vortical motions caused by the relaxation of diapycnal mixing events, may explain the observed dispersion in some cases.

Stratified Ekman layers

Under fair weather conditions the time-averaged, wind-driven current forms a spiral in which the current vector decays and turns clockwise with increasing depth. These spirals resemble classical Ekman spirals. They differ in that their rotation depth scale exceeds the e-folding depth of the speed by a factor of 2 to 4 and they are compressed in the downwind direction. A related property is that the time-averaged stress is evidently not parallel to the time-averaged vertical shear. We develop the hypothesis that the flat spiral structure may be a consequence of the temporal variability of stratification. Within the upper 10-20 m of the water column this variability is associated primarily with the diurnal cycle and can be treated by a time-dependent diffusion model or a mixed-layer model. The latter can be simplified to yield a closed solution that gives an explicit account of stratified spirals and reasonable hindcasts of midlatitude cases. At middle and higher latitudes the wind-driven transport is found to be trapped mainly within the upper part of the Ekman layer, the diurnal warm layer. At tropical latitudes the effects of diurnal cycling are in some ways less important, and Ekman layer currents are likely to be significant to much greater depths. In that event the lower part of the Ekman layer is likely to be affected by stratification variability that may be nonlocal.

Lateral mixing and the North Atlantic Tracer Release Experiment: Observations and numerical simulations of Lagrangian particles and a passive tracer

Mixing and stirring of Lagrangian particles and a passive tracer were studied by comparison of float and tracer observations from the North Atlantic Tracer Release Experiment (NATRE). Statistics computed from the NATRE floats were found to be similar to those estimated by Ledwell et al. [this issue] from the tracer dispersion. Mean velocities computed from the floats were (u¯,υ¯)=(−1.2±0.3, −0.9±0.2) cm/s for the (zonal, meridional) components, and large-scale effective eddy diffusivities were (κe11, κe22) = (1.5±0.7, 0.7±0.4) × 103 m2/s. The NATRE observations were used to evaluate theoretical models of tracer and particle dispersal. The tracer dispersion observed by Ledwell et al. [this issue] was consistent with an exponential growth phase for about the first 6 months and a linear growth at larger times. A numerical model of mesoscale turbulence that was calibrated with float statistics also showed an exponential growth phase of tracer and a reduced growth for longer times. Numerical results further show that Garretts [1983] theory, relating the effective small-scale diffusivity to the rms strain rate and tracer streak width, requires a scale factor of 2 when the observed growth rate of streak length is used as a measure of the strain rate. This scale factor will be different for different measures of the strain rate and may also be affected by temporal and spatial variations in the mesoscale strain field.

The latmix summer campaign: Submesoscale stirring in the upper ocean

AbstractLateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast diff...

Stirring by Small-Scale Vortices Caused by Patchy Mixing

Evidence is presented that lateral dispersion on scales of 1–10 km in the stratified waters of the continental shelf may be significantly enhanced by stirring by small-scale geostrophic motions caused by patches of mixed fluid adjusting in the aftermath of diapycnal mixing events. Dye-release experiments conducted during the recent Coastal Mixing and Optics (CMO) experiment provide estimates of diapycnal and lateral dispersion. Microstructure observations made during these experiments showed patchy turbulence on vertical scales of 1–10 m and horizontal scales of a few hundred meters to a few kilometers. Momentum scaling and a simple random walk formulation were used to estimate the effective lateral dispersion caused by motions resulting from lateral adjustment following episodic mixing events. It is predicted that lateral dispersion is largest when the scale of mixed patches is on the order of the internal Rossby radius of deformation, which seems to have been the case for CMO. For parameter values relevant to CMO, lower-bound estimates of the effective lateral diffusivity by this mechanism ranged from 0.1 to 1 m 2 s 1 . Revised estimates after accounting for the possibility of long-lived motions were an order of magnitude larger and ranged from 1 to 10 m 2 s 1 . The predicted dispersion is large enough to explain the observed lateral dispersion in all four CMO dye-release experiments examined.

Geostrophic Adjustment of an Isolated Diapycnal Mixing Event and Its Implications for Small-Scale Lateral Dispersion

Abstract In this first of two companion papers, the time-dependent relaxation of an isolated diapycnal mixing event is examined in detail by means of numerical simulations, with an emphasis on the energy budget, particle displacements, and their implications for submesoscale oceanic lateral dispersion. The adjustment and dispersion characteristics are examined as a function of the lateral extent of the event L relative to the Rossby radius of deformation R. The strongest circulations and horizontal displacements occur in the regime R/L ≈ O(1). For short times, less than an inertial period, horizontal displacements are radial. Once the adjustment is completed, displacements become primarily azimuthal and continue to stir fluid over several to tens of inertial periods. The cumulative effect of many such events in terms of the effective lateral dispersion that they induce is examined in the companion paper.

Three-dimensional mapping of fluorescent dye using a scanning, depth-resolving airborne lidar

Abstract Results are presented from a pilot study using a fluorescent dye tracer imaged by airborne lidar in the ocean surface layer on spatial scales of meters to kilometers and temporal scales of minutes to hours. The lidar used here employs a scanning, frequency-doubled Nd:YAG laser to emit an infrared (1064 nm) and green (532 nm) pulse 6 ns in duration at a rate of 1 kHz. The received signal is split to infrared, green, and fluorescent (nominally 580–600 nm) channels, the latter two of which are used to compute absolute dye concentration as a function of depth and horizontal position. Comparison of dye concentrations inferred from the lidar with in situ fluorometry measurements made by ship shows good agreement both qualitatively and quantitatively for absolute dye concentrations ranging from 1 to >10 ppb. Uncertainties associated with horizontal variations in the natural seawater attenuation are approximately 1 ppb. The results demonstrate the ability of airborne lidar to capture high-resolution thre...

Numerical Simulations of Lateral Dispersion by the Relaxation of Diapycnal Mixing Events

Abstract In this second of two companion papers, numerical simulations of lateral dispersion by small-scale geostrophic motions, or vortical modes, generated by the adjustment of mixed patches following diapycnal mixing events are examined. A three-dimensional model was used to solve the Navier–Stokes equations and an advection/diffusion equation for a passive tracer. Model results were compared with theoretical predictions for vortical mode stirring with results from dye release experiments conducted over the New England continental shelf. For “weakly nonlinear” cases in which adjustment events were isolated in space and time, lateral dispersion in the model was consistent to within a constant scale factor with the parameter dependence predicted by Sundermeyer et al., where h and L are the vertical and horizontal scales of the mixed patches, ΔN 2 is the change in stratification associated with the mixed patches, f is the Coriolis parameter, ϕ is the frequency of diapycnal mixing events, and νB is the bac...

Submesoscale Water-Mass Spectra in the Sargasso Sea

Submesoscale stirring contributes to the cascade of tracer variance from large to small scales. Multiple nested surveys in the summer Sargasso Sea with tow-yo and autonomous platforms captured submesoscale water-mass variability in the seasonal pycnocline at 20‐60-m depths. To filter out internal waves that dominate dynamic signals on these scales, spectra for salinity anomalies on isopycnals were formed. Salinitygradient spectra are approximately flat with slopes of 20.2 6 0.2 over horizontal wavelengths of 0.03‐10km. While the two to three realizations presented here might be biased, more representative measurements in the literature are consistent with a nearly flat submesoscale passive tracer gradient spectrum for horizontal wavelengths in excess of 1km. A review of mechanisms that could be responsible for a flat passive tracer gradient spectrum rules out (i) quasigeostrophic eddy stirring, (ii) atmospheric forcing through a relict submesoscale winter mixed layer structure or nocturnal mixed layer deepening, (iii) a downscale vortical-mode cascade, and (iv) horizontal diffusion because of shear dispersion of diapycnal mixing. Internal-wave horizontal strain appears to be able to explain horizontal wavenumbers of 0.1‐7 cycles per kilometer (cpkm) but not the highest resolved wavenumbers (7‐30cpkm). Submesoscale subduction cannot be ruled out at these depths, though previous observations observe a flat spectrum well below subduction depths, so this seems unlikely. Primitive equation numerical modeling suggests that nonquasigeostrophic subinertial horizontal stirring can produce a flat spectrum. The last need not be limited to mode-one interior or surface Rossby wavenumbers of quasigeostrophic theory but may have a broaderband spectrum extending to smaller horizontal scales associated with frontogenesis and frontal instabilities as well as internal waves.

Vortex Stability in a Large-Scale Internal Wave Shear

AbstractThe effect of a large-scale internal wave on a multipolar compound vortex was simulated numerically using a 3D Boussinesq pseudospectral model. A suite of simulations tested the effect of a background internal wave of various strengths, including a simulation with only a vortex. Without the background wave, the vortex remained apparently stable for many hundreds of inertial periods but then split into two dipoles. With increasing background wave amplitude, and hence shear, dipole splitting occurred earlier and was less symmetric in space. Theoretical considerations suggest that the vortex alone undergoes a self-induced mixed barotropic–baroclinic instability. For a vortex plus background wave, kinetic energy spectra showed that the internal wave supplied energy for the dipole splitting. In this case, it was found that the presence of the wave hastened the time to instability by increasing the initial perturbation to the vortex. Results suggest that the stability and fate of submesoscale vortices i...