Dave Hebert

Two Years in the Life of a Mediterranean Salt Lens

Abstract A lens of Mediterranean water (Meddy) was tracked in the eastern North Atlantic for two years with SOFAR floats. The Meddy was first found between the Canary Islands and the Azores in October 1984. It center moved in an irregular pattern, at speeds of a few cm s−1, and translated 1100 km to the south in two years. This Meddy was surveyed four times by CTD and velocity profilers, and once with the microstructure profiler EPSONDE. When observed during the first two surveys the Meddy had a core that was stably and smoothly stratified in both salinity and temperature, nearly uniform in the horizontal, and was saltier than the surrounding ocean by 0.65 psu. The Meddy was eroded from its edges, top and bottom, and lost salt and hat with an e-folding time of about one year. The salinity at the center remained at its original value during the first year and decreased during the second year. Evidence was seen for mixing by lateral intrusions, double diffusion, and turbulence; the intrusions are thought to...

Evolution of a mediterranean salt lens : scalar properties

Abstract The evolution of a Mediterranean salt lens (Meddy) over a two year period is examined. Several nondimensional numbers can be used to describe the overall decay in the structure of the Moddy. Two Rossby numbers, one using the central relative vorticity and another using the radius and velocity of the azimuthal velocity maximum, did not change over the two year period. However, the Burger number N2H2/(f2L2) increased as the Meddy decayed. Another Burger number, the ratio of total kinetic energy to total available potential energy, decreased from 1.1 to 0.6 over a one year period. The rates at which the Meddy lost salt and heat are consistent with estimates of horizontal fluxes by intrusions. A horizontal diffusivity of O(5 m2 s−1) is needed if this flux by intrusions is parameterized by an eddy coefficient. Simple models of the evolution of an isolated eddy by horizontal and vertical mixing of mass and momentum are examined. These simple attempts to explain the evolution of the Meddy suggest more c...

Decay of a Near-Inertial Wave

Abstract The decay of a downward propagating near-inertial wave was observed over four days. During this short period, the energy of the near-inertial wave decreased by 70%. The shear layers produced by the wave were regions of enhanced turbulent dissipation rates. The authors estimate that 44% of the observed change in the near-inertial energy was lost to turbulence. Estimates of the wave energy lost at the survey site due to the wave propagating out of the region were smaller. Energy lost by horizontal advection of the wave out of the survey region was more difficult to estimate; the horizontal extent of the near-inertial energy was unknown. Advection could account for more than half of the observed energy lost. However, the authors did not detect the near-inertial wave during a 40 km×40 km ADCP survey after completing the six-day station.

Direct Observations of Along-Isopycnal Upwelling and Diapycnal Velocity at a Shelfbreak Front*

Abstract By mapping the three-dimensional density field while simultaneously tracking a subsurface, isopycnal float, direct observations of upwelling along a shelfbreak front were made on the southern flank of Georges Bank. The thermohaline and bio-optical fields were mapped using a towed undulating vehicle, and horizontal velocity was measured with a shipboard acoustic Doppler current profiler. A subsurface isopycnal float capable of measuring diapycnal flow past the float was acoustically tracked from the ship. The float was released near the foot of the shelfbreak front (95–100-m isobath) and moved 15 km seaward as it rose from 80 to 50 m along the sloping frontal isopycnals over a 2-day deployment. The floats average westward velocity was 0.09 m s−1, while a drifter drogued at 15 m released at the same location moved westward essentially alongfront at 0.18 m s−1. The float measured strong downward vertical velocities (in excess of 0.02 m s−1) associated with propagation of internal tidal solibores in...

Intrusions: What Drives Them?

Abstract The driving mechanism for the observed interleaving of water masses is generally assumed to be double-diffusive mixing. However, some observations of intrusions have been made in regions where the mean stratification is stable to double-diffusive mixing. It has been hypothesized that a finite amplitude disturbance must occur to produce regions where the stratification allows double-diffusive mixing or that an instability due to differences in the molecular diffusivity of salinity and temperature produces the desired stratification for double-diffusive mixing to start. There is also the possibility of a differential vertical flux of salt and heat due to incomplete mixing by turbulence. The basis of this idea is described in this paper. Growth rates, vertical scales, and cross-frontal slopes of intrusions predicted by this process are compared to those predicted by double-diffusive mixing.

The front on the Northern Flank of Georges Bank in spring: 2. Cross‐frontal fluxes and mixing

buoyancy fluxes are downward and most intense, reaching values of 5 � 10 � 7 W/kg, near the bottom at the edge of the bank and decrease both on- and off-bank. Horizontal, crossbank buoyancy fluxes are partitioned into mean, tidal pumping, and nontidal eddy components and are computed as a function of cross-isobath/vertical position by averaging in the along-isobath direction. The tidal pumping component is dominant over most of the cross-bank section with a peak value of � 1 � 10 � 4 W/kg, directed off-bank near the bank edge. A diagnosed tidal vertical velocity field is used with mean buoyancy gradients to compute the along-isopycnal skew flux. The horizontal component of this skew flux has similar spatial structure and magnitude to that of the observed tidal pumping flux. The divergent component of the skew flux, at depths above the bottom boundary layer, appears to be convergent north of the bank edge and divergent at the bank edge, suggesting that tidally driven advective processes drive buoyant bank water downward and off-bank at mid-depth and force the upwelling of deep, dense water near the bottom at the bank edge. INDEX TERMS: 4528 Oceanography: Physical: Fronts and jets; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4560 Oceanography: Physical: Surface waves and tides (1255); 4544 Oceanography: Physical: Internal and inertial waves; KEYWORDS: Georges Bank, tidal mixing front, GLOBEC, internal tide, skew eddy flux, turbulent mixing

Internal gravity waves in the upper eastern equatorial Pacific: Observations and numerical solutions

On the basis of data from a towed thermistor chain collected near 140°W on the equator during April 1987, the zonal wavenumber and vertical structure of internal gravity waves were observed to vary significantly between wave events. Our hypothesis is that this variability is due to changes in the vertical structure of mean horizontal velocity and density. Assuming that the observed waves were the fastest growing modes for shear instability, we solve the Taylor-Goldstein equation, using different analytical basic states, including a zonal and meridional flow, to simulate the different conditions during 4 nights of intense internal wave activity. We find that while the observed waves are of finite amplitude, linear shear instability is sufficient to explain the wavelength and vertical structure of vertical displacement for most of the waves. The fastest growing, unstable, mode-one solutions have e-folding growth times of less than 10 min. These solutions show wave phase speeds and vertical structures to be highly dependent upon the velocity structure of the uppermost 40 m. Near the base of the mixed layer at a flow inflection point the kinetic energy of the mean flow is extracted for wave growth. Wave vertical displacement is maximum near this inflection point. Zonal phase speeds range from −0.8 to −0.1 m/s. The propagation direction of waves with growth rates of 75% of the maximum growth rate can range from about 45° north to 45° south of the zonal direction. The vertical wave-induced Reynolds stress divergence could explain a discrepancy in zonal momentum budgets of the upper 90 m of this region. Estimates of this stress divergence show that only about 100 days of intense internal wave activity is needed per year for these internal waves to explain estimated residuals of the mean zonal momentum budgets of this region at 50- to 100-m depth.

The Cross-Stream Potential Vorticity Front and Its Role in Meander-Induced Exchange in the Gulf Stream

Abstract The potential vorticity (PV) front observed in the upper thermocline at the northern edge of the Gulf Stream has been thought of as an inhibitor to lateral motion of parcels on isopycnals. The role of the PV front in relation to lateral motion and cross-stream exchange was investigated by monitoring the evolving PV field in the vicinity of Lagrangian parcels at the northern edge of the stream. The observational study involved shipboard acoustic Doppler current profiler (ADCP) and conductivity–temperature–depth surveys to sample the velocity and density fields around water parcels as they flowed through meanders. The observations reveal that the PV field in the vicinity of parcels moves laterally across the stream and evolves as parcels negotiate meander crests and troughs, allowing parcels to move across the stream without changing their Lagrangian PV. This lateral motion of the PV front suggests that it is more a response to than an inhibitor of the lateral motion of parcels in the meanders. The...

On the Geostrophic Adjustment of an Isolated Lens: Dependence on Burger Number and Initial Geometry*

Geostrophic adjustment of an isolated axisymmetric lens was examined to better understand the dependence of radial displacements and the adjusted velocity on the Burger number and the geometry of initial conditions. The behavior of the adjustment was examined using laboratory experiments and numerical simulations, which were in turn compared to published analytical solutions. Three defining length scales of the initial conditions were used to distinguish between various asymptotic behaviors for large and small Burger numbers: the Rossby radius of deformation, the horizontal length scale of the initial density defect, and the horizontal length scale of the initial pressure gradient. Numerical simulations for the fully nonlinear time-dependent adjustment agreed both qualitatively and quantitatively with analogous analytical solutions. For large Burger numbers, similar agreement was found in laboratory experiments. Results show that a broad range of final states can result from different initial geometries, depending on the values of the relevant length scales and the Burger number computed from initial conditions. For Burger numbers much larger or smaller than unity, differences between different initial geometries can readily exceed an order of magnitude for both displacement and velocity.

Mixing Structure of High-Frequency Internal Waves in the Upper Eastern Equatorial Pacific

Abstract A thermistor chain towed from 140° to 110°W along the equator revealed the presence of high-frequency internal waves in the upper 125 m having zonal wavelengths of 150–250 m. Turbulence dissipation rates, e, observed from a free-falling profiler were high when wave packets were present. Unfortunately, the frequency of the vertical profiles of e taken did not resolve the internal wave cycle, so a dynamical link between the waves and the mixing could not be directly observed with vertical profiler data. It is presumed that either wave-induced shear instability or advective instability destabilized the waves and led to increased e. The thermistor data, which were sampled at 20 Hz or approximately 12.5 cm horizontally, resolved part of the inertial subrange of turbulence and are used to determine the structure of turbulence within an internal wave cycle. A temperature gradient variance method for estimating e relies on a fully resolved Batchelor spectrum that, for this experiment, would have required...