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Andrew P. Mack and D. Hebert

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, ε, observed from a free-falling profiler were high when wave packets were present. Unfortunately, the frequency of the vertical profiles of ε 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 ε. 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 ε relies on a fully resolved Batchelor spectrum that, for this experiment, would have required resolution of scales less than 2–10 mm. Nevertheless, the authors use the observed, underestimated temperature gradient variance in this study as a surrogate for ε. That is, an activity index A T was used as an indicator of the turbulence dissipation rate. Observations of A T as a function of internal wave phase and depth reveals a consistent structure of turbulent mixing within the wave cycle. This structure, having relatively higher A T associated with wave crests near 20-m depth and wave troughs near 60-m depth, is consistent with purely wave-induced shear instability based on its criterion and is not consistent with purely advective instability. The index, A T, as a function of U z/N (U z is the mean vertical shear of zonal velocity and N is the mean buoyancy frequency) and wave slope (defined as the product of the wavenumber and the wave displacement amplitude) demonstrates agreement between mixing and a neutral stability curve for the combined effects of advective and shear instabilities. However, the background shear and stratification are such that the vast majority of observed waves are associated with purely shear instability.

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J. Rajamony, D. Hebert, and T. Rossby

Abstract

Lagrangian trajectories may be estimated with an Acoustic Doppler Current Profiler (ADCP) as an alternative to tracking parcels with floats. By using the depth of isopycnals from a conductivity–temperature–depth (CTD) instrument and the velocities at various depths from the ADCP, velocities may be mapped on isopycnals. These isopycnal velocities may be integrated with respect to time to determine Lagrangian trajectories. This method of tracking water parcels with CTD and ADCP was implemented in two stages—a real-time tracking concurrent with a survey of the vicinity and a refinement of the tracking based on data from the vicinity.

Trajectories of isopycnal parcels in a kinematic model and floats in Gulf Stream meanders were estimated by this method; these estimated trajectories were compared with computed trajectories in the model and with acoustically determined trajectories of the floats in the meanders, respectively. The comparison shows that the differential GPS renders ADCP data accurate enough to estimate Lagrangian trajectories over timescales of a few days. This method can be used as an alternative to tracking isopycnal parcels with devices such as the isopycnal Swallow float. A distinct advantage of this method over shipboard tracking of floats is that the density and velocity measurements in the vicinity of an isopycnal parcel can be used to determine the Lagrangian trajectories of other parcels distributed in the horizontal and vertical.

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J. Rajamony, D. Hebert, and T. Rossby

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 relative contributions of the components of PV to the front indicated a rich structure of changes in shear and stretching across the stream. Also, temperature changes of Lagrangian parcels were linked to small-scale mixing processes associated with observed intrusive features. Estimated diffusion coefficients suggest that horizontal mixing could be the main mechanism of mixing in the upper thermocline at the northern edge of the stream.

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D. Hebert, J. N. Moum, and D. R. Caldwell

Abstract

In spite of the effects of several form of temporal variability that tend to mask geographical patterns in turbulence intensity, our evidence indicates that the turbulence is enhanced above the equatorial undercurrent in comparison to latitudes north and south of it. This evidence consists of three meridional transects of micro-structure observations across the equator (at 140°W in 1984 and 1987. and at 110°W in 1987) along with an equatorial station at 140°W and a longitudinal transect along the equator from 140°W to 110°W. All three meridional transects show a peak in averaged estimates of the turbulent kinetic energy dissipation rates, ε, at the equator, although in 1984 the peak was not significant at the 95% level. The major sources a temporal variability were the diurnal buoyancy flux variation and the wind stress variations, which had a typical period of a few days. After the diurnal variability is removed by averaging, it can be shown that, for similar wind stress, ε is larger over the undercurrent than away from it. Examination of the 16-m, 1-hour averaged ε, in terms of the vertical shear of horizontal velocity and the stratification (determined over similar space and time scales), indicated a tendency of this mean ε to vary with the Richardson number, Ri, when Ri<1. However, closer examination showed that the dependence of ε on Ri varied with depth. Therefore, a simple parameterization for mixing rates on Ri is not valid for all depth.

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J. N. Moum, D. Hebert, C. A. Paulson, D. R. Caldwell, M. J. McPhaden, and H. Peters

Abstract

Appearing in this issue of the Journal of Physical Oceanography are three papers that present new observations of a distinct, narrow band, and diurnally varying signal in temperature records obtained in the low Richardson number shear flow above the core of the equatorial undercurrent. Moored data suggest that the intrinsic frequency of the signal is near the local buoyancy frequency, while towed data indicate that the horizontal wavelength in the zonal direction is 150–250 m. Coincident microstructure profiling shows that this signal is associated with bursts of turbulent mixing, it seems that this narrowband signal represents the signature of instabilities that ultimately cause the turbulence observed in the equatorial thermocline. Common problems in interpreting the physics behind the signature are discussed here.

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J. N. Moum, D. Hebert, C. A. Paulson, and D. R. Caldwell

Abstract

High correlations between turbulent dissipation rates and high-wavenumber internal waves and the high values of turbulent dissipation associated with internal wave activity suggest that internal waves are the main direct source of mixing in the thermocline above the core of the Equatorial Undercurrent. An extensive dataset obtained using a microstructure profiler and thermistor chain towed along the equator was analyzed to examine the correspondence between turbulent mixing and high-wavenumber internal waves. In the low Richardson number (Ri) thermocline below the mixed layer but above the core of the Equatorial Undercurrent, and when winds were moderate and steadily westward, it was found that:

• the spectrum of vertical isotherm displacement was dominated by a narrow wavenumber band (corresponding to 150–250-m zonal wavelength) of internal waves;

• both turbulence and internal waves varied diurnally—hourly averaged values of turbulent dissipation rate and wave potential energy were greater by a factor of 100 at night; and

• correlations between turbulent dissipation rate and several measures of internal wave activity (wave isotherm displacement, wave slope, and wave potential energy) were high.

Little or no high wavenumber internal wave activity was observed when winds were light or eastward: Superposing plane waves with the observed characteristics on the observed background field suggests that they are inherently unstable to both adjective and shear instability above the core of the Equatorial Undercurrent. These waves are due either to locally generated internal gravity waves or to Kelvin-Helmholz-type instabilities generated in the shear flow; from our measurements these two phenomena could not be distinguished.

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D. Hebert, J. N. Moum, C. A. Paulson, and D. R. Caldwell

Abstract

In the low Richardson number shear flow above the Pacific Equatorial Undercurrent, a single vertical microstructure profile intersected the overturning crest of a packet of high horizontal wavenumber waves. The observed dissipation rates within the overturning wave were so high that if they were representative of the volume-averaged rate, the total wave energy would have been dissipated within a single buoyancy period. The chaotic structure (and temperature fluctuations with horizontal scales less than 2 m) of the two wave crests and troughs west of the overturning wave crest suggest that recent mixing had occurred there. Wave crests and troughs east of the overturning wave crest showed little or no sign of turbulent mixing.

Similar high horizontal wavenumber waves, believed to be shear-instability waves, have been observed in low Richardson number regions of the midlatitude seasonal thermocline. Although the equatorial waves have a horizontal wavelength appropriate for shear-instability waves, their vertical scale is much larger than the vertical extent of the low Richardson region, unlike that found for simple shear-instability waves.

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R. A. Richardson, G. G. Sutyrin, D. Hebert, and L. M. Rothstein

Abstract

The upper ocean response to idealized surface wind forcing that is representative of conditions observed during the TOGA-COARE Intensive Observation Period is studied by numerical simulations using a second-moment closure model. A set of experiments is described with a variety of squall-like wind stress distributions and linear initial stratification in the ocean. Several physical regimes of turbulent mixing and decay during and after wind forcing are described. Differences in the structure of the upper and lower parts of the mixing layer are analyzed. The results indicate an exponential decay of turbulent kinetic energy (TKE) with time after surface forcing is removed, and TKE source terms continue to play an important role.

The velocity and density structure after the squall are found to be universal, with a nearly constant Richardson number throughout the mixing layer. It is demonstrated that this implies that the mixed layer depth is determined by the initial buoyancy frequency and total momentum input from the wind stress in the same manner as in the bulk mixed layer models. It does not depend essentially on the squall duration or the time evolution of the wind stress during the squall.

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