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  • Author or Editor: H. W. Wijesekera x
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H. W. Wijesekera, C. A. Paulson, and A. Huyer

Abstract

Measurements of a fresh surface anomaly (fresh lens) produced by rainfall during a westerly wind burst have been analyzed. The measurements were made in December 1992 as part of the Coupled Ocean–Atmosphere Response Experiment in the western equatorial Pacific (2°S, 156°E). Measurements included radar estimates of rainfall, upper-ocean temperature (T), salinity (S), horizontal velocity, and microstructure. In situ observations of the fresh lens were made 5 to 7 hours after its formation. In the 5 hours after formation, the lens deepened to a depth of 40 m as indicated by its salinity anomaly. Salinity and temperature were highly correlated within the lens, consistent with its initial formation by cold rainfall. The T–S relation exhibited curvature, which can be explained by surface cooling and upper-ocean mixing subsequent to formation of the lens. The lens exhibited a horizontal velocity anomaly in the direction of wind, which extended down to a depth of 40 m. The horizontal velocity anomaly is consistent with momentum being trapped near the surface due to rain-induced stratification. Vertical velocity, estimated from the divergence of zonal velocity, showed downwelling at the leading edge of the lens and upwelling at the trailing edge. The magnitude of vertical velocity at a depth of 20 m is 20 m day−1. Richardson numbers within the lens were low (0.25 to 0.5), suggesting that turbulent mixing was governed by critical-Ri instability. Wavenumber spectra of T and S in the upper 20 m exhibit a −5/3 range, which extends to wavenumbers below the range of local isotropy. Spectral levels were used to estimate turbulent dissipation rates of T and S, which were in turn used to estimate turbulent fluxes of heat and salt. Turbulent fluxes were also estimated from microstructure observations between depths of 10 and 60 m. Fluxes within the fresh lens were nearly uniform from 2 m to 35 m depth, then decreased to near zero at 45 m. The lifetime of fresh lenses during westerly wind bursts appears to be less than one day.

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D. W. Wang, H. W. Wijesekera, E. Jarosz, W. J. Teague, and W. S. Pegau

Abstract

Breaking surface waves generate layers of bubble clouds as air parcels entrain into the upper ocean through the action of turbulent motions. The turbulent diffusivity in the bubble cloud layer is investigated by combining measurements of surface winds, waves, bubble acoustic backscatter, currents, and hydrography. These measurements were made at water depths of 60–90 m on the shelf of the Gulf of Alaska near Kayak Island during late December 2012, a period when the ocean was experiencing winds and significant wave heights up to 22 m s−1 and 9 m, respectively. Vertical profiles of acoustic backscatter decayed exponentially from the wave surface with e-folding lengths of about 0.6 to 6 m, while the bubble penetration depths were about 3 to 30 m. Both e-folding lengths and bubble depths were highly correlated with surface wind and wave conditions. The turbulent diffusion coefficients, inferred from e-folding length and bubble depth, varied from about 0.01 to 0.4 m2 s−1. Analysis suggests that the turbulent diffusivity in the bubble layer can be parameterized as a function of the cube of the wind friction velocity with a proportionality coefficient that depends weakly on wave age. Furthermore, in the bubble layer, on average, the shear production of the turbulent kinetic energy estimated by the diffusion coefficients is a similar order of magnitude as the dissipation rate predicted by the wall boundary layer theory.

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W. J. Teague, H. W. Wijesekera, W. E. Avera, and Z. R. Hallock

Abstract

Closely spaced observations of nonlinear internal waves (NLIWs) were made on the outer continental shelf off New Jersey in June 2009. Nearly full water column measurements of current velocity were made with four acoustic Doppler current profilers (ADCPs) that were moored about 5 km apart on the bottom along a line approximately normal to the bathymetry between water depths of 67 and 92 m. Density profiles were obtained from two vertical strings of temperature and conductivity sensors that were deployed near each of the interior ADCP moorings. In addition, a towed ScanFish provided profiles and fixed-level records of temperature and salinity through several NLIW packets near the moorings. Several case studies were selected to describe the propagation of the NLIWs. One to three solitary waves of depression were observed in five selected packets. There were also occurrences of multiple-phase dispersive wave packets. The average propagation speed corrected for advection of the observed waves was 0.51 ± 0.09 m s−1. The waves were directed primarily shoreward (~northwestward) along the mooring line with average wavelengths and periods of about 300 m and 10 min, respectively. Wave amplitudes and energies decreased with decreasing water depth. The observed wave parameters can be locally described by a two-layer Korteweg–de Vries (KdV) model, except for the decreasing amplitudes, which may be due to shear-induced dissipation and/or bottom drag. The various complementary observations utilized in this study present a unique description of NLIWs.

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T. M. Dillon, J. A. Barth, A. Y. Erofeev, G. H. May, and H. W. Wijesekera

Abstract

A new high-frequency turbulence measuring instrument, MicroSoar, has been developed, tested, and used to make scalar variance dissipation rate measurements. MicroSoar was mounted on the undercarriage of SeaSoar, a depth-programmable winged platform, and towed by a ship, at speeds up to 7 kt, in a depth range of the sea surface to 120 m. Sensors carried by MicroSoar were a fast thermistor, a pressure sensor, a microscale capillary conductivity sensor, and a three-axis accelerometer. With appropriate assumptions about the local TS relation, measurements of microscale conductivity fluctuations can often be used to directly determine temperature variance dissipation rate (χ T), the Cox number (C x), and the scalar diathermal turbulent diffusivity (K T). Compared to conventional quasi-free-fall tethered vertically profiling instruments, MicroSoar's major advantage lies in its ability to sample large fluid volumes and large geographic areas in a short time, and to provide, rapidly and simply, two-dimensional (horizontal–vertical) representations of the distribution of oceanic mixing rates.

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H. W. Wijesekera, D. W. Wang, E. Jarosz, W. J. Teague, W. S. Pegau, and J. N. Moum

Abstract

Momentum transport by energy-containing turbulent eddies in the oceanic mixed layer were investigated during high-wind events in the northern Gulf of Alaska off Kayak Island. Sixteen high-wind events with magnitudes ranging from 7 to 22 m s−1 were examined. Winds from the southeast prevailed from one to several days with significant wave heights of 5–9 m and turbulent Langmuir numbers of about 0.2–0.4. Surface buoyancy forcing was much weaker than the wind stress forcing. The water column was well mixed to the bottom depth of about 73 m. Spectral analyses indicate that a major part of the turbulent momentum flux was concentrated on 10–30-min time scales. The ratio of horizontal scale to mixed layer depth was from 2 to 8. Turbulent shear stresses in the mixed layer were horizontally asymmetric. The downwind turbulent stress at 10–20 m below the surface was approximately 40% of the averaged wind stress and was reduced to 5%–10% of the wind stress near the bottom. Turbulent kinetic energy in the crosswind direction was 30% larger than in the downwind direction and an order of magnitude larger than the vertical component. The averaged eddy viscosity between 10- and 30-m depth was ~0.1 m2 s−1, decreased with depth rapidly below 50 m, and was ~5 × 10−3 m2 s−1 at 5 m above the bottom. The divergence of turbulent shear stress accelerated the flow during the early stages of wind events before Coriolis and pressure gradient forces became important.

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H. W. Wijesekera, D. W. Wang, W. J. Teague, E. Jarosz, W. E. Rogers, D. B. Fribance, and J. N. Moum

Abstract

Several acoustic Doppler current profilers and vertical strings of temperature, conductivity, and pressure sensors, deployed on and around the East Flower Garden Bank (EFGB), were used to examine surface wave effects on high-frequency flows over the bank and to quantify spatial and temporal characteristic of these high-frequency flows. The EFGB, about 5-km wide and 10-km long, is located about 180-km southeast of Galveston, Texas, and consists of steep slopes on southern and eastern sides that rise from water depths over 100 m to within 20 m of the surface. Three-dimensional flows with frequencies ranging from 0.2 to 2 cycles per hour (cph) were observed in the mixed layer when wind speed and Stokes drift at the surface were large. These motions were stronger over the bank than outside the perimeter. The squared vertical velocity w 2 was strongest near the surface and decayed exponentially with depth, and the e-folding length of w 2 was 2 times larger than that of Stokes drift. The 2-h-averaged w 2 in the mixed layer, scaled by the squared friction velocity, was largest when the turbulent Langmuir number was less than unity and the mixed layer was shallow. It is suggested that Langmuir circulation is responsible for the generation of vertical flows in the mixed layer, and that the increase in kinetic energy is due to an enhancement of Stokes drift by wave focusing. The lack of agreement with open-ocean Langmuir scaling arguments is likely due to the enhanced kinetic energy by wave focusing.

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Eric D. Skyllingstad, W. D. Smyth, J. N. Moum, and H. Wijesekera

Abstract

The response of the upper ocean to westerly wind forcing in the western equatorial Pacific was modeled by means of large-eddy simulation for the purpose of comparison with concurrent microstructure observations. The model was initialized using currents and hydrography measured during the Coupled Ocean–Atmosphere Response Experiment (COARE) and forced using measurements of surface fluxes over a 24-h period. Comparison of turbulence statistics from the model with those estimated from concurrent measurements reveals good agreement within the mixed layer. The shortcomings of the model appear in the stratified fluid below the mixed layer, where the vertical length scales of turbulent eddies are limited by stratification and are not adequately resolved by the model. Model predictions of vertical heat and salt fluxes in the entrainment zone at the base of the mixed layer are very similar to estimates based on microstructure data.

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D. A. Cherian, E. L. Shroyer, H. W. Wijesekera, and J. N. Moum

Abstract

We describe the seasonal cycle of mixing in the top 30–100 m of the Bay of Bengal as observed by moored mixing meters (χpods) deployed along 8°N between 85.5° and 88.5°E in 2014 and 2015. All χpod observations were combined to form seasonal-mean vertical profiles of turbulence diffusivity K T in the top 100 m. The strongest turbulence is observed during the southwest and postmonsoon seasons, that is, between July and November. The northeast monsoon (December–February) is a period of similarly high mean K T but an order of magnitude lower median K T, a sign of energetic episodic mixing events forced by near-inertial shear events. The months of March and April, a period of weak wind forcing and low near-inertial shear amplitude, are characterized by near-molecular values of K T in the thermocline for weeks at a time. Strong mixing events coincide with the passage of surface-forced downward-propagating near-inertial waves and with the presence of enhanced low-frequency shear associated with the Summer Monsoon Current and other mesoscale features between July and October. This seasonal cycle of mixing is consequential. We find that monthly averaged turbulent transport of salt out of the salty Arabian Sea water between August and January is significant relative to local EP. The magnitude of this salt flux is approximately that required to close model-based salt budgets for the upper Bay of Bengal.

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H. W. Wijesekera, E. Jarosz, W. J. Teague, D. W. Wang, D. B. Fribance, J. N. Moum, and S. J. Warner

Abstract

Pressure differences across topography generate a form drag that opposes the flow in the water column, and viscous and pressure forces acting on roughness elements of the topographic surface generate a frictional drag on the bottom. Form drag and bottom roughness lengths were estimated over the East Flower Garden Bank (EFGB) in the Gulf of Mexico by combining an array of bottom pressure measurements and profiles of velocity and turbulent kinetic dissipation rates. The EFGB is a coral bank about 6 km wide and 10 km long located at the shelf edge that rises from 100-m water depth to about 18 m below the sea surface. The average frictional drag coefficient over the entire bank was estimated as 0.006 using roughness lengths that ranged from 0.001 cm for relatively smooth portions of the bank to 1–10 cm for very rough portions over the corals. The measured form drag over the bank showed multiple time-scale variability. Diurnal tides and low-frequency motions with periods ranging from 4 to 17 days generated form drags of about 2000 N m−1 with average drag coefficients ranging between 0.03 and 0.22, which are a factor of 5–35 times larger than the average frictional drag coefficient. Both linear wave and quadratic drag laws have similarities with the observed form drag. The form drag is an important flow retardation mechanism even in the presence of the large frictional drag associated with coral reefs and requires parameterization.

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C. A. Luecke, H. W. Wijesekera, E. Jarosz, D. W. Wang, J. C. Wesson, S. U. P. Jinadasa, H. J. S. Fernando, and W. J. Teague

Abstract

Long-term measurements of turbulent kinetic energy dissipation rate (ε), and turbulent temperature variance dissipation rate (χ T) in the thermocline, along with currents, temperature, and salinity were made at two subsurface moorings in the southern Bay of Bengal (BoB). This is a part of a major international program, conducted between July 2018 and June 2019, for investigating the role of the BoB on the monsoon intraseasonal oscillations. One mooring was located on the typical path of the Southwest Monsoon Current (SMC), and the other was in a region where the Sri Lanka dome is typically found during the summer monsoon. Microstructure and finescale estimates of vertical diffusivity revealed the long-term subthermocline mixing patterns in the southern BoB. Enhanced turbulence and large eddy diffusivities were observed within the SMC during the passage of a subsurface-intensified anticyclonic eddy. During this time, background shear and strain appeared to influence high-frequency motions such as near-inertial waves and internal tides, leading to increased mixing. Near the Sri Lanka dome, enhanced dissipation occurred at the margins of the cyclonic feature. Turbulent mixing was enhanced with the passage of Rossby waves and eddies. During these events, values of χ T exceeding 10−4 °C2 s−1 were recorded concurrently with ε values exceeding 10−5 W kg−1. Inferred diffusivity peaked well above background values of 10−6 m2 s−1, leading to an annually averaged diffusivity near 10−4 m2 s−1. Turbulence appeared low throughout much of the deployment period. Most of the mixing occurred in spurts during isolated events.

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