Albert J. Plueddemann

On the Exchange of Momentum over the Open Ocean

AbstractThis study investigates the exchange of momentum between the atmosphere and ocean using data collected from four oceanic field experiments. Direct covariance estimates of momentum fluxes were collected in all four experiments and wind profiles were collected during three of them. The objective of the investigation is to improve parameterizations of the surface roughness and drag coefficient used to estimate the surface stress from bulk formulas. Specifically, the Coupled Ocean–Atmosphere Response Experiment (COARE) 3.0 bulk flux algorithm is refined to create COARE 3.5. Oversea measurements of dimensionless shear are used to investigate the stability function under stable and convective conditions. The behavior of surface roughness is then investigated over a wider range of wind speeds (up to 25 m s−1) and wave conditions than have been available from previous oversea field studies. The wind speed dependence of the Charnock coefficient α in the COARE algorithm is modified to , where m = 0.017 m−1 ...

Western Arctic Shelfbreak Eddies: Formation and Transport

Abstract The mean structure and time-dependent behavior of the shelfbreak jet along the southern Beaufort Sea, and its ability to transport properties into the basin interior via eddies are explored using high-resolution mooring data and an idealized numerical model. The analysis focuses on springtime, when weakly stratified winter-transformed Pacific water is being advected out of the Chukchi Sea. When winds are weak, the observed jet is bottom trapped with a low potential vorticity core and has maximum mean velocities of O(25 cm s−1) and an eastward transport of 0.42 Sv (1 Sv ≡ 106 m3 s−1). Despite the absence of winds, the current is highly time dependent, with relative vorticity and twisting vorticity often important components of the Ertel potential vorticity. An idealized primitive equation model forced by dense, weakly stratified waters flowing off a shelf produces a mean middepth boundary current similar in structure to that observed at the mooring site. The model boundary current is also highly v...

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

The Office of Naval Researchs Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Marthas Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over sca...

Structure and variability of Langmuir circulation during the Surface Waves Processes Program

A cooperative, multiplatform field experiment was conducted in the eastern North Pacific during February and March of 1990 as part of the Surface Waves Processes Program (SWAPP). One of the experimental objectives was to investigate Langmuir circulation so that its role in the evolution of the oceanic surface boundary layer could be better understood. The concurrent use of different observational techniques, ranging from simple surface drifters to complex Doppler sonar systems, resulted in new information about Langmuir circulation structure and variability. Estimates of Langmuir cell spacing indicated that a broad range of scales, from about 2 to 200 m, was excited during periods of strong surface forcing and that the energy containing scales evolved with time. Estimates of cell spacing based on Doppler velocities from a surface-scanning sonar directed crosswind showed this scale evolution, but estimates based on backscattered intensity did not. This was attributed to the fact that the intensity-based estimates were only indirectly related to circulation strength. The near-surface convergent velocities from the sonar were used to form an objective, quantitative measure of the temporal variations in Langmuir circulation strength. As expected, the circulation strength increased dramatically during strong wind events. However, circulation strength and wind stress did not decrease simultaneously, and Langmuir circulation was detectable for up to a day after abrupt reductions in wind stress. Energy from the surface wave field, which decayed more slowly than the wind, was apparently responsible for maintaining the circulation. The variation of circulation strength was found to be better related to (u*Us)½ than to u*, where u* = (τ/ρ)½ is the friction velocity, τ is the wind stress, and Us is the surface wave Stokes drift. This scaling is consistent with wave-current interaction theories of Langmuir cell generation.

Characterization of the patterns of diel migration using a Doppler sonar

Abstract Measurements of backscattered intensity and Doppler velocity were made over a depth interval of 60–1200 m using 67 kHz Doppler sonar mounted on the Research Platform FLIP. Data were analysed for a 13-day period during which FLIP drifted over a distance of approximately 115 km. The existence of vertically migrating layers is evident in both the intensity and vertical velocity fields. Analysis of the temporal variations of intensity and vertical velocity from a long-term mean state provides complementary characterizations of migration patterns. Three distinct scattering layers are observed with daytime depts of about 300, 560 and 1000 m. Both intensity and velocity data are consistent with vertical migration rates of between 1.0 and 4.0 cm s−1. The patterns of vertical migration of the three layers show a high degree of persistence over a time period of 13 days and horizontal scales of order 50 km. Day-to-night (12 h) differences in profiles of backscattered intensity are significant compared to difference between successive days or successive nights, confirming a day-to-night resdistribution of scaterers. Day-to-day (24 h) variability in the intensity and velocity fields is generally small compared to the magnitude of the anomalies defining the scattering layers. The exception is an isolated region of high variability between 100 and 250 m depth during daylight hours that is associated with a relative minimum in backscattered intensity.

Observations of Turbulence in the Ocean Surface Boundary Layer: Energetics and Transport

Observations of turbulent kinetic energy (TKE) dynamics in the ocean surface boundary layer are presented here and compared with results from previous observational, numerical, and analytic studies. As in previous studies, the dissipation rate of TKE is found to be higher in the wavy ocean surface boundary layer than it would be in a flow past a rigid boundary with similar stress and buoyancy forcing. Estimates of the terms in the turbulent kinetic energy equation indicate that, unlike in a flow past a rigid boundary, the dissipation rates cannot be balanced by local production terms, suggesting that the transport of TKE is important in the ocean surface boundary layer. A simple analytic model containing parameterizations of production, dissipation, and transport reproduces key features of the vertical profile of TKE, including enhancement near the surface. The effective turbulent diffusion coefficient for heat is larger than would be expected in a rigid-boundary boundary layer. This diffusion coefficient is predicted reasonably well by a model that contains the effects of shear production, buoyancy forcing, and transport of TKE (thought to be related to wave breaking). Neglect of buoyancy forcing or wave breaking in the parameterization results in poor predictions of turbulent diffusivity. Langmuir turbulence was detected concurrently with a fraction of the turbulence quantities reported here, but these times did not stand out as having significant differences from observations when Langmuir turbulence was not detected.

Eddies in the Canada Basin, Arctic Ocean, Observed from Ice-Tethered Profilers

Five ice-tethered profilers (ITPs), deployed between 2004 and 2006, have provided detailed potential temperature and salinity S profiles from 21 anticyclonic eddy encounters in the central Canada Basin of the Arctic Ocean. The 12–35-m-thick eddies have center depths between 42 and 69 m in the Arctic halocline, and are shallower and less dense than the majority of eddies observed previously in the central Canada Basin. They are characterized by anomalously cold and low stratification, and have horizontal scales on the order of, or less than, the Rossby radius of deformation (about 10 km). Maximum azimuthal speeds estimated from dynamic heights (assuming cyclogeostrophic balance) are between 9 and 26 cm s 1 , an order of magnitude larger than typical ambient flow speeds in the central basin. Eddy –S and potential vorticity properties, as well as horizontal and vertical scales, are consistent with their formation by instability of a surface front at about 80°N that appears in historical CTD and expendable CTD (XCTD) measurements. This would suggest eddy lifetimes longer than 6 months. While the baroclinic instability of boundary currents cannot be ruled out as a generation mechanism, it is less likely since deeper eddies that would originate from the deeper-reaching boundary flows are not observed in the survey region.

Acoustic travel‐time perturbations due to shallow‐water internal waves and internal tides in the Barents Sea Polar Front: Theory and experiment

During August 1992, a combined acoustics/physical oceanography experiment was performed to study both the acoustical properties and the ocean dynamics of the Barents Sea Polar Front in the region near Bear Island. Oceanographic observations from shipboard hydrography and moored sensors allowed the construction of the internal wave frequency spectrum for the area. A rapidly sampled tomographic section from a 224‐Hz, 16‐Hz‐bandwidth acoustic source to a 16‐element vertical receiving array enabled the monitoring of travel‐time fluctuations over the internal wave frequency band. To describe the measured acoustic fluctuations, theoretical expressions have been developed for the travel‐time variances which are functions of the internal wave oceanographic field, the local acoustic propagation characteristics, and the acoustical system’s properties. Both ray and mode theory expressions are generated, as the experiment was performed in shallow water and both ray and mode arrivals were resolvable. Comparison of the...

Significance of Langmuir circulation in upper ocean mixing : comparison of observations and simulations

Received 4 February 2009; revised 27 March 2009; accepted 20 April 2009; published 28 May 2009. [1] Representing upper ocean turbulence accurately in models remains a great challenge for improving weather and climate projections. Langmuir circulation (LC) is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum, heat, and mass in the oceanic surface layer. We present a direct comparison between observations and large eddy simulations, based on the wave-averaged Navier-Stokes equation, of an LC growth event. The evolution of cross-wind velocity variance and spatial scales, as well as mixed layer deepening are only consistent with simulations if LC effects are included in the model. Our results offer a validation of the large eddy simulation approach to understanding LC dynamics, and demonstrate the importance of LC in ocean surface layer mixing. Citation: Kukulka, T., A. J. Plueddemann, J. H. Trowbridge, and P. P. Sullivan (2009), Significance of Langmuir circulation in upper ocean mixing: Comparison of observations and simulations, Geophys. Res. Lett., 36, L10603, doi:10.1029/ 2009GL037620.

Bio‐optical variability associated with phytoplankton dynamics in the North Atlantic Ocean during spring and summer of 1991

Bio-optical data recorded from April 30 to July 19, 1991, using a mooring located in the open ocean (59°35.6°N, 20°57.9°W) are described and interpreted. Five multi-variable moored systems (MVMS) were deployed in the upper 90 m to obtain concurrent, co-located measurements of horizontal currents, water temperature, photosynthetically available radiation (PAR), transmission of light at 660 nm (c660), and stimulated chlorophyll fluorescence. In addition, meteorological and subsurface temperature data (12 depths from 80 to 310 m) were collected. When the mooring was deployed, surface waters were weakly stratified and there was little evidence of a phytoplankton bloom. Soon after the deployment, a marked increase in phytoplankton concentration occurred simultaneously with an increase of near-surface water temperature. The most striking observation was a period (year days 128–140) of strong mixed layer depth variability (daily amplitude of about 40 m) during which phytoplankton standing stock reached its maximum. During this period, phytoplankton biomass was mixed down to deeper waters at nighttime. As a result, the variability of the bio-optical parameters was extremely high, and deepwater phytoplankton concentration was much greater than would have been expected from the productivity estimates. Later, phytoplankton concentrations declined sharply in response to extremely stormy weather around year day 140. Once the storm passed (after day 143), surface waters stratified and the phytoplankton stock increased again, but the depth integrated biomass concentration did not reach as high values as before the storm. During this strong thermal stability period, fluorescence and c660 signals in near-surface waters were much higher than at depth, and displayed a diel cycle which was well correlated with PAR.