Clinton D. Winant

Effect of a kelp forest on coastal currents

Abstract Ocean currents supply a kelp ecosystem with nutrients, planktonic food, and larvae. We have found that these currents in a kelp forest (Macrocystis pyrifera) are slower than currents outside. At the Pt. Loma, San Diego, California, site that we studied, current velocities were about a third of those outside. A comparison of frequency spectra shows that semi-diurnal frequencies are preferentially passed by the kelp. This effect of a kelp forest on the currents that nurture it is similar to that of a terrestrial forest on local winds.

The Marine Layer off Northern California: An Example of Supercritical Channel Flow

Abstract During the spring and summer, northerly winds driven by the North Pacific high pressure system are prevalent over the Northern California continental shelf, only interrupted for periods of a few days, when weak or southerly winds occur. In the course of the Coastal Ocean Dynamics Experiment (CODE), fixed station and observations were made to describe the temporal and spatial structure of the lower atmosphere, and their relation to the strong upwelling of coastal waters in a region extending up to 40 km offshore and 100 km along the coast. These observations suggest that atmospheric conditions during the spring and summer usually fall into one of three categories: the surface wind can be everywhere weak (Pattern 1), it can blow at large speeds in a uniform pattern (Pattern 2), or finally the structure of the northerly surface wind can be complex, with large changes in the wind speed and corresponding changes in the surface pressure over short spatial scales (Pattern 3), The latter pattern, which o...

Seasonal patterns of wind stress and wind stress curl over the Gulf of Mexico

Meteorological observations from an array of stations deployed along the periphery of the Gulf of Mexico, between 1990 and 1993, are used to describe the seasonal fluctuations in patterns of atmospheric variables from a contemporary set of measurements. Seasonal maps of wind stress based on these measurements resemble wind stress maps based on ship observations, as published by Elliott [1979], rather than maps based on analyses of numerical weather forecasts, as published by Rhodes et al. [1989], particularly near the western boundary of the gulf. Seasonal maps of wind stress curl are characterized by positive curls over the western and southwestern gulf. The central result of this study is to document the important role of the mountain chain which extends along the southwestern section of the gulf in channeling the wind toward the Isthmus of Tehuantepec.

FVCOM validation experiments : comparisons with ROMS for three idealized barotropic test problems

[1] The unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM) is evaluated using three idealized benchmark test problems: the Rossby equatorial soliton, the hydraulic jump, and the three-dimensional barotropic wind-driven basin. These test cases examine the properties of numerical dispersion and damping, the performance of the nonlinear advection scheme for supercritical flow conditions, and the accuracy of the implicit vertical viscosity scheme in barotropic settings, respectively. It is demonstrated that FVCOM provides overall a second-order spatial accuracy for the vertically averaged equations (i.e., external mode), and with increasing grid resolution the model-computed solutions show a fast convergence toward the analytic solutions regardless of the particular triangulation method. Examples are provided to illustrate the ability of FVCOM to facilitate local grid refinement and speed up computation. Comparisons are also made between FVCOM and the structured-grid Regional Ocean Modeling System (ROMS) for these test cases. For the linear problem in a simple rectangular domain, i.e., the winddriven basin case, the performance of the two models is quite similar. For the nonlinear case, such as the Rossby equatorial soliton, the second-order advection scheme used in FVCOM is almost as accurate as the fourth-order advection scheme implemented in ROMS if the horizontal resolution is relatively high. FVCOM has taken advantage of the new development in computational fluid dynamics in resolving flow problems containing discontinuities. One salient feature illustrated by the three-dimensional barotropic winddriven basin case is that FVCOM and ROMS simulations show different responses to the refinement of grid size in the horizontal and in the vertical.

On the Mean Circulation in the Mid-Atlantic Bight

Abstract Two possible mechanisms which may drive the observed mean alongshelf flow in the Mid-Atlantic Bight are described. Runoff from concentrated sources could conceivably force this flow; however, the one-layer homogeneous model results of Csanady (1978) and Beardsley and Hart (1978) imply that the observed shelf flow is not driven by runoff alone. On the other hand. the Semtner and Mintz (1977) numerical model of the North Atlantic strongly suggests that the shelf circulation is just a boundary layer component of the ocean circulation and thus driven by the large-scale wind stress and heat flux distributions. This model result supports Csanadys (1978) conclusion that the physical mechanism which creates the alongshelf pressure gradient thought to drive the alongshelf flow must be of oceanic origin.

A Comparison of Some Shallow Wind-Driven Currents

Abstract Four sets of current measurements made in water depths ranging between 28 and 38 m over periods ranging from three to five weeks are examined and compared. The response of the water column to wind forcing is examined by computing regression coefficients between the surface wind stress and two different parameterizations of bottom stress in terms of measured currents. Coefficients computed for the different data sets vary by as much as a factor of 4. While such variations might be due to instrumental differences, it seems more likely that the assumed dynamical balance between surface and bottom stress is incomplete, i.e., other forces such as the alongshore pressure gradient are quantitatively important even when the water depth is comparable to the turbulent Ekman layer thickness.

Diurnal variability in currents and temperature on the continental shelf between central and southern California

[1]xa0Long time series are used to describe the diurnal variability in the meteorological and astronomical tidal forcing and the temperature and current response on the continental shelf of California between Port San Luis and Port Hueneme. Spectral characteristics are used to attribute the response to each of the forcing mechanisms. The diurnal temperature field is decomposed into two dominant vertical modes. A surface-enhanced mode is coherent with the meteorological forcing, suggesting that it is a direct response to local heating and wind forcing. The amplitude of the second mode is maximum at middepth. Spectra of this mode exhibit multiple peaks at frequencies corresponding to the astronomical tidal forcing. The phase difference between the middepth mode and the tidally dominated sea level is not consistent across the region. We conclude that this mode is a localized baroclinic response to the astronomical tidal forcing. The diurnal velocity field is dominated by a clockwise component. At the surface this component is coherent across the study region, and it is also coherent with the diurnal wind stress, suggesting that it is a direct response to the diurnal heating and wind forcing. Coherence with the surface-enhanced temperature mode further confirms this. A weaker, barotropic counterclockwise component is also present in the diurnal velocity field. Spectra of this component exhibit multiple peaks at periods corresponding to the astronomical tidal forcing. The phase difference between this component and the surface elevation varies little between the observational sites. We conclude that this velocity component represents a barotropic response to astronomical forcing that is coherent over the study area.

Covariation of coastal water temperature and microbial pollution at interannual to tidal periods

[1]xa0Water temperature and fecal indicator bacteria levels covary along the shoreline of Huntington and Newport Beach, California at interannual to tidal periods. During summer, cooler than average waters caused by interannual variability in sea surface temperature (SST), synoptic upwelling, and tidal-period cooling are coincident with elevated levels of microbial pollution in the surf zone. This relationship can be explained by the effects of weakening in stratification on the fate of a waste water plume and the prolonged persistence of fecal indicator bacteria in colder waters. During winter, warmer than average water caused by basin scale oscillations and atmospheric and oceanographic processes that contribute to the Multivariate El Nino Southern Oscillation Index are indicative of elevated total coliform levels in the surf zone. The elevated coliform levels can be ascribed to increased rainfall, and the resultant storm water inflow to the surf zone.

Wind-Driven Subinertial Circulation inside a Semienclosed Bay in the Gulf of California

AbstractMoored current and pressure observations were obtained at Bahia Concepcion, a semienclosed bay located on the eastern side of the Baja California peninsula in Mexico, to describe the wind-driven subinertial circulation. In winter and early spring, the bay is well mixed and forced by persistent winds toward the southeast, aligned with the central axis. The authors’ observations show that the sea surface rises downwind in response to wind stress and that there exists a crosswind drift at the surface that is consistent with Ekman dynamics. This feature is typical of a bay that is deeper than one Ekman depth and hence affected by the rotation of the earth. There is a persistent along-bay circulation toward the end of the bay along its western side with return flow on the opposite side. Drifters released near the surface across a transect move westward and downwind toward the closed end, where they recirculate cyclonically. Wind-driven linear theoretical models successfully predict the observed cross-b...