Coastal Ocean Dynamics Experiment Research Articles

Analysis of the Surface Dispersion in the Mediterranean Sub-Basins

Surface dispersion properties give an immediate characterization of the spreading of passive and active tracers in the ocean, like pollutant and marine species. The Mediterranean sub-basins (Tyrrhenian, Adriatic, Ionian, Levantine and Aegean) are known as complex dynamic regions due to the presence of coherent structures on different motion scales. This paper focus on dispersion of the surface Mediterranean flow using the surface current data derived from two different drifter designs: the Coastal Ocean Dynamics Experiment (CODE) and the Surface Velocity Program (SVP) drifters. The absolute dispersion for small time scales (15 days) and spatial scale larger than DI, our results emphasize a similarity in all the sub-basins with the presence of a quasi Random-walk regime and a quasi diffusive regime for the absolute and relative dispersion, respectively.

Assessment of the Water-Following Capabilities of CODE Drifters Based on Direct Relative Flow Measurements

AbstractDirect measurements of the relative water flow near the top and bottom of Coastal Ocean Dynamics Experiment (CODE) drifters were made in the northeast Pacific Ocean and the Mediterranean Sea in wind speeds as large as 15 m s−1. These measurements confirmed that the CODE drifter is a good Lagrangian drifter with a mean downwind slip of about 0.1% of the wind speed. Substantial mean vertical shears across the drifter (top 1 m below the surface) were observed, reaching an amplitude of 12 cm s−1 and corresponding to strong stratification (due to the proximity of river runoff) and strong winds.

Lagrangian Dispersion Characteristics in The Western Mediterranean

Dispersion characteristics in the Western Mediterranean are analyzed using data from Coastal Ocean Dynamics Experiment (CODE) and Surface Velocity Program (SVP) surface drifters deployed in the period 1986–2017. Results are presented in terms of absolute dispersion A2 (mean-squared displacement of drifter individuals) and of relative dispersion (D2; mean square separation distance of drifter pairs). Moreover, the dispersion characteristics are estimated for different initial separation distances (D0) between particles: smaller, larger, or comparable with the internal Rossby radius of deformation. Results show the presence of a quasiballistic regime for absolute dispersion at small time scales and the nonlocal relative dispersion regime related to the submesoscale activities for scales smaller than the internal Rossby radius. At intermediate times, two anomalous absolute dispersion regimes (elliptic and hyperbolic regimes) related with the flow topology are observed, although the relative dispersion involves the Richardson and shear/ballistic regimes only for D0 smaller than the Rossby radius. During the subsequent 20–30 days, absolute dispersion shows quasirandom walk regime and relative dispersion follows the diffusive regime for scales larger than 100 km for which pair velocities are uncorrelated.

A Biodegradable Surface Drifter for Ocean Sampling on a Massive Scale

AbstractTargeted observations of submesoscale currents are necessary to improve science’s understanding of oceanic mixing, but these dynamics occur at spatiotemporal scales that are currently challenging to detect. Prior studies have recently shown that the submesoscale surface velocity field can be measured by tracking hundreds of surface drifters released in tight arrays. This strategy requires drifter positioning to be accurate, frequent, and to last for several weeks. However, because of the large numbers involved, drifters must be low-cost, compact, easy to handle, and also made of materials harmless to the environment. Therefore, the novel Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifter was designed following these criteria to facilitate massive sampling of near-surface currents during the Lagrangian Submesoscale Experiment (LASER). The drifting characteristics were determined under a wide range of currents, waves, and wind conditions in laboratory settings. Results showed that the drifter accurately follows the currents in the upper 0.60 m, that it presents minimal wave rectification issues, and that its wind-induced slip velocity is less than 0.5% of the neutral wind speed at 10 m. In experiments conducted in both coastal and deep ocean conditions under wind speeds up to 10 m s−1, the trajectories of the traditional Coastal Ocean Dynamics Experiment (CODE) and the CARTHE drifters were nearly identical. Following these tests, 1100 units were produced and deployed during the LASER campaign, successfully tracking submesoscale and mesoscale features in the Gulf of Mexico. It is hoped that this drifter will enable high-density sampling near metropolitan areas subject to stress by the overpopulation, such as lakes, rivers, estuaries, and environmentally sensitive areas, such as the Arctic.

Improved Surface Velocity and Trajectory Estimates in the Gulf of Mexico from Blended Satellite Altimetry and Drifter Data

AbstractThis study investigates the results of blending altimetry-based surface currents in the Gulf of Mexico with available drifter observations. Here, subsets of trajectories obtained from the near-simultaneous deployment of about 300 Coastal Ocean Dynamics Experiment (CODE) surface drifters provide both input and control data. The fidelity of surface velocity fields are measured in the Lagrangian frame by a skill score that compares the separation between observed and hindcast trajectories to the observed absolute dispersion. Trajectories estimated from altimetry-based velocities provide satisfactory average results (skill score > 0.4) in large (~100 km) open-ocean structures. However, the distribution of skill score values within these structures is quite variable. In the DeSoto Canyon and on the shelf where smaller-scale structures are present, the overall altimeter skill score is typically reduced to less than 0.2. After 3 days, the dataset-averaged distance between hindcast and drifter trajectories, , is about 45 km—only slightly less than the average dispersion of the observations, km. Blending information from a subset of drifters via a variational method leads to significant improvements in all dynamical regimes. Skill scores typically increase to 0.8 with reduced to less than half of . Blending available drifter information with altimetry data restores velocity field variability at scales not directly sampled by the altimeter and introduces ageostrophic components that cannot be described by simple Ekman superposition. The proposed method provides a means to improve the fidelity of near-real-time synoptic estimates of ocean surface velocity fields by combining altimetric data with modest numbers of in situ drifter observations.

Wind Effects on Drogued and Undrogued Drifters in the Eastern Mediterranean

Abstract The wind effects on drogued and undrogued drifters are assessed using Coastal Ocean Dynamics Experiment (CODE) and Surface Velocity Program (SVP) drifter datasets and ECMWF wind products in the eastern Mediterranean. Complex and real linear regression models are used to estimate the relative slip of undrogued SVP drifters and to extract the wind-driven currents from the drifter velocities. The frequency response of the wind-driven currents is studied using cross-spectral analysis. By comparing the velocities of cotemporal and nearly collocated undrogued and drogued SVP drifters, it appears that undrogued SVP drifters have a general downwind slippage of about 1% of the wind speed. Time-lagged complex correlations and cross-spectral results show that the wind response is almost simultaneous. The velocities of SVP drifters drogued to 15 m are poorly correlated with the winds (R2 ≈ 3%): wind-driven currents have a magnitude of 0.7% of the wind speed and are 27°–42° to the right of the wind. For undrogued SVP drifters, the correlation with the winds increases to R2 ≈ 22% and the angle between winds and currents decreases to 17°–20°. The magnitude of the wind-driven currents is about 2% of the wind speed. For CODE designs, wind-driven currents are 1% of the wind speed at an angle of about 28° to the right of the wind (R2 ≈ 8%). Spectral and cospectral analyses reveal that the drifters sampled more anticyclonic than cyclonic motions. The inner coherence spectra show that wind and currents are more correlated at temporal scales spanning 3–10 days. They also confirm that the wind response is quasi-simultaneous and that currents are generally to the right of the wind.

Wind effects on drogued and undrogued drifters in the Eastern Mediterranean

The wind effects on drogued and undrogued drifters are assessed using Coastal Ocean Dynamics Experiment (CODE) and Surface Velocity Program (SVP) drifter datasets and ECMWF wind products in the eastern Mediterranean. Complex and real linear regression models are used to estimate the relative slip of undrogued SVP drifters and to extract the wind-driven currents from the drifter velocities. The frequency response of the wind-driven currents is studied using cross-spectral analysis. By comparing the velocities of cotemporal and nearly collocated undrogued and drogued SVP drifters, it appears that undrogued SVP drifters have a general downwind slippage of about 1% of the wind speed. Time-lagged complex correlations and cross-spectral results show that the wind response is almost simultaneous. The velocities of SVP drifters drogued to 15 m are poorly correlated with the winds (R2 ≈ 3%): wind-driven currents have a magnitude of 0.7% of the wind speed and are 27°–42° to the right of the wind. For undrogued SVP drifters, the correlation with the winds increases to R2 ≈ 22% and the angle between winds and currents decreases to 17°–20°. The magnitude of the wind-driven currents is about 2% of the wind speed. For CODE designs, wind-driven currents are 1% of the wind speed at an angle of about 28° to the right of the wind (R2 ≈ 8%). Spectral and cospectral analyses reveal that the drifters sampled more anticyclonic than cyclonic motions. The inner coherence spectra show that wind and currents are more correlated at temporal scales spanning 3–10 days. They also confirm that the wind response is quasi-simultaneous and that currents are generally to the right of the wind.

A novel technique for detection of the toxic dinoflagellate, Karenia brevis, in the Gulf of Mexico from remotely sensed ocean color data

Karenia brevis, a toxic dinoflagellate that blooms regularly in the Gulf of Mexico, frequently causes widespread ecological and economic damage and can pose a serious threat to human health. A means for detecting blooms early and monitoring existing blooms that offers high spatial and temporal resolution is desired. Between 1999 and 2001, a large bio-optical data set consisting of spectral measurements of remote-sensing reflectance ( R rs( λ)), absorption ( a( λ)), and backscattering ( b b( λ)) along with chlorophyll a concentrations and K. brevis cell counts was collected on the central west Florida shelf (WFS) as part of the Ecology and Oceanography of Harmful Algal Blooms (ECOHAB) and Hyperspectral Coastal Ocean Dynamics Experiment (HyCODE) programs. Reflectance model simulations indicate that absorption due to cellular pigmentation is not responsible for the factor of ∼3–4 decrease observed in R rs( λ) for waters containing greater than 10 4 cells l −1 of K. brevis. Instead, particulate backscattering is responsible for this decreased reflectivity. Measured particulate backscattering coefficients were significantly lower when K. brevis concentrations exceeded 10 4 cells l −1 compared to values measured in high-chlorophyll (>1.5 mg m −3), diatom-dominated waters containing fewer than 10 4 cells l −1 of K. brevis. A classification technique for detecting high-chlorophyll, low-backscattering K. brevis blooms is developed. In addition, a method for quantifying chlorophyll concentrations in positively flagged pixels using fluorescence line height (FLH) data obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) is introduced. Both techniques are successfully applied to Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and MODIS data acquired in late August 2001 and validated using in situ K. brevis cell concentrations.

Coastal ocean optical influences on solar transmission and radiant heating rate

An extensive set of physical and optical measurements is utilized to characterize the processes and quantify parameters that contribute to the variability of solar transmission, sea surface albedo, and radiant heating rate (RHR). This study is among the first to utilize multidisciplinary observations coupled with radiative transfer simulations to investigate the impact of optical properties on solar transmission, albedo, and heating in nearshore coastal waters. The data were collected from a shallow‐water coastal mooring as part of the Hyperspectral Coastal Ocean Dynamics Experiment (HyCODE) in summer 2001. Over the 41‐day time series, the average loss in solar radiation was 274 W m−2 for mean surface radiation of 365 W m−2 (average solar transmission of 21%). Quantitative coherence and principle component analyses suggest that cloud cover, chlorophyll concentration (Chl), and colored dissolved organic matter (CDOM) have the greatest impacts on solar transmission variability on timescales of ∼1 week. Radiative transfer simulations show that Chl, absorption, and attenuation have the most significant impact on solar transmission, whereas solar angle and cloud cover greatly influence albedo.

A modeling study of shelf circulation off northern California in the region of the Coastal Ocean Dynamics Experiment 2. Simulations and comparisons with observations

This is the second part of a modeling study of wind‐forced flow on the continental shelf off northern California in the region (37°–40°N) of the Coastal Ocean Dynamics Experiment (CODE). Gan and Allen [2002] analyzed the shelf flow response to idealized wind stress forcing in a process‐oriented study. The study here applies forcing from observed winds and heat flux for April–May 1982 and compares the model results with moored current and temperature measurements. The Princeton Ocean Model (POM) is used in a three‐dimensional limited area domain with a high‐resolution curvilinear grid (approximately 1 km horizontal spacing, 60 vertical levels) and realistic coastline and bottom topography. The objectives of the study are to simulate the response of the shelf circulation field to time‐varying observed wind stress and heat flux, to compare model results with oceanographic observations to establish confidence in the model, and to subsequently analyze the model fields and the model dynamical balances to help understand the behavior of the observed flow. The model variables show overall good agreement with corresponding observations. Similar to the conclusions by Gan and Allen [2002], it is found that the alongshore variability of upwelling is mainly controlled by the interaction of the wind‐forced shelf flow with the coastline and bottom topography. Different dynamical regimes in the regions north and south of the coastal capes formed by Pt. Reyes and Pt. Arena and in the more uniform region between these capes are identified and investigated. The results demonstrate that the coastal capes play a dominant role in causing alongshore variability of the upwelling flow, including the setup of an alongshore pressure gradient that forces northward currents during relaxation of southward upwelling favorable winds. An analysis of the balance of terms in the equation for potential temperature indicates that across‐shore temperature advection is the major contributor to the cooling of coastal water during upwelling, with a larger magnitude to the south of the capes. To the north of the capes, however, alongshore temperature advection is the dominant contributor to the colder water near the coast.

A modeling study of shelf circulation off northern California in the region of the Coastal Ocean Dynamics Experiment: Response to relaxation of upwelling winds

A two‐part modeling study of the wind‐forced flow on the continental shelf off northern California in the region (37°–40°N) of the Coastal Ocean Dynamics Experiment (CODE) is pursued. This paper involves a process‐oriented study with idealized wind stress forcing. Gan and Allen [2002] involves forcing with observed winds and heat flux for April–May 1982 and comparison of model results with CODE observations. A characteristic, but previously unexplained, response observed during CODE following the weakening, or relaxation, of southward upwelling favorable winds is the time‐dependent development of northward currents over the inner shelf next to the coast. The presence of northward winds is not necessary for this occurrence. The objective in this paper is to investigate the dynamics of the shelf flow response to upwelling wind relaxation events under idealized conditions. In the basic case experiment a spatially uniform upwelling favorable southward wind stress of 0.1 Pa is applied to the ocean initially at rest. The stress is held constant for 10 days and then decreased linearly to zero over 3 days. In response to the southward wind stress, southward alongshore currents develop on the shelf accompanied by upwelling of cold, dense water near the coast. Considerable spatial variability in the shelf flow, clearly related to the alongshore variations in coastline and bottom topography, is found. The alongshore currents tend to separate from the coast south of capes, and the coldest surface water is found at those locations. As the winds decrease, northward currents, similar to those observed, develop on the inner shelf next to the coast at many alongshore locations. Examination of the alongshore momentum balances shows that the northward currents are forced by a northward pressure gradient force associated with negative alongshore pressure gradients. These pressure gradients are set up by the interaction of the wind‐forced flow with the alongshore variations in shelf topography. In general, negative alongshore pressure gradients, intensified off Pt. Reyes and Pt. Arena by the gradient wind balance, are found south of capes. The negative pressure gradients geostrophically balance onshore flow at depth, and upwelling is strengthened in these locations. North of capes, positive pressure gradients that are primarily in balance with nonlinear advective effects are found. After the winds cease the forced across‐shelf circulation weakens, and the resulting unbalanced negative pressure gradients south of the capes accelerate the alongshore currents northward. Processes with similar dynamics are found embedded in the more complex coastal ocean response to observed time varying winds by Gan and Allen [2002].

Nearshore physical processes and bio‐optical properties in the New York Bight

Temporal and spatial variability of physical, biological, and optical properties on scales of minutes to months and meters to ∼50 km are examined using an extensive data set collected on the New York Bight continental shelf during the Hyperspectral Coastal Ocean Dynamics Experiment. Measurements from a midshelf mooring and bottom tripod (∼25 km offshore, 24 m water depth) and two nearshore profiling nodes (∼5 km offshore, 15 m water depth) are utilized to quantify and correlate midshelf and nearshore variability. Towed shipboard undulating profilers and a high‐frequency radar (CODAR) array provide complementary spatial data. We show that phytoplankton and dissolved matter each accounted for roughly 50% of total absorption (440 nm) at midshelf. In contrast, particulate compared to gelbstoff absorption dominated total absorption at the nearshore location. A relatively high‐salinity, low‐temperature, low particulate coastal jet decreased turbidity nearshore and advected lower‐salinity, higher‐chlorophyll waters to the midshelf region, resulting in increased biomass at midshelf. Small‐scale (order of a few kilometers) convergence and divergence zones formed from the interaction of semidiurnal tides with mean currents and a water mass/turbidity front. The front resulted in increased decorrelation scales from nearshore (∼1 day) toward midshelf (2–3 days) for optical and biological parameters. We conclude that optical and biological variability and distributions at midshelf and nearshore locations were influenced mainly by semidiurnal tides and the coastal jet. We present insights into nearshore coastal processes and their effects on biology and optics as well as for the design of future nearshore interdisciplinary coastal programs.

Observations of high‐frequency internal waves in the Coastal Ocean Dynamics Region

Current meter data from the second Coastal Ocean Dynamics Experiment (CODE II) for July 1982 are analyzed for internal waves in the 6 to 40 cycles per day (cpd) frequency band. It is found that the wave field is anisotropic and that the current ellipses are oriented in approximately the cross‐isobath direction. The squares of the ratio of the major to minor axes of the current ellipses (the “ellipticity”) are consistent with a continuum of internal waves propagating onshore but are not consistent with a single wave propagating onshore. The reduction of internal wave energy across the shelf is consistent with propagation from the deep ocean or shelf break, as is the correlation between vertical velocities and velocities parallel to the minor axis. However, there is evidence for the generation of additional internal wave energy on the shelf in the evolution of the current ellipses across the shelf and in the bluing of the internal wave spectra across the shelf. Internal wave energy levels are elevated by a factor of 1.5 to 5 above Garrett and Munk [1972] levels at the moorings on the 130 and 365 m isobaths. The first vertical mode dominates at the 130 m isobath, the only mooring for which the vertical modal analysis was done.

Surface heat flux variability over the northern California shelf

Surface heat flux components are estimated at a midshelf site over the northern California shelf using moored measurements from the 1981–1982 Coastal Ocean Dynamics Experiment (CODE) and the 1988–1989 Shelf Mixed Layer Experiment (SMILE). Time series of estimated fluxes extend from early winter through summer upwelling conditions, allowing examination of seasonal variations as well as synoptic events. On a seasonal timescale, the surface heat flux is strongly influenced net surface heat flux are the annual variation in incident shortwave solar radiation (insolation) and the atmospheric spring transition. Between mid‐November 1988 and late February 1989, insolation is weak and the mean daily averaged heat flux is nearly zero (absolute value less than 10 W m−2), with a standard deviation of ∼50 W m−2. Beginning in March, insolation increases markedly, and typical daily‐averaged heat fluxes increase to greater than 100 W m−2 by the spring transition in April or May. In June and July, the average heat flux is near 200 W m−2, with a standard deviation of ∼90 W m−2. In winter, the daily‐averaged heat flux varies on periods of several days. Net heat flux losses can range up to 130 W m−2. These losses are not identified with any one type of event. For example, comparable heat flux losses can occur for very low relative humidities (RHs), moderate winds, and clear skies, and for high RHs, high winds, and cloudy skies. In summer, surface heat flux variability is strongly influenced by upwelling and relaxation events. Upwelling is characterized by clear skies and high equatorward winds, while relaxation is characterized by the presence of clouds and low or northward winds. These conditions lead to opposing changes in insolation and in longwave radiative cooling and latent heat flux. Variability in insolation dominates, and the daily‐averaged heat flux into the ocean is greatest during upwelling events (up to 350 W m−2 or more) and least during relaxation events (sometimes less than 100 W m−2).

Coastal Boundary Conditions and the Baroclinic Structure of Wind-Driven Continental Shelf Currents*

Abstract The generation of continental shelf currents by wind forcing is investigated by analytical and numerical methods. The investigation is motivated by observations from the Coastal Ocean Dynamics Experiment. A central assumption is that the vertical structure of the response over the inshore half of the shelf is controlled by the vertical distribution of the turbulent stress. This suggests a two-layer model of the wind-driven circulation, in which the upper layer represents a surface wind-mixed layer, and the lower layer represents the remainder of the fluid. The response of this idealized dynamical model to wind forcing is examined and compared with observations in the 2–7-day period band. For the alongshore velocity gain relative to local wind stress, an onshore surface maximum and an offshore interior maximum are robustly reproduced by the model. These features are evidently related to a dynamical transition over the inner half of the shelf, in which the alongshore wind stress is balanced more by...

Bulk parameterization of momentum, heat, and moisture fluxes over a coastal upwelling area

Aircraft measurements of turbulent fluxes of momentum, sensible heat, and moisture were used to calculate bulk aerodynamic coefficients over the northern California coastal shelf during wintertime (Shelf MIxed Layer Experiment, SMILE) and summertime (Coastal Ocean Dynamics Experiment, CODE). Low‐level (30 m) data from 15 SMILE flights and 15 CODE flights with varying stability conditions, wind directions, and intensities were reduced to the standard reference height of 10 m using diabatic flux‐profile relations following Monin‐Obukhov similarity theory. The fluxes were parameterized by bulk aerodynamic formulas after reduction to neutral stability. Results for summer and winter were in reasonably good agreement if the bulk coefficients are averaged over intervals of 1 m s−1 and linear regression with speed is performed using the averaged values. The neutral drag coefficient showed a statistically significant linear dependence on wind speed at 10‐m height, while the neutral Stanton and Dalton numbers (the heat and moisture transfer coefficients) were independent of wind speed and were not significantly different from each other. The response of the ocean to the atmospheric forcing and the feedback of the resulting ocean circulation on the wind forcing were studied using a vertically resolved numerical model of coastal upwelling. The model was run using buoy observations of wind speed, air temperature, atmospheric pressure, and relative humidity from SMILE and CODE. The resulting coastal upwelling produced a negative feedback through decreased sea surface temperature at the coast and increased atmospheric stability which decreased the effective drag coefficient (and hence the wind stress) by at most 30% near the coast, with an average decrease of about 12% within 100 km from the coast. The effect of this decrease in wind stress, however, produced an insignificant change in the sea surface temperature field, which indicates that parameterization of coastal drag coefficients for modeling coastal upwelling can be done without taking into account surface temperature feedback.

Effects of Wind Stress and Wind Stress Curl Variability on Coastal Upwelling

Abstract Aircraft measurements during the winter 1989 Shelf Mixed Layer Experiment (SMILE) and summer 1982 Coastal Ocean Dynamics Experiment (CODE) were used to characterize the spatial variation of the low-level wind and wind stress over the northern California shelf. The curl of the wind stress was calculated from directly measured turbulent stress components. The accuracy of the computed curl was estimated to be adequate to map the spatial structure. Wintertime measurements showed a concentration of large positive curl [over 1 Pa (100 km)−1] west of Point Arena, regardless of wind direction, due to the effects of the coastal topography on the wind fields. Results from summertime measurements showed a similar local maximum of positive curl west of Point Arena. Larger curl values [over 3.5 Pa (100 km)−1], however, were observed across a hydraulic jump propagating from Stewarts Point for highly supercritical marine boundary-layer flow. A two-layer, vertically integrated numerical model of coastal upwellin...

The Horizontal Momentum Balance in the Marine Atmospheric Boundary Layer during CODE-2

Abstract The horizontal momentum balance in the marine atmospheric boundary layer during the Coastal Ocean Dynamics Experiment (CODE) is analyzed, using meteorological data from an array of surface moorings. Previous studies have indicated the presence of orographically generated mesoscale features that are induced by strong southward flow around Point Arena. The present analysis demonstrates that during periods of strong southward flow, the cross-shore momentum equation is dominated by a balance between the ageostrophic acceleration associated with the flow curvature around Point Arena, and the cross-shore pressure gradient, while the along-shore momentum equation is dominated by a balance between vertical stress divergence and alongshore pressure gradient. These balances are consistent with results from a shallow water model of the marine layer. The calculations provide evidence for orographic modification of the horizontal structure of the boundary layer under a broader range of southward flow conditio...

Current Dynamics over the Northern California Inner Shelf

Abstract Subtidal current dynamics at a northern California inner-shelf site are analysed using moored current observations in 30 m of water, in conjunction with wind and bottom pressure measurements acquired during the summer of 1981 as part of the first Coastal Ocean Dynamics Experiment. The subtidal flow is driven locally by both an alongshelf wind stress and an alongshelf pressure gradient, which tend to be similar in magnitude but opposite in direction. Model depth-average alongshelf currents are about twice as large as observed. Analyses suggest that this discrepancy is due to a larger drag on the inner-shelf currents than suggested by bottom tripod measurements at the site, due to the presence of large rock outcrops over the inner shelf in this region. A notable characteristic of the observations is the weakness of the alongshelf flow over the inner shelf; alongshelf current standard deviations are a factor of 4 smaller than at midshelf. Analyses suggest this is due to a decrease in the wind stress...

On the effect of the alongshore pressure gradient on numerical simulations over the Northern California Continental Shelf

Chen and Wang's (1990) two‐dimensional shelf circulation model has been extended to include the alongshore (the dimension absent in the model) pressure gradient. The alongshore pressure gradient, calculated independently from a linear coastal‐trapped wave model, is included as an external “forcing”. The extended model has been used to simulate the temperature and velocity fields observed during the Coastal Ocean Dynamics Experiment 2 (CODE 2) in the spring and summer of 1982 on the Northern California Continental Shelf. The results were compared to the CODE 2 observations and to the results of the purely two‐dimensional and coastal trapped wave models. Results using the extended model show some improvement in the cross‐shore and alongshore velocity fluctuations. The differences between the models are explained in terms of the opposition between the wind stress and the pressure gradient in the alongshore direction, which reduces the magnitude of the alongshore velocity. The reduced magnitude of the alongshore flow decreases the alongshore bottom stress which, in turn, reduces the offshore transport in the bottom boundary layer.