Showing papers in "Journal of Physical Oceanography in 1994"

Modeling Wave-Enhanced Turbulence in the Ocean Surface Layer

Abstract: Until recently, measurements below the ocean surface have tended to confirm “law of the wall” behavior, in which the velocity profile is logarithmic, and energy dissipation decays inversely with depth. Recent measurements, however, show a sublayer, within meters of the surface, in which turbulence is enhanced by the action of surface waves. In this layer, dissipation appears to decay with inverse depth raised to a power estimated between 3 and 4.6. The present study shows that a conventional model, employing a “level 2½” turbulence closure scheme predicts near-surface dissipation decaying as inverse depth to the power 3.4. The model shows agreement in detail with measured profiles of dissipation. This is despite the fact that empirical constants in the model are determined for situations very different from this near-surface application. The action of breaking waves is modeled by a turbulent kinetic energy input at the surface. In the wave-enhanced layer, the downward flux of turbulent kinetic en...

Interaction between the Subtropical and Equatorial Ocean Circulations: The Subtropical Cell

Abstract: Three versions of a 2 1/2-layer ocean model are used to study the subtropical cell (STC), a shallow, meridionalcirculation cell consisting of subtropical subduetion, equatorward advection of cool subsurface water into thetropics, upwelling at the equator, and poleward advection of warm surface water back to midlatitudes. Thethree versions are a steady-state analytic model, a numerical model with constant layer temperatures, and anumerical model with variable layer temperatures and active thermodynamics. Two different pammeterizationsof mixed-layer processes are utilized to determine how water moves between the lwo layers. In the simplerparameterization, entrainment and detrainment rates, we and wd, are specified so that the upper-layer thicknessh1 relaxes back to an externally prescribed thickness; in the other, they are related to the surface heat flux Q.in both versions detrainment is cut off at the latitude yd = 18° to prevent subduction from occurring in thetropics. Solutions are obtained in ...

Forced Stage Response to a Moving Hurricane

Abstract: The upper ocean's response to three hurricanes [Norbert (1984), Josephine (1984) and Gloria (1985)] is examined using field observations and a numerical ocean model. Our goal is to describe the physical processes that determine the structure and amplitude of hurricane-driven upper-ocean currents. All three of these Northern Hemisphere hurricanes produced a rightward-biased response of the mixed-layer current and transport. This asymmetry arises because the wind stress vector rotates clockwise on the right side of the track and remains nearly parallel with the inertially rotating mixed-layer current during most of the hurricane passage. The maximum observed mixed-layer current varied from 0.8 m s−1 in response to Josephine, which was a large but comparatively weak hurricane, to 1.7 m s−1 in response to Gloria, which was very large and also intense. These cases have been simulated with a three-dimensional numerical model that includes a treatment of wind-driven vertical mixing within the primitive ...

Heating rate within the upper ocean in relation to its bio-optical state

Abstract: Solar radiation absorption and local heating within the upper layers of the open ocean are strongly influenced by the abundance of phytoplankton as depicted by the chlorophyll concentration According to whether this concentration is high or low, the heat deposition occurs within a layer that may vary in thickness from low than 10 m to more than 100 m A simple parameterization, accounting for this dependence, is developed It allows the vertical profiles of heating rate to be predicted from the phytoplanktonic pigment concentration, as it can (and will) be remotely detected from space, by using ocean color sensors This computationally efficient parameterization has been validated in reference to the results of a full spectral model In the simplified computation, the solar spectrum is partitioned into two domains, below and above the wavelength 075 µm For the infrared waveband, not influenced by biological materials the irradiance profile is described by a single exponential function For the

The Deacon Cell and the other meridional cells of the Southern ocean

Abstract: The meridional circulation cells of the Southern Ocean are investigated using the results from a fine-resolution primitive equation model. Zonal integration along depth levels shows the classical series of meridional cells but integration along density layers shows a number of differences, including the virtual disappearance of the Deacon cell. To investigate the differences, the meridional transport is calculated as a function of both density and depth. The results show that the Deacon cell is associated with systematic changes in the depth of density surfaces between the western boundary current region off South America and the return flow in the interior of the ocean. Water flowing on each density surface produces a meridional cell with a vertical excursion of a few hundred meters. Thew cells combine, without water crossing density surfaces, to produce a single integrated Deacon cell extending from the surface to below 2000 m. The results also show that, at each latitude, water on each of the ...

A Hybrid Vertical Mixing Scheme and Its Application to Tropical Ocean Models

Abstract: A novel hybrid vertical mixing scheme, based jointly on the Kraus–Turner-type mixed layer model and Price's dynamic instability model, is introduced to aid in parameterization of vertical turbulent mixing in numerical ocean models. The scheme is computationally efficient and is capable of simulating the three major mechanisms of vertical turbulent mixing in the upper ocean, that is, wind stirring, shear instability, and convective overturning. The hybrid scheme is first tested in a one-dimensional model against the Kraus–Turner-type bulk mixed layer model and the Mellor–Yamada level 2.5 (MY2.5) turbulence closure model. As compared with those two models, the hybrid model behaves more reasonably in both idealized experiments and realistic simulations. The improved behavior of the hybrid model can be attributed to its more complete physics. For example, the MY2.5 model underpredicts mixed layer depth at high latitudes due to its lack of wind stirring and penetrative convection, while the Kraus–Turn...

Trapping of a Coastal Density Front by the Bottom Boundary Layer

Abstract: The dynamics of a surface-to-bottom density front on a uniformly sloping continental shelf and the role of density advection in the bottom boundary layer are examined using a three-dimensional, primitive equation numerical model. The front is formed by prescribing a localized freshwater inflow through the coastal boundary. The resulting freshwater plume turns anticyclonically and moves along the coast, generating offshore transport in the bottom boundary layer, which advects freshwater offshore and creates a sharp surface-to-bottom density front with a surface-intensified alongshelf jet over the front. The offshore buoyancy flux in the bottom boundary layer moves the front offshore until it reaches a depth where the vertical shear within the front leads to a reversal in the cross-shelf velocity at the shoreward edge of the front. Consequently, the offshore buoyancy flux in the bottom boundary layer vanishes shoreward of the front. Within the front, a steady balance is established in the bottom bo...

Interannual Sea Level in the Northern and Eastern Indian Ocean

Abstract: Monthly Indian and Pakistani sea level records, adjusted for the effect of atmospheric pressure, were used to examine interannual sea level variability in the northern Indian Ocean. The interannual sea level is correlated along the boundary. The observations hint that interannual sea level propagates along the boundary, but the evidence is not conclusive. Calculations with the Comprehensive Ocean-Atmosphere Data Set wind stress as input to a simple model suggest that the interannual sea level signal occurs along more than 8000 km of Indian Ocean coastline extending from southern Java to Bombay and is generated remotely by zonal interannual winds blowing along the equator. The eastern Indian Ocean boundary is broken between Indonesia and Australia and examination of north- western Australian interannual sea level shows that it is not well correlated with that in the northern Indian Ocean. The northwestern Australian sea level is larger in amplitude, related to ENSO, and of Pacific origin. Calculat...

Diagnosing Climate Change and Ocean Ventilation using Hydrographic Data

Abstract: Changes in atmospheric forcing can affect the subsurface water column of the ocean by three different mechanisms. First, warmed mixed-layer water that is subducted into the ocean interior will cause subsurface warming; second, the subducted surface water can be freshened through changes in evaporation and precipitation; and third, the properties at a given depth may be changed by the vertical displacement of isotherms and isohalines without changes of water masses. These vertical displacements of the water column can be caused either by changes in the rates of renewal of water masses or by dynamical changes (such as changes in wind stress). A method for analysing the subsurface temporal changes in hydrographic data is described in terms of these three processes: “pure warming,” “pure freshening,” and “pure heave.” Linear relations are derived for the relative strength of each process in terms of the observed changes of potential temperature and salinity in two different coordinate frames: (i) con...

Warm Events in the Tropical Atlantic

Abstract: Sea surface temperature in the eastern equatorial Atlantic Ocean undergoes anomalous warming events of 1°–2°C every few years. The warm anomalies reach their maximum strength in Northern Hemisphere summer, when equatorial upwelling normally brings cold thermocline water to the surface. By compositing surface observations from a 28-year record, we are able to identify consistent features in anomalies of SST and winds. The composites show that the SST anomalies in northern summer are confined to the eastern equatorial region, with reduced zonal winds to the west and reduced northward trade winds to the east. Accompanying these changes in winds are enhanced convection near the equator caused by a southward shift and intensification of the intertropical convergence zone. Later in the year, SST south of the equator becomes elevated. As a result, by spring of the year following the equatorial anomaly, convection in the western side of the basin is much higher than normal. To understand the ocean dynami...

Infragravity-Frequency (0.005–0.05 Hz) Motions on the Shelf. Part I: Forced Waves

Abstract: In Part I, the energy levels of ocean surface waves at infragravity frequencies (nominally 0.005–0.05 Hz) locally forced by swell in 13-m water depth were shown to be predicted accurately by second-order nonlinear wave theory. However, forced infragravity waves were consistently much less energetic than free infragravity waves. Here, in Part II, observations in depths between 8 and 204 m, on Atlantic and Pacific shelves, are used to investigate the sources and variability of free infragravity wave energy. Both free and forced infragravity energy levels generally increase with increasing swell energy and decreasing water depth, but their dependencies are markedly different. Although free waves usually dominate the infragravity frequency band, forced waves contribute a significant fraction of the total infragravity energy with high energy swell and/or in very shallow water. The observed h−1 variation of free infragravity energy with increasing water depth h is stronger than the h−1/2 dependence pre...

Unsteady, Turbulent Convection into a Homogeneous, Rotating Fluid,with Oceanographic Applications

Abstract: Turbulent convection into a homogeneous, rotating fluid has been generated in laboratory tanks, for both laterally confined and unconfined domains. When a given experiment was in a solid-body rotation, a source located at the top surface of the water column was activated to release denser saltwater into the underlying, less-dense fluid of total depth H. As a result, a downward propagating 3D turbulent front was formed. Eventually, at a transition depth zc, rotational effects dominated the turbulence and many quasi-2D vortices were generated, which then penetrated downward beneath the upper 3D turbulent layer. Measurements in the confined experiments gave zc ≈ (12.7 ± 1.5) (B0/f3)1/2; the mean diameter (Dv) of the quasi-2D vortices as Dv≈(15.0±1.5) (B0/f3)1/2, their downward speed of propagation (uc) as uc ≈ (1.0 ± 0.1) (B0/f)1/2, and the maximum swirl velocity (uv) of an individual vortex as uv ≈ (4.0 ± 0.4)(B0/f)1/2 (where B0 is the surface buoyancy flux and f the Coriolis parameter). All are in...

A storm surge prediction model for the northern Bay of Bengal with application to the cyclone disaster in April 1991

Abstract: A numerical model for simulating and predicting tides and storm surges in regions that include areas of open sea combined with estuarine channels and intertidal banks is described. The model makes use of modified depth-averaged equations with a numerical scheme in which the solution of 1D equations for narrow channels, 2D equations for the open sea, and an approach to modeling inundation are combined within a unified framework. This simplifies the construction of realistic models for complex coastal areas and deltas. The model is applied to the northern shelf of the Bay of Bengal and the Ganges Delta, including the coast of Bangladesh devastated by a cyclone surge in April 1991 in which about 140 000 people died. Hindcast and “forecast” simulations of this event, using forcing derived from a semianalytical cyclone model with data supplied by the Joint Typhoon Warning Center (JTWC), are described. It is shown that the timing of cyclone landfall and its coincidence with high tide determine the area...

A Rotated EOF Analysis of Global Sea Surface Temperature Variability with Interannual and Interdecadal Scales

Abstract: Spatiotemporal variability of preferred global-scale sea surface temperature anomaly patterns is documented, applying a varimax-rotated empirical orthogonal function (R-EOF) analysis to monthly mean SST anomalies. The present study focuses especially on the interdecadal variability of leading R-EOF modes. It is first found that temporal variability of R-EOF1 has a quasi periodicity of 2–5 years and coincides quite well with the occurrence of the ENSO event; hence this mode can be identified with the ENSO mode and distinguished from the other modes dominated by interdecadal variability. The authors find that R-EOF2 typically shows the dominance of interdecadal variability and signals of the ENSO phenomenon are removed. This mode is characterized by increasing Indian Ocean SST and decreasing central North Pacific SST around 40°–50°N in the recent ten or more years. A further indication is that both R-EOF3 and R-EOF4, which show the dominance of interdecadal variability, are fundamentally regarded a...

Parameterization of the Cool Skin of the Ocean and of the Air-Ocean Gas Transfer on the Basis of Modeling Surface Renewal

Abstract: Heat and gas transport in molecular sublayers at the air-sea interface is governed by similar laws. A model of renewal type based on the physics of molecular sublayers allows the derivation of a parameterization of the temperature difference across the cool skin of the ocean and of the coefficient of the direct air-sea gas transfer. The surface Richardson number controls the transition from convective instability to wind-induced instability (“rollers” on breaking wavelets) and the Keulegan number controls the transition from the regime of rollers to long-wave breaking. A critical value of the surface Richardson number and of a nondimensional constant can be evaluated by comparing the parameterizations of the cool skin with field data. The critical value of the Keulegan number is determined from the wind speed at which long-wave breaking appears. The parameterizations have been compared with cool skin data obtained from campaigns in the tropical and subtropical Atlantic Ocean, while the gas transf...

Multiple Equilibria of an Asymmetric Two-Basin Ocean Model

Abstract: An ocean general circulation model is used to examine the role of model geometry and surface buoyancy and wind stress forcing in the asymmetry of the global thermohaline circulation. The model domain is a highly idealized Atlantic and Pacific, linked by a circumpolar ocean in the south, and the integrations are performed under mixed boundary conditions diagnosed from spinups under various temperature and salinity profiles constructed from the present-day climatology. The model exhibits a tendency to favor either a “conveyor”-type circulation with sinking in the northern North Atlantic and upwelling in the North Pacific, or a “southern sinking” state with deep sinking in the Antarctic only. This bias is not dictated solely by the hydrological cycle, nor apparently by the greater northern extension of the Atlantic basin, but presumably by the overall asymmetry of the geometry. Equilibria with northern sinking in both basins can appear, however, when the winds in the Southern Ocean are reduced or th...

The Eastern Boundary of the Subtropical North Atlantic

Abstract: A quasi-meridional hydrographic section carried out between 60° and 20°N offshore from the European and African continental slopes is analyzed in terms of water masses and zonal transports in and out of the eastern boundary. Outstanding features of the meridional distribution of water masses are focused on, such as the transition between North Atlantic and South Atlantic Central Waters at 20°–25°N, the properties and anticyclonic circulation of the Rockall Channel mode water, and the northern boundary of the large-scale Mediterranean Water plume at about 50°N. An eastward transport of about 11×10 6 m 3 s −1 is found to enter the eastern boundary layer at densities lower than 27.25 and feed southward alongshore currents. The relation of the incoming transport to the water mass distribution and its eventual splitting into several outflowing components are discussed. Apart from the downward entrainment of upper water known to occur in the Gulf of Cadiz, there is no sign of the so-called “eastern boundary ventilation” mechanism in the central water density range. Yet a significant transport is found to escape the winter mixed layer toward the interior of the subtropical gyre, suggesting the horizontal southward currents across the sloping bottom of the mixed layer to be the main cause of ventilation at the eastern boundary.

A GCM Study of Tropical–Subtropical Upper-Ocean Water Exchange

Abstract: Experiments with an oceanic general circulation model indicate that the tropical and subtropical oceanic circulations are linked in three ways. Far from coast in the oceanic interior, equatorial surface waters flow poleward to the southern part of the subtropical gyre, and then are subducted and returned in the thermocline to the upper part of the core of the Equatorial Undercurrent. There is, in addition, a surface western boundary current that carries waters from the equatorial region to the northern part of the subtropical gyre. After subduction, that water reaches the equator by means of a subsurface western boundary current and provides a substantial part (2/3 approximately) of the initial transport of the Equatorial Undercurrent. The eastward flow in the Equatorial Undercurrent is part of an intense equatorial cell in which water rises to the surface at the equator, drifts westward and poleward, then sinks near 3° latitude to flow equatorward where it rejoins the undercurrent.

Energy Dissipation by Breaking Waves

Abstract: Recent field measurements by Agrawal et al. have provided evidence of a shallow surface mixed layer in which the rate of dissipation due to turbulence is one to two orders of magnitude greater than that in a comparable turbulent boundary layer over a rigid wall. It is shown that predictions by Phillips of the energy lost by breaking surface waves in an equilibrium regime and laboratory measurements by Rapp and Melville of the mixing and turbulence due to breaking together lead to estimates of the enhanced dissipation rate and the thickness of the surface layer consistent with the field measurements. Wave-age-dependent scaling of the dissipation layer is proposed. Laboratory measurements of dissipation rates in both unsteady and quasi-steady breaking waves are examined. It is shown that an appropriately defined dimensionless rate of dissipation in unsteady breaking waves is not constant, but increases with a measure of the wave slope. Differences between dissipation rates in quasi-steady and unste...

Surface Eddy Momentum Flux and Velocity Variances in the Southern Ocean from Geosat Altimetry

Abstract: Satellite altimetry has previously been used to map the magnitude of the surface eddy variability of the global oceans, but the direction of the time-variable velocities have been more difficult to determine. Here, a technique is presented for resolving both magnitude and direction of residual surface geostrophic velocities at Geosat altimeter crossover points; providing a two-year time series with a temporal resolution of 17 days and horizontal resolution of around 100 km. The time series of residual velocity components are then used to determine surface eddy statistics in the Southern Ocean and to investigate the role of transient eddies in the Southern Ocean momentum balance. The surface eddy statistics from Geosat crossover points show a complex spatial distribution in the surface Reynolds stresses (u′2, v′2, u′v′). In contrast to the assumptions of isotropic variability in previous analyses of altimeter data, velocity variance ellipses are found to be distinctly anisotropic in many regions. ...

A mechanism for the accumulation of floating marine debris North of Hawaii

Abstract: A mechanism for the accumulation of floating marine debris in the North Pacific, especially north of the Hawaiian Islands, is investigated. First, about 50 pseudo marine debris markers floating at the sea surface are arranged in the North Pacific. Their trajectories are simulated by surface currents consisting of Stokes drift, Ekman drift, and geostrophic currents. The simulation by Ekman drift alone shows remarkable convergence at midlatitudes and moderate convergence north of the Hawaiian Islands. On the other hand, that by Stokes drift or geostrophic currents does not present any remarkable high-density area and shows basin-scale movement of marine debris. However, from the simulation by combined surface currents, marine debris gathers north of the Hawaiian Islands. This can be explained by a simple mechanism consisting of a three-step process. First, marine debris accumulates in the Ekman convergence zone related to westerly and trade winds. Second, the debris moves eastward by geostrophic cu...

Reflection of Ocean Surface Gravity Waves from a Natural Beach

Abstract: The energy of seaward and shoreward propagating ocean surface gravity waves on a natural beach was estimated with data from an army of 24 bottom-mounted pressure sensors in 13-m water depth, 2 km from the North Carolina coast. Consistent with a parameterization of surface wave reflection from a plane sloping beach by Miche, the ratio of seaward to shoreward propagating energy in the swell-sea frequency band (0.044–0.20 Hz) decreased with increasing wave frequency and increasing wave height, and increased with increasing beach-face slope. Although most incident swell-sea energy dissipated in the surf zone, reflection was sometimes significant (up to 18% of the incident swell-sea energy) when the beach face was steep (at high tide) and the wave field was dominated by low-energy, low-frequency swell. Frequency-directional spectra show that reflection of swell and sea was approximately specular. The ratio of seaward to shoreward propagating energy in the infragravity frequency band (0.010–0.044 Hz) v...

Instability of the thermohaline circulation with respect to mixed boundary conditions: is it really a problem for realistic models?

Abstract: This comment discusses two issues raised by a recent study of the stability of the thermohaline circulation (Tziperman et al.). A numerical problem is pointed out that may have affected the results of the reported calculations. In addition, it is argued that the state transition of the model circulation found in the above study is triggered by a convective instability mechanism, not the advective mechanism discussed by its authors. Should this be the case, then the conclusion of Tziperman et al. that the ocean is close to a stability transition point must be questioned.

Modeling Spectral Dissipation in the Evolution of Wind Waves. Part I: Assessment of Existing Model Performance

Abstract: This study examines the performance of a state-of-the-art spectral wind wave model that uses a full solution to the nonlinear interaction source term. The situation investigated here is fetch-limited wind wave evolution, for which a significant observational database exists. The authors consider both the evolutionary characteristics such as the predicted development of wave energy and peak wave frequency with fetch, as well as the predicted local features of the directional wavenumber spectrum: the spectral shape of the dominant wave direction slice, together with the directional spreading function. In view of the customary practice of constraining the shape of the spectral tail region, this investigation required relaxing the constrained tail assumption. This has led to new insight into the dynamic role of the spectral tail region. The calculations have focused on the influence of two of the source terms in the spectral evolution (radiative transfer) equation for the energy density spectrum—thos...

Circulation in the Bering Sea basin observed by satellite-tracked drifters : 1986-1993

Abstract: From 1986 through April 1993, 86 satellite-tracked buoys were deployed in the North Pacific and Bering Sea. Most of the buoys were drogued at 40 m. A composite current pattern is derived using these data. The two principal currents (the Alaskan Stream and Kamchatka Current) are clearly evident. Eddy kinetic-mean kinetic energy ratios are low in the stream and along the western Bering Sea basin. An eastward flowing current occurred along the north flank of the Aleutian Islands, this flow was modified by inflow at the passes. Westward flow occurred north of 56°N; its source was the Bering Slope Current. The Kamchatka Current originated near 175°E along the Russian coast. Numerous eddies and meanders were observed in the Kamchatka Current; eddies were also present on the eastern side of the basin.

A Simple Box Model of Stochastically Forced Thermohaline Flow

Abstract: A modified Stommel two-box model is considered as a minimal representation of the buoyancy-driven ocean circulation. In the limit of fast temperature relaxation only the salinity evolves in time while the temperature is clamped to the prescribed ambient value. The box model has no intrinsic variability: just two linearly stable and one unstable equilibria. A finite perturbation is needed to shift the system from one stable equilibrium to the other. The minimum amplitude and duration in time of the perturbation are calculated. A stochastic component of the freshwater flux forcing is then added to model the effect of changes in the global hydrological cycle due to the “weather.” The stochastic forcing is a source of extrinsic time dependence. The salinity gradient obeys an equation analogous to the trajectory of a viscous particle in a double-welled potential, subject to Brownian agitation. If the amplitude of the stochastic driving is above a certain threshold, then there is a finite probability o...

Three-Dimensional Structure of the Wind-Driven Circulation in the Subtropical North Pacific

Abstract: The subduction rate is calculated for the North Pacific based on Levitus climatology data and Hellerman and Rosenstein wind stress data. Because the period of effective subduction is rather short, subduction rates calculated in Eulerian and Lagrangian coordinates are very close. The subduction rate defined in the Lagrangian sense consists of two parts. The first part is due to the vertical pumping along the one-year trajectory, and the second part is due to the difference in the winter mixed layer depth over the one-year trajectory. Since the mixed layer is relatively shallow in the North Pacific, the vertical pumping term is very close to the Ekman pumping, while the sloping mixed layer base enhances subduction, especially near the Kuroshio Extension. For most of the subtropical North Pacific, the subduction rate is no more than 75 m yr−1, slightly larger than the Ekman pumping. The water mass volume and total amount of ventilation integrated for each interval of 0.2σ unit is computed. The corre...

Variability and Multiple Equilibria of the Thermohaline Circulation Associated with Deep-Water Formation

Abstract: In this study the variability of the thermohaline circulation on decadal and centennial time scales that is related to the process of deep-water formation is investigated. This is done within the context of a simple geostrophic three-layer ocean model with a rectangular closed basin. When slowly varying the atmospheric forcing the model response shows sudden transitions, characterized by local changes in convective activity. Many different equilibria were found within the thermally driven regime (in Stommel's sense, i.e., downwelling occurs near the poles). Next, the deep-water formation process was analyzed with a one-dimensional box model. In this box model four different regimes can be identified: convective, nonconvective, periodic, and a regime where both convection and no convection are possible with the same mixed boundary conditions. These regimes were identified in the circulation of the ocean model. A region was traced where convection is possible according to the authors’ analysis but ...

Wave- and Wind-Driven Flow in Water of Finite Depth

Abstract: The authors first derive both Coriolis-induced and viscosity-induced stresses for arbitrary water depth and arbitrary wave direction. Opportunity is taken here to succinctly and rigorously derive the Longuet-Higgins virtual tangential stress due to wave motion. It is shown that the virtual stress is a projection on the surface slope of two viscous normal stresses acting on the vertical and horizontal planes. Then a simple Eulerian model is presented for the steady flow driven by waves and by waves and winds This simple Eulerian model demonstrates that the wave forcing can he easily incorporated with other conventional forcing, rather than resorting to a complicated and lengthy perturbation analysis of the Lagrangian equations of motion. A further focus is given to the wave-driven flow when the various limits of the wave-driven steady flow are discussed. The wave-driven steady flow given by the model yields a unified formula between Ursell and Hasselmann's inviscid but rotational theory and the Lo...

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...