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C. D. Winant

Abstract

Observations of horizontal currents and temperature made along a mid-shelf isobath during the summer on the Southern California shelf are used to determine longshore coherent length scales. The observations are characterized by weak atmospheric forcing and strong density stratification of the water column. For periods longer than a day, coherent longshore length scales of currents are around 25 km, while coherent length scales of temperature were longer than the instrumental array (57.5 km). For periods ranging from a day to an hour, near-surface motions were dominated by diurnal fluctuations which were coherent over lengths of 5 km, whereas near-bottom motions, dominated by semidiurnal fluctuations, showed evidence of a signal propagating up coast at phase speeds of order 1 m s−1. The observed scales of motion in different frequency bands are used to estimate the dispersive properties of the shelf motions. The dispersion coefficient is found to vary with scale, as the current variance associated with larger, lower-frequency motions becomes available to dispersion, and this variation is compared to the behavior of dispersion coefficients deduced by Okubo (1971).

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C. D. Winant and R. C. Beardsley

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.

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R. C. Beardsley and C. D. Winant

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 Csanady's (1978) conclusion that the physical mechanism which creates the alongshelf pressure gradient thought to drive the alongshelf flow must be of oceanic origin.

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C. E. Dorman and C. D. Winant

Abstract

The Santa Barbara Channel is a region characterized by coupled interaction between the lower-level atmosphere, the underlying ocean, and the elevated topography of the coastline. The nature of these interactions and the resulting weather patterns vary between summer and winter.

During summer, synoptic winds are largely controlled by the combined effect of the North Pacific anticyclone and the thermal low located over the southwestern United States, resulting in persistent northwesterly winds. A well-defined marine atmospheric boundary layer (MABL) with properties distinct from the free atmosphere above is a conspicuous feature during the summer. The wind has different characteristics in each of three zones. Maximum winds occur in the area extending south and east from Pt. Conception (zone 1), where they initially increase as they turn to follow the coast, then decrease farther east. Winds are usually weak in zone 2, located in the easternmost part of the channel, offshore from the Oxnard plain. Winds are also weak in zone 3, sometimes reversing to easterly at night, in a narrow band located along the mainland coast. Summer air temperature at the surface follows the SST closely and varies significantly with location. Summer sea level pressure gradients are large, with the lowest pressure occurring on the northeast end of the Santa Barbara Channel. Diurnal variations are strongest in summer, although the modulation is weakest in zone 1. The diurnal variation is parallel to the coast in all of zone 3 but the Oxnard plain, where it is perpendicular to the coast. The height of the marine layer varies between 300 m in late afternoon and 350 m in late morning.

In winter, synoptic conditions are driven by traveling cyclones and sometimes accompanied by fronts. These are usually preceded by strong southeast winds and followed by strong northwest winds. Atmospheric parameters are distributed more uniformly than in summer, and diurnal variations are greatly reduced. Sea level air temperature and pressure are more spatially uniform than in the summer.

Spatial variations in the observed fields in the summer are consistent with a hydraulic model of the MABL as a transcritical expansion fan. The summertime situation is governed by a coupled interaction between the atmosphere and the underlying water. The ocean influences the density of the MABL to the extent that it behaves distinctly from the free atmosphere above, resulting in strong winds polarized in the direction parallel to the coast. In turn, these winds provoke an upwelling response in the coastal ocean, which in part determines the surface properties of the water.

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S. J. Lentz and C. D. Winant

Abstract

Subinertial currents on the southern California shelf are investigated using observations from a current meter array deployed near San Diego during the summer, fall and winter of 1978/79. This region is characterized by weak winds (order 1–2 m s−1) and thus other driving mechanisms for the subinertial currents may be important. A simplified depth-averaged longshore momentum equation consisting of a balance between current accelerations, the adjusted sea level (ASL) gradient and the wind and bottom stress is examined. The ASL gradient is estimated using sea level and atmospheric pressure observations separated by 350 km. A linear parameterization of the bottom stress is used with a drag coefficient of 5 × 10−4 m s−1. During fall and winter, the first-order balance of terms in the longshore momentum equation over the inner-shelf (15 m depth) is between the wind and bottom stress. Over the outer-shelf (60 m) the primary balance is between the ASL gradient and the bottom stress. At midshelf (30 m) both driving mechanisms are important. An estimate of the depth-average longshore velocity in turns of wind stress and the ASL gradient reproduce the major features of the observed depth-averaged velocity (correlations from 0.59 to 0.85). These simple dynamics fail to describe the summer observations, except during one strongly forced event. The vertical structure in summer is complicated by the presence of a thermocline and strong shears over the shelf. Examination of observed and estimated winds within the Southern California Bight and to the south along Baja California indicates that winds along Baja California (∼500 km south of San Diego) may play an important role in generating the ASL gradient fluctuations in the Southern California Bight.

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Guillermo Auad, Myrl C. Hendershott, and Clinton D. Winant

Abstract

The Santa Barbara Channel (SBC) is a coastal basin about 100 km long bounded by the Southern California mainland on the north and by a chain of islands on the south. The SBC is at most 50 km wide and just over 600 m deep. The nature of current and wind variance peaks in the 2–4-day and 4–6-day bands in the channel are analyzed from January to July 1984. For both bands the dominant empirical mode of the currents is highly coherent with the dominant empirical mode of the winds over this region. Surface intensification of currents is revealed by measurements made between 25 and 300 m. In contrast the deeper currents are characterized by bottom trapping. Evidence for baroclinic bottom-trapped topographic Rossby waves is found on the northern shelf at the western mouth of the channel in both frequency bands. At 30 m the distribution of phases shows currents at the center of the western mouth leading the southern interisland passes by about 0.3 day and the eastern mouth by about 0.6 day. In both bands co- and quadrature vectors of currents and winds describe this wind–current system in detail. It is speculated from spatial and temporal eigenfunctions of currents and winds and from available satellite images that the dominant current mode described above is a channelwide response to upwelling north of Point Conception (northwestward of the SBC). The upwelling-related currents cause a net inflow of mass into the western end of the channel, which is compensated by an outflow passing through both the interisland passes and through the eastern mouth of the channel. As a result of the narrowness and shallowness of the passes and of the shallowness of the southern shelf in general, high flow speeds are attained there that, the authors speculate, seem to force deep high-frequency motions both at the center of the SBC and at the northern half of its western mouth.

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C. D. Winant, C. E. Dorman, C. A. Friehe, and R. C. Beardsley

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 occurs with generally northerly winds, is characterized by a strong low-level inversion and the spatial structure of the surface wind is correlated with the coastal topography. The inversion acts as a material interface, and the marine layer behaves as a supercritical channel flow, when the Froude number is greater than one: oblique expansion waves and hydraulic jumps, associated with changes in the orientation of the coastline, account for the observed spatial structure of the flow. Observations from mid-latitudes on the eastern side of other ocean basins suggest that similar supercritical conditions in the marine layer may prevail there also.

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Carl A. Friehe, Robert C. Beardsley, Clinton D. Winant, and Jerome P. Dean

Abstract

Intercomparisons of meteorological data—wind speed and direction, surface temperature and surface pressure—were obtained for NCAR Queen Air overflights of four buoys during the CODE-1 experiment. The overflights were at a nominal altitude of 33 m. Wind and air temperature sensors were at 10 m on two National Data Buoy Office (NDBO) buoys and at 3.5 m on two Woods Hole Oceanographic Institution (WHOI) buoys. The buoy wind speeds were adjusted to the aircraft altitude using diabatic flux-profile relations and bulk aerodynamic formulas to estimate the surface fluxes and stability. For the experimental period (22 April-23 May 1981) and location (northern coast of California), the atmospheric surface layer was generally stable, with the Monin-Obukhov length on average 500 m with large variability.

The results of the intercomparisons of the above variables were in general good. Average differences (aircraft - buoy) and standard deviations were +0.1 m s−1 (±1.8) for wind speed, 3.3 deg (±11.2) for wind direction, +0.02°C (±1.7) for air temperature and +0.8 mb (+1.0) for surface pressure. The aircraft downward-looking infrared radiometer indicated a surface temperature 1°C lower than the buoy hull (NDBO) and 1 m immersion (WHOI) sea temperature sensors.

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Andreas Münchow, Charles S. Coughran, Myrl C. Hendershott, and Clinton D. Winant

Abstract

A towed acoustic Doppler current profiler (ADCP) system was tested. The instrument was deployed from ships of opportunity and towed at depths between 5 and 25 m. The towed system carries upward- and downward-looking ADCPs. The instrument platform is stable in most operating conditions at ship speeds up to 4.5 m s−1. Large discrepancies are found, however, between the ship's velocity obtained from bottom-tracking ADCP pulses and that from navigational data. These are explained with a magnetic compass bias that varies with the ship's heading direction. Both the ship and the tow platform induce magnetic fields that bias the ADCP compass. An in situ compass calibration scheme is thus necessary and requires accurate navigational data. In our main study area, it is found that the Global Position System provides absolute and relative positions to within 88 and 4 m, respectively. These accuracies are sufficient for calibration purposes. With our calibration scheme the towed ADCP system performs as well as vessel-mounted systems. The case of deployment from ships of opportunity and the capacity of the tow system to carry additional instruments makes it a valuable research tool. Furthermore, the capability of our system to profile the water column above and below the platform with different frequencies and thus different vertical resolutions enhances its flexibility and usefulness, especially to study surface and bottom boundary-layer processes.

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A. L. Ponte, G. Gutiérrez de Velasco, A. Valle-Levinson, K. B. Winters, and C. D. Winant

Abstract

Moored current and pressure observations were obtained at Bahía Concepción, 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-bay circulation but fail to predict the along-bay flow pattern. The role of spatial inhomogeneities of wind stress (suggested by synoptic observations of the wind) and nonlinearities related to advection of momentum is investigated with theoretical and numerical modeling. Both mechanisms can contribute to the observed pattern of along-bay circulation. Even though the observations discussed were taken during the relatively well-mixed season, density fluctuations are shown to play, at times, an active role dynamically.

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