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Paul A. Spyers-Duran and Clinton D. Winant

Abstract

A comparison of sea surface temperatures is made between aircraft precision radiation thermometer (PRT-5) and aircraft deployed expendable bathythermographs (AXBT) drops. These observations were obtained using the NCAR King Air aircraft for an experiment in the Gulf of California during March 1984. The average difference between the sea surface temperatures reported by the first temperature observed in each AXBT drop and the PRT-5 is −0.07°C with a standard deviation of 0.57°C. The difference in temperature between the two observations increases at lower wind speeds. Based on 116 case studies, differences of 1–2°C exist between the surface and the upper meter of the ocean when wind speeds are less than 5 m s−1.

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Clinton D. Winant and Guillermo Gutiérrez de Velasco

Abstract

The tidal and residual circulations in Laguna San Ignacio (LSI), a well-mixed evaporative lagoon located on the Pacific coast of the Baja California peninsula in Mexico, is described based on surveys and moored observations. At tidal periods pressure and axial current fluctuations are about one-quarter of a period out of phase, and so the tidal wave is close to standing. Pressure fluctuations increase and axial currents decrease with distance from the ocean. The fluctuating axial momentum balance is nonlinear and involves local acceleration, advection, barotropic pressure gradients, and friction. The structure of the residual circulation depends on the internal Froude number Fri, a measure of the relative strength of tidal and buoyancy forcing. Most of the time, Fri is large and the residual flow is laterally variable, driven by the tidally averaged nonlinear advective terms. The sense of this residual circulation is shown to depend on the lateral structure of the tidal stress and is away from the ocean in the deep channels when the tidal wave is standing, as in LSI, and in the opposite direction for a progressive wave. During neap tides, when Fri is small, the residual circulation is vertically stratified, with a dense near-bottom flow toward the ocean and relatively fresh inflow at the surface.

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Guillermo Gutiérrez de Velasco and Clinton D. Winant

Abstract

Near the ocean, the residual circulation in Laguna San Ignacio, located on the Pacific coast of Baja California in Mexico, has been shown to be driven by tides. Here the subtidal circulation in the portion of the lagoon farther from the ocean is shown to be driven by the wind. The pressure difference between two stations, one near the closed end and the other midway along the central axis, is correlated well with the wind stress, in the sense that sea level rises downwind. Where the bathymetry is relatively simple, with a deep channel separating two shoal areas, the flow is upwind at the deepest part of the section, driven by the axial pressure gradient. In areas where the bathymetry is more complex, the direction of the observed flow is parallel neither to the local bathymetry nor to the applied wind stress. Linear theory qualitatively explains the major features of the wind-driven flow, including the relative strength of the pressure gradient to the wind stress, the direction of the flow, and the vertical structure, even in topographically complex areas. The residual circulation, after the wind-driven component has been removed, is assumed to be driven by the salinity gradient. That flow changes direction with depth. Where the bathymetry is simple, the near-bottom flow is toward relatively fresh water; flow is in the opposite direction closer to the surface. A linear model driven by a prescribed horizontal density gradient predicts downgradient flow at all depths near the deepest point of any section—a prediction that is qualitatively different from the observed flow.

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John M. Bane, Clinton D. Winant, and James E. Overland

A number of observational programs have been carried out on the United States continental shelf to describe coastal-ocean circulation with emphasis on mesoscale processes. In several of these studies the atmosphere was found to play a central role in determining the coastal circulation through either local or remote forcing. Because of these results, the Coastal Physical Oceanography (CoPO) planning effort has designated three coastal air-sea interaction areas to focus on in a national program to study the physical processes on the continental shelf. These areas are shelf frontogenesis, interaction of stable layers with topography, and forcing by severe storms. The long-term objective of the air-sea interaction component of CoPO is to better understand the structure, dynamics, and evolution of the various mesoscale and synoptic-scale processes that significantly affect coastal/shelf circulation through air-sea interactions. Within this body of knowledge will be an improved quantification of the air-sea exchanges of dynamically important quantities set in the framework of mesoscale and synoptic-scale processes.

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Amy F. Waterhouse, Arnoldo Valle-Levinson, and Clinton D. Winant

Abstract

The spatial structure of tidal amplitude and phase in a simplified system of connected estuaries, an idealized version of Florida’s Intracoastal Waterway, is analyzed with a linear analytical model. This model includes friction, the earth’s rotation, and variable bathymetry. It is driven at the connection with the ocean by a co-oscillating tide. Model results compare well with observations of pressure and currents in a section of the Intracoastal Waterway on the east coast of Florida. The comparison suggests that the waterway is highly frictional, causing the amplitude of the water elevation and tidal velocity to decrease away from the inlets to a minimum in the middle of the waterway. The local phase relationship between velocity and water elevation changed nonlinearly from 90° with no friction to 45° with maximum friction. In moderately to highly frictional basins, the phase lag was consistently less than 45°.

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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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Kathleen A. Edwards, Audrey M. Rogerson, Clinton D. Winant, and David P. Rogers

Abstract

During summer, significant changes in marine atmospheric boundary layer (MABL) speed and depth occur over small spatial scales (<100 km) downstream from topographic features along the California coast. In June and July 1996, the Coastal Waves 96 project collected observations of such changes at capes with an instrumented aircraft. This paper presents observations from the 7 June flight, when the layer-averaged speed increased 9 m s−1 and depth decreased by 500 m over a 75-km downwind from Cape Mendocino, accompanied by enhanced surface fluxes and local cloud clearing. The acceleration and thinning are reproduced when the flow is modeled as a shallow transcritical layer of fluid impinging the bends of a coastal wall, leading to the interpretation that they are produced by an expansion fan. Model runs were produced with different coastlines and imposed pressure gradients, with the best match provided by a coastline in which the cape protruded into the flow and forced a response in the subcritical region upstream of the cape. A hydraulic jump was produced at a second bend, near where the aircraft's lidar observed the MABL height to increase. Light variable winds observed within Shelter Cove were replicated in model flows in which the flow separated from the coastline. Though highly idealized, the shallow-water model provided a satisfactory representation of the main features of the observed flow.

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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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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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