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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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Arthur J. Miller, Douglas S. Luther, and Myrl C. Hendershott

Abstract

The fortnightly and monthly tides are discussed in the light of recent sea level observations and numerical modeling results. Within the tide gauge network of the low-latitude Pacific, the fortnightly tide is shown to possess a large-scale phase lag of roughly 10–40 degrees. Although the nonequilibrium part of the fortnightly tide is traditionally thought to be dominated by Rossby wave dynamics, it is shown, via global shallow-water modeling studies, that this large-scale phase lag is explicable in terms of remotely forced gravity waves whose origin is mainly in the Arctic Ocean. Although future observations outside the low-latitude region of the Pacific may eventually reveal Rossby wave excitation, the fortnightly tidal signal in the tide gauge network at hand appears to reveal at most only weak excitation of Rossby waves compared to the phase lag due to remotely forced gravity waves. The observed monthly tide appears to be only slightly closer to equilibrium than the fortnightly tide. The reason for this remains unclear since the monthly tide is less affected by the remotely forced gravity waves than the fortnightly tide.

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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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