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  • Author or Editor: J. H. Filloux x
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J. H. Filloux

Abstract

A two-month ocean-floor pressure record obtained 330 km to the east of the main island of Hawaii by means of a Bourdon tube-type transducer with optical readout is discussed in detail. An approach to subtraction of the drift component associated with plastic flow of the heavily strained transducer is assessed. In spite of a 40 m progressively accumulated error, it is shown that fluctuations with periods as long as a few cycles per record length are resolved with a remarkable precision. The lunar fortnightly tide, with its period around 14 days, for instance, appears to be in error by no more than 0.3 cm, on the assumption that the transfer function between gravitational driving corrected for earth tides and sea floor pressure has a modulus of 0.7 and a negligible phase shift.

Eleven tidal constituents in each of the diurnal and semidiurnal bands, as well as constituents M3 and Mf are tabulated. These estimated tidal constants come within a few percent of those published for Hilo, revealing a relative uniformity of tidal behavior for this area. The use of tide constants from Hilo to cheek or to constrain mathematical models of the Pacific tides thus appears acceptable.

Because of its inherent high-frequency response and its high accuracy (response up to 1 cycle per second; resolution 0.0206 cm in present data) the instrumentation used in the experiment described here can contribute to the investigation of a variety of problems of ocean geophysics. For instance, the low-frequency end of the surface wind-generated wave spectrum is clearly resolved and is shown to vary slowly from day to day with considerable variations over weekly or longer time spans.

The close approach on 20 July 1978 of Tropical Cyclone Fico to the area of our sea-floor station provided an opportunity to investigate its effect on sea-floor pressure fluctuations. Although somewhat disappointing, this attempt does stress the great advantage to be gained by using an array of stations rather than individual ones to identify and to sort out the many processes at play.

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J. H. Filloux
and
R. L. Snyder

Abstract

This paper is the first in a series reporting the results of a study of tides, setup and bottom friction in the Bight of Abaco, Bahamas. The paper describes three month-long field experiments. employing 15 tide gages and four weather stations distributed throughout the Bight.

The amplitude and phase of five principal tidal constituents and the M4 and M6 overtides are estimated for all stations and errors computed from a generalization/hybridization of the algorithm of Munk and Hasselman (1964) for tidal doublets. The resulting tidal distributions constitute an unusually complete data base against which to optimize the numerical models reported in Parts II and III of the series.

The relatively small amplitude of the override constituents along the western margin of the Bight suggests that these constituents are locally generated. Residual fluctuations are highly coherent with the wind field. Significant differential setup effects are apparent.

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R. L. Snyder
,
M. Sidjabat
, and
J. H. Filloux

Abstract

This is the second in a series of papers reporting the results of a study of tides, setup and bottom friction in the Bight of Abace, Bahamas. The extensive field data reported in Part I of the series (Filloux and Snyder, 1979) are compared with tidal computations using a modified elliptic model first developed by Sidjabat (1970). This model, a multiconstituent generalization of the “hamonic method” of Dronkers (1964), is based on a polynomial representation for the magnitude of the current which provides a tractable resolution of bottom friction, resulting in a coupled set of time-independent equations governing the individual constituents. This resolution naturally spots the bottom friction into a part which can be absorbed on the left-hand (operator) side of the constituent equations and a part which can be treated as multilinear source terms on the right-hand side. The contribution to the left-hand side is large enough that the resulting coupled set may be solved iteratively, converging rapidly. The method provides an efficient and physically transparent alternative to time-stepping methods, particularly for parameter studies such as that described in the paper.

Comparison of model computations with the Bight field data supports the following conclusions:

1) Bottom friction is important to the dynamics of all tidal constituents; it is the source of the M6 overtide. 2) The M4 and M6 overtides are locally generated.

3) While it is possible to fit the principal tides and the M4 overtide with quadratic bottom friction (using a constant drag coefficient and neglecting the contribution of non-tidal motions to the rms current), it is not possible to simultaneously fit the M6 overtide. The resulting frictional interaction is too strong a source of the overtide (by about a factor of 2).

4) The addition of linear bottom friction (and corresponding reduction of the quadratic friction) allows an acceptable fit to both the principal tides and the M4 and M6 overtides.

5) Inclusion of a non-tidal contribution to the rms current also allows an acceptable fit to both the principal tides and the M4 and M6 overtides. The necessary rms non-tidal current (∼0.28 m s−1) is, however, somewhat larger than seems reasonable.

6) In either case the implied bottom stress is significantly larger than reported by other investigators.

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