Frank Snodgrass

Tides off‐shore: Transition from California coastal to deep‐sea waters‡

Abstract Tidal pressures and currents were measured with self‐contained capsules dropped to the sea floor for one month at distances of 175, 190, and 500 nautical miles from San Diego. These observations, together with a one‐week bottom pressure record by Filloux at 750 n miles, and three half‐week bottom current records by Isaacs et al, at intermediary distances, were analyzed for tidal components by cross‐correlation with a noise‐free reference time series. (For short records this method has some merit over classical tide analysis.) It was found that the tide decays seaward to e‐1 times the coastal amplitude over a distance of order 1000 km for the semidiurnal species, slower for the diurnal species. Tidal currents turn counterclockwise, and are polarized with maximum flow parrallel to shore in the direction of tidal propagation (320°T) at local high tide. The current amplitude is roughly 2 cm/sec for the semidiurnal component, 1 cm/sec for the diurnal component. Superimposed baroclinic tidal currents l...

MODE Bottom Experiment

Abstract Pressure fluctuations on the deep seafloor at frequencies below inertial and tidal have been measured. Between 0.1 and 1 cycle per day the variance is about 2 mb2, spectra diminish with increasing frequency as ω−n, n=1.5 to 2, and a signal-to-instrument noise ratio of 10 dB is achieved. Fluctuations are in phase and highly coherent within the MODE area (>0.95 at 200 km) and even with inferred (atmosphere plus sea level) Bermuda subsurface pressures (0.8 at 700 km). Station differences (to which MODE-sized eddies would make the principal contribution) are relatively small. The large horizontal scale of the recorded bottom pressure fluctuations resembles that of atmospheric pressure, yet the coherence locally between atmospheric and bottom pressure is slight; the recorded fluctuations may be related to a barotropic ocean response to a variable wind stress on the subtropical gyre. Bottom temperature records show “sudden” (1 day) changes of order 30 millidegrees Celcius separated by long intervals (2...

M 2 amphidrome in the northeast pacific

Abstract A previously proposed amphidrome between San Diego and Hawaii is confirmed by offshore measurements of tides across the amphidrome, showing opposite phases and opposite sense of current rotation.

Measurements of southern swell at Guadalupe Island

Abstract We have obtained some energy spectra of pressure fluctuations on the sea bottom at a depth of 60 fathoms off Guadalupe Island, Mexico. The spectra consist of peaks whose frequency increases by about 10 per cent per day; sequences repeat once every three or four days. The waves come from the southwest quarter, and the distance of generation is estimated at 8000 nautical miles. The signals are attributed to cyclones in the storm belt of the Southern Hemisphere. The earliest arrivals are of the order of 1 mm high and 1 km long. They may have originated in the Indian Ocean and passed just east of New Zealand, or to the west through the Tasman Sea. Later arrivals were generated in the South Pacific. Measurements taken at many other occasions at various localities in the north-east Pacific indicate that this radiation from the Southern Hemisphere is present at all seasons. Apparently it constitutes the principal background in the ocean wave spectrum for the low frequencies under consideration.

MODE:IGPP Measurements of Bottom Pressure and Temperature

Abstract We review the MODE deployment of IGPP bottom instruments, together with preliminary tests on the Pacific seafloor and in the laboratory. Pressure and temperature were measured with quartz-crystal transducers in different configurations. Spectra of instrument noise in the laboratory and on the seafloor were estimated from duplicate transducers. These estimates are prerequisite to the forthcoming discussions of MODE tides, the bottom experiment, and internal waves. There are two puzzling features: (i) the temperature noise continuum on the seafloor is generally 20 dB above that in the laboratory, and (ii) the pressure noise spectrum has a tidal line structure. Instrument drifts during MODE are of the order of a few millibars and a millidegree Celcius, respectively.

Evaluation of deep sea tide gauge sensors

A deep sea recording capsule carrying two quartz-crystal pressure sensors and two quartz-crystal temperature sensors was operated in 2.2 km depth off Brest during November-December 1973 as part of an inter national deep sea tide gauge intercalibration experiment. The dual instrumentation provides information for in situ evaluation of instrument calibrations. Differences in the recordings give estimates of the instrumental noise spectra. Instrumental noise of the temperature-corrected pressure sensor was found to be 10-20 db below the sea level continuum spectrum at frequencies below 0.2 cph. At higher frequencies the pressure signal-to-noise ratio decreased to 0-6 db at 3 cph (the Nyquist). The temperature sensor noise spectrum was everywhere 20-40 db below the signal spectrum. Problems associated with using dual instrumentation to evaluate sensor performance are discussed.