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S. D. Meyers, M. Liu, J. J. O'Brien, M. A. Johnson, and J. L. Spiesberger

Abstract

Variations in the thermocline depth of the northeast Pacific Ocean during 1970–1989 are investigated using a reduced-gravity numerical model forced by the local surface wind stress and at the southern land-ocean boundary by a coastal Kelvin wave signal. Three experiments are presented with forcings by wind only, Kelvin wave only, and a combination of both. The wind forcing generates an anticyclonic gyre circulation with mostly annual variations. The Kelvin waves along the coast excite Rossby waves that propagate into the basin interior, producing changes in upper-layer thickness (related to changes in thermocline depth) that last for years after the Kelvin signal has passed. Two sequential upwelling Kelvin waves in 1973 and 1975 produce upwelling Rossby waves that reduce the mean upper-layer thickness by approximately 10–20 m during 1976. This shift is reinforced by later upwelling events lasting until the early 1980s. The authors present a new hypothesis that the previously known climate shift observed in winter sea surface temperature is influenced by changes in the depth of the permanent thermocline induced by remotely forced Rossby waves. Acoustic thermometry might be a sensitive means to detect Rossby waves, motivating a thermometric approach to studying interannual variations in the ocean.

The role of the Rossby waves in the travel time variations of acoustic signals on geodesic paths from Hawaii to the North American coast are calculated using the scheme of Roed. Wind forcing produces mainly annual variations in the acoustic travel time anomalies, except for the high latitude paths above 40°N that exhibit sudden shifts in travel times due to changes in the magnitude of the wind stress. The Rossby waves are primarily responsible for interannual variations.

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John L. Spiesberger, Paul J. Bushong, Kurt Metzger, and Theodore G. Birdsall

Abstract

Pulse-like acoustic transmissions (133 Hz, 60 ms resolution), between a bottom-mounted source near Oahu and a bottom-mounted receiver at about 4000 km range near the coast of Northern California, are recorded during a 5-day interval in 1983 and a 21-day interval in 1987. Measurements of the acoustic travel-time change, based on the acoustic phase, are made every two minutes to a precision of about 135 μs. Power spectra of the acoustic phases, at periods less than about 34 hours, exhibit many significant peaks at nontidal periods whose equivalent rms travel times are between 1 and 10 ms. The periods and amplitudes of these peaks change significantly over intervals of five days. The 1983 dataset is used to demonstrate that the travel times along different ray paths oscillate in phase with each other at periods near the prominent nortidal periods of 15 and 20 hours. This observation leads to the conclusion that the ocean process is either barotropic or it consists of the first or the second baroclinic mode with a horizontal scale of at least 50 km. Ocean fluctuations, which are confined to a distance of about 10 km from the source or the receiver, are improbable candidates for the origin of the nontidal peaks whose rms travel times are 10 ms.

Several hypotheses are considered for the nontidal oscillations. The internal-wave field is too baroclinic and too weak to generate the nontidal acoustic oscillations. Simple linear models, forced by peak values of the atmospheric pressure and the wind, are unable to generate an ocean response of sufficient size to account for the acoustic spectra. It is possible that the small nontidal peaks (1 ms rms) are caused by resonant gravity-wave modes. The large nontidal peaks have rms travel times as large as that due to the semidiurnal tide (10 ms rms). However, historical observations from tide and bottom pressure gauges may rule out the possibility that shallow-water gravity wave modes cause the large travel-time oscillations unless the modes are excited intermittently or unless the structure of the modal shapes computed by Platzman are incorrect. If the modes are responsible for some of the nontidal peaks, then some excitation mechanism must be found for them other than pressure and wind forcing of the ocean through linear models. In conclusion, the origin of the nontidal peaks is unknown.

The internal wave field and the acoustic noise impose an ultimate limit on the ability of a tomographic array (which utilizes acoustic frequencies above 100 Hz) to detect basin-scale currents. For an array consisting of an acoustic source and ten acoustic receivers, the threshold at which a basin-scale current can be detected with a confidence of 95 percent is given by u ≈ 33.6T μm s−1 where the amplitude of the current is u and the period of the oscillation, T, is expressed in days. Other fluctuations in the ocean may place more severe limits on the ability to detect basin-scale currents.

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J. L. Spiesberger, T. G. Birdsall, K. Metzger, R. A. Knox, C. W. Spofford, and R. C. Spindel

Abstract

Phase-coded signals with 60 rms resolution were transmitted twice weekly for several months from acoustic sources at ∼2000 m depth in the Sargasso Sea to three bottom-mounted receives designed as West, East, and North stations at ranges approximately between 1000 and 2000 km. The transmission paths to West and East stations were entirely in the Sargasso Sea. The path to North station crossed the Gulf Stream and so traversed one of the most time- and range-dependent environments found anywhere in the ocean. Arrivals at all three stations were stable and could be identified from range-dependent ray traces. Travel times at West station clearly change is response to the warming of the seasonal thermocline from spring to summer. The travel-time change with predictions. Travel-time changes at North station primarily respond to the north-south meandering of the Gulf Stream.

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