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Howard Freeland

Abstract

During the fall of 1994 a conductivity–temperature–depth/hydrographic survey was carried out as part of the World Ocean Circulation Experiment one-time survey, Line P15N. The survey included standard water properties required by WOCE. Line P15N extended southward from the Aleutian Islands along 165°W into the vicinity of the Samoa Passage. The line was adjusted to follow the axis of the passage, and time was found to complete a cross-section survey across the passage. This paper will present geostrophic computations of flow velocities through the gap, including transport estimates, and will present longitudinal plots of properties. The longitudinal plots show evidence of hydraulic control at the sill in the Samoa Passage. A best estimate of northward transport of water colder than 1.2°C (potential temperature) is determined to be 8.4 Sv.

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Howard J. Freeland

Abstract

Data obtained during the Australian Coastal Experiment, previously used to verify the dispersion relation for coastal-trapped waves at low frequencies, are analyzed at frequencies in the diurnal tidal band. It is shown that only a single (the first) mode is required to describe the cross shelf and slope distribution of alongshore currents in contrast to the low frequency limit where at least two and arguably three modes were needed. Furthermore, it is shown that the alongshore structure of these modes yields a good fit to the dispersion relation and its slope in the ω-k plane.

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Howard J. Freeland

Abstract

Observations of deep currents in the northeast Pacific Ocean are reported that indicate that although the eddy kinetic energy level is, as expected, generally low, the deep northeast Pacific is subject to occasional intensely energetic events. These events are energetic enough to dominate the distribution of kinetic energy in the water column and the depth-averaged kinetic energy. The assumption can no longer be made that deep flows are weak when estimating near-surface flows.

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Patrick F. Cummins and Howard J. Freeland

Abstract

Velocity measurements from the interior of the Alaskan gyre are presented from a current meter mooring deployed in 4000 m of water at 49°33′N, 138°38′W, in the vicinity of Ocean Weather Station P. The mooring held five current meters, which spanned the depth of the water column. The data reveal surface-intensified motions with flow fluctuations in the upper layers of the water column characterized by long-period, O(100 d) time scales of variability. At abyssal depths, the flow displays shorter, O(20 d) time scales of variability. The data are compared with observations from one of the NEPAC moorings in the northeast Pacific (42°N, 152°W). Similar characteristics in kinetic energy levels, in the vertical structure of the flow, and in the vertical variation of eddy time scales are found at this location.

The current measurements are considered in terms of the linear theory of directly wind-driven variability of Müller and Frankignoul. A comparison with simulated currents from a quasigeostrophic numerical model demonstrates that stochastic atmospheric forcing of the ocean can account for the observed variability. The numerical experiments and a simple extension of the linear theory suggest that the presence of bottom topography is important for the partition of energy between vertical modes and for the vertical variation of the time scales of the flow.

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John A. Church and Howard J. Freeland

Abstract

The sea level on the southern Australian coast is examined for the source of the coastal-trapped wave energy observed during the Australian Coastal Experiment. Sea level, adjusted for atmospheric pressure, and atmospheric pressure are observed to propagate eastward at about 10 m s−1. At the lowest frequency examined (24-day period), some energy travels south along the west coast of Tasmania, but does not reach the east coast of mainland Australia, while some energy travels through Bass Strait to reach the east coast of mainland Australia. At the most energetic frequency (8-day period), adjusted sea levels are coherent over the 3700 km of coastline from southern Australia to the east coast, and much of the wind-forced coastal-trapped wave energy appears to travel through Bass Strait to the mainland east coast. We have not identified a mechanism for energy transfer through Bass Strait, and we do not know what fraction of the coastal-trapped wave energy incident on western Bass Strait actually reaches the east coast. It is suggested that at low frequencies the long wavelength waves are not affected by relatively small gaps in the coastline, but that at higher frequencies the wavelength is smaller and breaks in the coastline become more important. The first and second coastal-trapped wave modes observed at Cape Howe during the Australian Coastal Experiment are most coherent with the sea level at Lakes Entrance at the eastern edge of Bass Strait. It is suggested that these coastal-trapped wave modes are generated when the east-west flow through Bass Strait has to adjust to the narrow shelf of the east Australian coast and that the second mode is preferentially generated because its length scale (k−1) more closely approximates the north-south extent of this east–west flow.

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T. Yao, Howard J. Freeland, and Lawrence A. Mysak

Abstract

Six current-meter mooring were deployed in a line approximately 600 km in length along the continental shelf of British Columbia. Analysis of the low frequency (periods exceeding a day) fluctuations in current for the winter 1981–82 period is discussed. Alongshore currents off Vancouver Island are mutually correlated with time lag less than a day. The region of mutual correlation does not extend north of Vancouver Island, across Queen Charlotte Sound. Coherence is observed between currents south and north of Queen Charlotte Sound only in a frequency band where there is mutual coherence with local wind. A comparison is made between observation and free coastal-trapped wave theory. Off northern Vancouver Island, where the shelf is narrower than off southern Vancouver Island, there is increased vertical shear, a feature of the second coastal-trapped wave mode. A consistency test is applied using the cross spectral matrix of alongshore components of velocity. In the dominant energy-containing frequency bands (periods ≳10 days), the structure of alongshore currents off Vancouver Island is consistent with the two lowest free coastal-trapped wave modes locked in phase.

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John A. Church, Howard J. Freeland, and Robert L. Smith

Abstract

The currents observed over the shelf and slope during the Australian Coastal Experiment (ACE) are used to determine the amplitudes (as functions of time) of the first three coastal-trapped wave (CTW) modes at three locations along the southeast coast of Australia. A statistical “eddy” mode is included to minimize contamination of the coastal-trapped wave currents from East Australian Current eddies. The first three CTW modes account for about 65% of the observed variance in the alongshelf currents on the shelf and slope at Cape Howe, about 40% at Stanwell Park, but only about 24% at Newcastle. Currents associated with the East Australian Current dominate the observations offshore from Newcastle. CTWs account for all but 10%, 37% and 27% of the currents observed at the most nearshore locations on the shelf at Cape Howe, Stanwell Park and Newcastle. The first two coastal-trapped wave modes propagate at close to the appropriate theoretical phse speeds, but the third coastal-trapped wave mode and the eddy mode are not coherent between the three current meter sections along the coast. Surprisingly, mode 2 carries a greater fraction of the coastal-trapped wave energy than does mode 1 at two of the sections. Modes 1 and 2 are coherent with each other at the 95% significance level. The major energy source for the CTWs is upstream (in the CTW sense) of the first line of current meters.

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John A. Church, Allan J. Clarke, Neil J. White, Howard J. Freeland, and Robert L. Smith

Abstract

No abstract available.

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John A. Church, Neil J. White, Allan J. Clarke, Howard J. Freeland, and Robert L. Smith

Abstract

The Australian Coastal Experiment (ACE) was designed to test coastal-trapped wave (CTW) theory and the generation of coastal-trapped waves by the wind. For the ACE dataset, we use CTW theory to attempt to hindcast the observed alogshelf currents and coastal sea levels at locations remote from the upstream (in the CTW sense) boundary of the ACE region. Local (in the ACE region) wind forcing is responsible for only about a quarter of the CTW energy flux at Stanwell Park (the center of the ACE region), and the remainder enters the ACE region from the south and propagates northward through the ACE region. Including the second-mode CTW improves the correlation between the hindcast and the observed near-bottom currents on the upper slope at Stanwell Park, but the use of the third-mode CTW cannot be justified. A linear bottom drag coefficient of r = 2.5 × 10−4 m s−1 works better than a larger drag coefficient, and simplifying the CTW equations by assuming the modes are uncoupled does not detract from the quality of the hindcasts. The hindcast and observed coastal sea levels are correlated at greater 2 than the 99% significance level. For the nearshore locations at Stanwell Park, the hindcast and observed alongshelf currents are correlated at greater than the 99% significance level, and the CTW model can account for about 40% of the observed variance. On the shelf at Stanwell Park, we find the hindcasts agree with the observations only if direct wind forcing within the ACE region and the correct (nonzero) upstream boundary conditions are included. However, even after attempting to remove the effects of the eddies and the East Australian Current, the CTW model is not useful for predicting the currents on the slope at Stanwell Park and on the shelf and slope at Newcastle (the northern boundary of the ACE region). The currents at these locations are dominated by the effect of the East Australian Current and its eddies.

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Michael W. Stacey, Stephen Pond, Paul H. LeBlond, Howard J. Freeland, and David M. Farmer

Abstract

A description of the low-frequency (ap;10 to 30 days period) current fluctuations in the Strait of Georgia is presented. Velocity time series from four cyclesonde moorings and five current meter mooring, spanning the time interval from June 1984 until January 1985, are analyzed. Emphasis is placed on identifying the forcing mechanisms and determining the spatial structure of low-frequency flow.

The nonlinear interaction of semidiurnal tidal constituents with bottom topography caused a near-bottom, low-frequency oscillation that was coherent over the span of the experimental array (ap;11 km). The tides were important elsewhere in the water column too, and altogether directly accounted for 37% of the low-frequency energy in the Strait.

There is evidence of significant wind forcing. An empirical orthogonal function analysis of the vertical structure of the current fluctuations yields strong evidence for the existence of wind-forced Ekman spirals. Typically, the orthogonal modes that dominate the variance near the surface rotate clockwise with depth and are coherent with the wind.

Longitudinal and transverse velocity correlations imply that at some depths the low-frequency current fluctuations are consistent with horizontally nondivergent, isotropic flow. They also suggest horizontal scales of less than 8 km.

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