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Kenneth H. Brink and J. S. Allen

Abstract

Observations of the velocity fields over the continental shelf and slope off Oregon and off Peru have shown that there is a phase difference in the onshore-offshore direction, with the velocity fluctuations nearshore leading those offshore in time. It is shown here that the effects of bottom Ekman layer friction and cross-shelf depth variation combine to result in such a phase lag in a model for forced or free long barotropic continental shelf waves. The model also shows that bottom friction results in a smaller phase lag between the alongshore components of velocity and wind stress than that predicted by a frictionless model, a feature found in the observations off Oregon.

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Glen Gawarkiewicz, Frank Bahr, Robert C. Beardsley, and Kenneth H. Brink

Abstract

Spring conditions at the shelf break in the Middle Atlantic Bight mark the transition period between the generally well-mixed shelf water in winter and the highly stratified shelf conditions during summer. A high-resolution hydrographic survey made during early May 1996 is used to describe the thermohaline and velocity structure of the shelfbreak front. The front was strongly affected by the presence of a slope eddy immediately offshore of the front. The eddy, which had a diameter of 25 km, was anticyclonic with onshore/offshore flows of 0.2 m s−1 on opposing sides. On the western side of the eddy, where the flow seaward of the front was predominantly onshore, the front was very steep and the frontal jet was particularly strong, with maximum near-surface velocities of 0.5 m s−1. On the eastern side of the eddy, the front was drawn offshore and was much less steep, with near-surface velocities of only 0.2 m s−1. A surprising feature was the presence of a second jet over the foot of the front, shoreward of the main, surface-trapped jet. These observations suggest that eddies over the slope have a large impact on shelfbreak frontal structure, and result in strong alongfront gradients within the frontal jet.

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Kenneth H. Brink, David W. Stuart, and John C. Van Leer

Abstract

Coordinated meteorological and oceanographic (CTD) measurements were made near Point Conception, California, during March–April 1981. The goal of the observations was to study coastal upwelling and the local characteristics of the assumed wind driving. Results showed substantial topographically-induced spatial structure in the near-surface winds, with weaker winds generally occurring within the Santa Barbara Channel. The 1981 “spring transition” event was monitored by mews of hydrographic and sea level measurements. The details of the event suggest that it was not entirely driven by lead wind stress. The mean sea surface temperature pattern suggests the existence of an upwelling center between Points Arguello and Conception. The individual sea surface temperature charts are all dominated by patchiness on a scale of 5–15 km. The nature of these structures is not well understood, but on the one occasion when a patch was isolated by a CTD survey, its structure penetrated to at 1east 50 db.

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Kenneth H. Brink, J. S. Allen, and Robert L. Smith

Abstract

An analysis is presented of low-frequency (<0.4 cpd) fluctuations in currents, temperature and tide gage data collected during the March-September 1976 segment of the CUEA JOINT-II experiment off the coast of Peru. The observations were made near 15°S, a region of particularly strong and persistent coastal upwelling. Conclusions about the dynamics of motions over the continental shelf and slope are reached by means of correlations, empirical orthogonal functions and other indicators. It is found that flow over the shelf, where stratification was weak, was generally dominated by vertical turbulent frictional effects and was strongly coupled to the effectively inviscid, baroclinic flow over the slope. The momentum balance was three dimensional, with the alongshore pressure gradient playing an important role. In contrast to behavior in other coastal upwelling regions, the alongshore velocity field over the shelf and slope was evidently not strongly driven by the local alongshore component of the wind stress. The mean wind stress throughout the period was equatorward (upwelling favorable), whereas the mean alongshore currents over the shelf were poleward. The alongshore current fluctuations, which propagated poleward along the coast, were initially poorly correlated with the local wind stress, but during the course of the experiment, the wind stress increased in magnitude and gained in importance as a driving mechanism. The temperature and onshore-offshore current fluctuations over the shelf and, therefore, presumably the upwelling circulation were, however, correlated with the local wind stress throughout the experiment.

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Robert C. Beardsley, David C. Chapman, Kenneth H. Brink, Steven R. Ramp, and Ronald Schlitz

Abstract

The Nantucket Shoals Flux Experiment (NSFE79) was conducted across the continental shelf and upper slope south of Nantucket from March 1979 to April 1980 to study the flow of shelf water from the Georges Bank/Gulf of Maine region into the Middle Atlantic Bight. The experiment included a moored array of current meters and bottom instrumentation deployed at six locations across the shelf and upper slope spanning a depth range from 46 to 810 m, and supporting hydrographic observations. A basic description of the moored current and temperature data is given here with an emphasis on the low-frequency variability.

In the summer period (April–August) when the local vertical stratification reached a maximum due to increased surface heating and reduced wind mixing, the mean flow over the shelf at all instruments was primarily along 1ocal isobaths towards the west. The subtidal current fluctuations were coherent both horizontally and vertically over the shelf, but not with current fluctuations observed over the upper slope. The wind stress during summer was weak and only moderately correlated with the subtidal current fluctuations.

In the winter period (October–March), when the seasonal thermocline was destroyed and the shelf water locally homogenized by increased surface cooling and wind mixing, the mean currents observed over the shelf were also primarily alongshelf towards the west at speeds comparable to those measured in summer. However, the low-frequency current fluctuations over the shelf were much more energetic in winter. These subtidal current fluctuations were highly coherent horizontally and vertically over the shelf and with surface wind-stress fluctuations (which increased in magnitude by a factor of 5 over the summer period). The most energetic subtidal current events observed over the shelf also tended to extend into the upper slope region.

The subtidal currents observed over the upper slope in summer were dominated by three bursts of large eastward currents which correspond to the passage of anticyclonic Gulf Stream warm-core rings near or through the moored army. The effect of these rings on the current field does not appear to penetrate shoreward of the shelf break. In winter only two rings passed near the array and their influence on the observed upper slope currents was unclear owing, in part, to the increased subtidal current variability caused by the stronger synoptic wind forcing in winter.

Multiple regression analysis was used to identify possible annual variations in the NSFE79 moored current and temperature data. Significant annual variations were found in the temperature field over the shelf and upper slope and in the low-frequency current variability over the shelf. No significant annual variation was observed in the alongshelf current over the shelf, however, suggesting that there is, at least on time scales of one month and more, a continuous flow of shelf water into the Middle Atlantic Bight from the Georges Bank/Gulf of Maine region. The mean westward volume flux between the 40 and 120 m isobaths observed in NSFE79 was 38.3 ± 6.9 × 104 m3 s−1.

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