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J. O. Blanton

Abstract

In three periods during 1970 representative of spring, summer and fall, the horizontal currents off Oshawa in Lake Ontario were measured at distances of 3, 6, 11 and 16 km offshore. All records were spectrally analyzed for an equal number of 2-hr values within each period. Within each season, there was a tendency for the total variance to descrease with depth and with distance offshore. Total energy at any given distance offshore was lowest in spring and highest in fall. At distances between 6 and 11 km offshore in the summer period, there was an abrupt increase offshore in the percent of total variance contained in rotary-type motion near the theoretical inertial period. The offshore increase in other seasons was much smaller in magnitude.

For all seasons, flow was predominantly westward. Nearshore currents reversed from west to east flow about 6 hr after the wind changed, but farther offshore the reversal lagged the wind by about 12 hr in summer and 36 hr in fall. These observations supplemented by data of the thermal structure of Lake Ontario indicate an apparent surplus of westerly momentum in the nearshore zone of Lake Ontario's north shore.

In view of the above results, it is concluded that single-point measurements of current flow in the near-shore zone are poor indicators of the flow structure surrounding the point.

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J. O. Blanton

Abstract

The upwelling-downwelling cycles observed along the north shore of Lake Ontario have periods of about 12 to 16 days in length. Currents associated with the downwelling cycle are typically stronger. The periods of the cycles are at least a factor of 2 larger than are periods expected from cyclone movements across the Great Lakes. Although the upwelling/downwelling cycles are generally a response to the wind, this discrepancy suggests a tendency for a more wave-like periodic response.

The kinetic energy in currents near the theoretical inertial period clearly delineates a nearshore zone of about 8 km in width. Internal wave periods observed are 14 and 17 h, but no 14 h periods are observed beyond 8 km. Most of the upwelling and downwelling is confined to this zone. The sloping shore model of Csanady appears to accurately predict the extent of this zone.

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J. O. Blanton and C. R. Murthy

Abstract

Observations of currents across a nearshore zone from 2 to 6 km offshore indicate that unsteady longshore flow and complete reversals in flow are usually accompanied by large values of lateral shear. These values often approach and may exceed 10−4 sec−1, near the value of the Coriolis parameter at mid-latitude. At times when lateral shear is high, other turbulent properties such as variance at a point are also high. The variations of lateral shear are highly temporal and can be qualitatively related to the cycles of cyclone-anti-cyclone activity in the area. High shear values usually do not coincide with high winds, but are usually related to the inability of the nearshore currents to adjust to a slowly varying wind regime. Simple momentum arguments suggest that the time for adjustment decreases as water depth nearshore decreases.

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A. H. Weber and J. O. Blanton

Abstract

A total of 339 389 marine weather observations have been analyzed to Produce Monthly mean wind fields for the South Atlantic Bight. The results of plotting wind vectors on a 1/2° latitude by 1/2° longitude grid yields four traditional seasonal flow regimes (winter, spring, summer and fall) and an additional regime designated as mariners’ fall. These seasonal wind regimes are discussed and related to the monthly mean ocean circulation in the Bight.

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J. O. Blanton, J. Amft, L-Y. Oey, and T. N. Lee

Abstract

We report a study of a coastal frontal zone of the southeastern United States based on a field experiment and numerical modeling. The study was conducted in the spring of 1985 during weak to moderate wind stress and strong input of buoyancy from solar radiation and river discharge. The study confirms that the structure and slope of the frontal zone depends on a combination of wind stress and cross-shelf advection of buoyancy.

A cross-shelf/depth two-dimensional (x, y), time-dependent numerical model illustrated the response of the frontal zone to the local wind stress regimes. A comparison of model results with field data showed that the model successfully predicted onsets of stratification and mixing. When alongshore wind stress was negative (southward), isopycnals in the frontal zone steepened due to a combination of horizontal advection and vertical convection. When stress was positive (northward), the offshore advection of low density water flattened the isopycnals and potential energy decreased, demonstrating that horizontal advection terms are important in the equation of conservation of buoyancy. The model predicts die offshore advection of lenses of less dense water during upwelling-favorable wind stress. These lenses are of the order of 20 km in cross-shelf scale and represent an efficient mechanism to export nearshore water. The lenses consist of a mixture of low-salinity coastal water and continental shelf water originating further offshore and advected onshore along the bottom.

The mean flow inside the frontal zone opposed the mean alongshore wind stress. Part of the alongshore flow was in geostrophy with the cross-shore pressure gradient; the other part was due to an alongshore pressure gradient force (kinematic) of about 1 × 10−6 m s−2 (equivalent sea surface slope = 1 × 10−7), which was trapped along the coast with an offshore width scale of O(10 km). It is likely that the alongshore extent of this pressure gradient was governed by the scale at which freshwater is injected to the continental shelf, i.e., 20–30 km. The pressure gradient force immediately outside of the frontal zone was about −5 × 10−7 m s−2 in the direction of the mean alongshore wind stress. It is hypothesized that, as a result of wind setup and freshwater influx, the northward pressure gradient forced over outer shelf/slope by the Gulf Stream decreases in magnitude onshore, and can even change sign across a nearshore frontal zone of O(10 km). The implied flow field near the frontal zone is therefore highly three-dimensional with |∂v/∂y|≈|∂u/∂x|, where (u, v) are velocities in the cross-shore (x) and alongshore (y) directions, respectively.

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