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  • Author or Editor: Anthony R. Kirincich x
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Anthony R. Kirincich

Abstract

In situ observations of turbulent momentum flux, or Reynolds stresses, were estimated from a 10-yr acoustic Doppler current profiler (ADCP) record of inner-shelf velocities at the Martha’s Vineyard Coastal Observatory (MVCO) using recently developed analysis techniques that account for wave-induced biases. These observations were used to examine the vertical structure of stress and turbulent mixing in the coastal ocean during tidal-, wave-, and wind-driven circulation by conditionally averaging the dataset by the level of forcing or stratification present. Bottom-intensified stresses were found during tidally driven flow, having estimated eddy viscosities as high as 1 × 10−2 m−2 s−1 during slack water. An assessment of the mean, low-wave, low-wind stress results quantified the magnitude of an unmeasured body force responsible for the mean circulation present in the absence of wind and wave forcing. During weak stratification and isolated wind forcing, downwind stresses matched the observed wind stress near the surface and generally decreased with depth linearly for both along- and across-shelf wind forcing. While consistent with simple models of circulation during across-shelf wind forcing, the linear slope of the stress profile present during along-shelf wind forcing requires the existence of an along-shelf pressure gradient that scales with the wind forcing. At increased levels of stratification, the observed downwind stresses generally weakened and shifted to the across-wind direction during across-shelf and mixed-direction (i.e., onshore and along shelf) wind forcing consistent with Ekman spiral modification, but were more variable during along-shelf wind forcing. No measurable stresses were found due to wave-forced conditions, confirming previous theoretical results.

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Anthony R. Kirincich and John A. Barth

Abstract

The spatial and temporal variability of inner-shelf circulation along the central Oregon coast during the 2004 upwelling season is described using a 70-km-long array of moorings along the 15-m isobath. Circulation at three stations located onshore of a submarine bank differed from that of a station north of the bank, despite the relatively uniform wind forcing and inner-shelf bathymetry present. During upwelling-favorable winds, strong southward alongshelf flow occurred north of the bank, no alongshelf flow occurred onshore of the northern part of the bank, and increasing southward flow occurred onshore of the southern part of the bank. During downwelling-favorable winds, strong northward flow occurred in the inner shelf onshore of the bank while weak flow occurred north of the bank. These alongshelf differences in inner-shelf circulation were due to the effects of the bank, which isolated the inner shelf onshore of the bank from the regional upwelling circulation that was evident at the northernmost station. As a result, circulation onshore of the bank was driven primarily by local wind forcing, while flow north of the bank was only partially driven by local winds. A secondary mode of variability, attributed to the movement of the regional upwelling jet due to remote forcings, contributed the bulk of the variability observed north of the bank. With the time-dependent wind forcing present, acceleration was an important term in the depth-averaged alongshelf momentum equation at all stations. During upwelling, bottom stress and acceleration opposed the wind stress north of the bank, while bottom stress was weaker onshore of the bank where the across-shelf momentum flux and the alongshelf pressure gradient balanced the residual of the acceleration and stresses. During downwelling, waters onshore of the bank surged northward at magnitudes much larger than that found north of the bank. These spatial variations developed as the season progressed and the regional upwelling circulation intensified, explaining known variations in growth and recruitment of nearshore invertebrate species.

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Anthony R. Kirincich and John A. Barth

Abstract

The event-scale variability of across-shelf transport was investigated using observations made in 15 m of water on the central Oregon inner shelf. In a study area with intermittently upwelling-favorable winds and significant density stratification, hydrographic and velocity observations show rapid across-shelf movement of water masses over event time scales of 2–7 days. To understand the time variability of across-shelf exchange, an inverse calculation was used to estimate eddy viscosity and the vertical turbulent diffusion of momentum from velocity profiles and wind forcing. Depth-averaged eddy viscosity varied over a large dynamic range, but averaged 1.3 × 10−3 m2 s−1 during upwelling winds and 2.1 × 10−3 m2 s−1 during downwelling winds. The fraction of full Ekman transport present in the surface layer, a measure of the efficiency of across-shelf exchange at this water depth, was a strong function of eddy viscosity and wind forcing, but not stratification. Transport fractions ranged from 60%, during times of weak or variable wind forcing and low eddy viscosity, to 10%–20%, during times of strong downwelling and high eddy viscosity. The difference in eddy viscosities between upwelling and downwelling led to varying across-shelf exchange efficiencies and, potentially, increased net upwelling over time. These results quantify the variability of across-shelf transport efficiency and have significant implications for ecological processes (e.g., larval transport) in the inner shelf.

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Anthony R. Kirincich, Steven J. Lentz, and John A. Barth

Abstract

Recent work by documents offshore transport in the inner shelf due to a wave-driven return flow associated with the Hasselmann wave stress (the Stokes–Coriolis force). This analysis is extended using observations from the central Oregon coast to identify the wave-driven return flow present and quantify the potential bias of wind-driven across-shelf exchange by unresolved wave-driven circulation. Using acoustic Doppler current profiler (ADCP) measurements at six stations, each in water depths of 13–15 m, observed depth-averaged, across-shelf velocities were generally correlated with theoretical estimates of the proposed return flow. During times of minimal wind forcing, across-shelf velocity profiles were vertically sheared, with stronger velocities near the top of the measured portion of the water column, and increased in magnitude with increasing significant wave height, consistent with circulation due to the Hasselmann wave stress. Yet velocity magnitudes and vertical shears were stronger than that predicted by linear wave theory, and more similar to the stratified “summer” velocity profiles described by S. Lentz et al. Additionally, substantial temporal and spatial variability of the wave-driven return flow was found, potentially due to changing wind and wave conditions as well as local bathymetric variability. Despite the wave-driven circulation found, subtracting estimates of the return flow from the observed across-shelf velocity had no significant effect on estimates of the across-shelf exchange due to along-shelf wind forcing at these water depths along the Oregon coast during summer.

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Anthony R. Kirincich, Steven J. Lentz, J. Thomas Farrar, and Neil K. Ganju

Abstract

The spatial structure of the tidal and background circulation over the inner shelf south of Martha's Vineyard, Massachusetts, was investigated using observations from a high-resolution, high-frequency coastal radar system, paired with satellite SSTs and in situ ADCP velocities. Maximum tidal velocities for the dominant semidiurnal constituent increased from 5 to 35 cm s−1 over the 20-km-wide domain with phase variations up to 60°. A northeastward jet along the eastern edge and a recirculation region inshore dominated the annually averaged surface currents, along with a separate along-shelf jet offshore. Owing in part to this variable circulation, the spatial structure of seasonal SST anomalies had implications for the local heat balance. Cooling owing to the advective heat flux divergence was large enough to offset more than half of the seasonal heat gain owing to surface heat flux. Tidal stresses were the largest terms in the mean along- and across-shelf momentum equations in the area of the recirculation, with residual wind stress and the Coriolis term dominating to the west and south, respectively. The recirculation was strongest in summer, with mean winds and tidal stresses accounting for much of the differences between summer and winter mean circulation. Despite the complex bathymetry and short along-shelf spatial scales, a simple model of tidal rectification was able to recreate the features of the northeastward jet and match an estimate of the across-shelf structure of sea surface height inferred from the residual of the momentum analysis.

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