Although there have been many circulation studies in the Mid-Atlantic Bight (MAB), the large extent of the area inevitably leads to a synoptic under sampling with numerous experiments having been conducted in limited regions and for limited durations. However, a tremendous amount of information has been gained from these studies, from which a well accepted general circulation pattern has emerged. While the general circulation pattern appears to be well understood, some of the smaller-scale details and variability, particularly in the across-shelf flow structure, have not been thoroughly examined. In recent years, high frequency (HF) radar networks have provided data that allows the study of smaller-scale surface flow characteristics and flow variability. Hence, the overall goal of this research was to examine sub-inertial surface flows, particularly across-shelf flow, in the central Mid-Atlantic Bight using HF data.
Since 2001, a network of long-range HF radars has been operating along the New Jersey coast. These HF radars have measured surface currents at spatial scales of 6 km and temporal scales of 3 hours. As this remote sensing technology tends to have intermittent gaps, the lack of a completely continuous time series with uniform data coverage requires careful analysis. However, a period of maximum coverage area and minimum temporal gaps (Aug 15 2002–Feb 5 2004) was selected and intensively analyzed. A spectral analysis was conducted as the spatial and temporal extent of the study area provides a level of detail not previously available. The analysis of time series confirmed the expected spectral characteristics of a shelf velocity time series with statistically significant peaks over a broad range of low frequency signals, as well as tidal and near-inertial time-bands. A seasonal division of the data showed a striking contrast in the distribution of spectral energy between the mixed and stratified seasons with near-inertial and diurnal constituents contributing strongly to the variance in the stratified period.
While not unexpected, the spectral analysis demonstrated the importance of low frequency processes in the surface layer. In terms of general circulation, the time-averaged, spatial mean velocity of 4 cm/s in the down-shelf along-shelf direction and 3 cm/s in the offshore across-shelf direction compared well to historical surface measurements in the study region. However, as the spatial resolution of the data set reveals, this simple measure masks significant spatial variations in the mean and seasonal flow structures, and their temporal/spatial correlation scales, which have not been well determined in the surface layer of the shelf in previous studies.
In addition, there is significant variability from the observed mean. This study took particular interest in the across-shelf flow patterns as many aspects of across-shelf transport are still poorly understood. Animations of the low frequency current field showed that several episodic patterns with strong across-shelf currents O(20-30cm/s) regularly occurred throughout the period study. Offshore flow events are found to occur six times more often than onshore flows, and these offshore events are highly episodic with five recurring spatial patterns. The most common offshore flow pattern was a shelf-wide flow that occurred throughout the year, but was strongest and most well-defined October through April, when the water column is typically less stratified. Other offshore flow patterns were characterized by smallerscale regions of localized offshore flow, with a flow at the bend or 'point' in the New Jersey coast being most common and most energetic. An empirical function analysis of the across-shelf flow suggested the bulk of the across-shelf variance was contained in the first two modes whose spatial structure and temporal amplitude captured the shelf wide and 'point' flow events. Although individual offshore flow events are unlikely to drive significant transport, multiple events, which often occur in rapid succession, may plausibly advect near-surface material tens of kilometers offshore.
Of these episodic events, shelf wide across-shelf flow events showed a distinct seasonal dependence related to local forcing. To quantify these observations, times series of the spatial mean surface current, wind stress, and coastal sea-level were analyzed using several types of correlation analyses. Results suggested that during the mixed period (Dec-Mar), across-shelf wind could drive across-shelf current and sea level setup, at times. While no similar effect was evident in the stratified period (Jun-Sep), there was evidence of an increased veering angle between the wind and the surface current with distance offshore. It was speculated that these seasonal differences resulted from a larger Ekman surface boundary layer depth during the mixed period.
|Commitee:||Kohut, Josh T., Lipphardt, Bruce L., Wong, Kuo-Chuin|
|School:||University of Delaware|
|School Location:||United States -- Delaware|
|Source:||DAI-B 70/01, Dissertation Abstracts International|
|Keywords:||Across-shelf circulation, Across-shelf flow, HF radar, Mid-Atlantic Bight, Shelf water, Surface currents, Surface velocity|
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