Woods Hole Oceanographic Institution

Paula S Fratantoni

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» Climate variability and the North Atlantic shelfbreak current

Middle Atlantic Bight Sheflbreak Circulation

Robert S. Pickart

Results From Prior NSF Research: Paula S

A dominant feature of the circulation in the coastal region south of New England is the shelfbreak frontal jet, a narrow baroclinic current that flows equatorward as part of a large-scale buoyancy driven coastal current system, extending from the east coast of Greenland to Cape Hatteras, North Carolina. On a large scale, the jet establishes a fast advective link between the subpolar and subtropical domains, enabling the equatorward transport of high latitude climate-driven signals.  Regionally, the jet and its associated hydrographic front represent a barrier limiting the exchange of waters between the relatively cold, fresh shelf and the warmer, saltier open ocean. Hence, the shelfbreak current plays a fundamental role in governing the mass and property budgets on the shelf as well as influencing fish populations, migrations, and feeding sources to a variety of species.

The shelfbreak jet is highly variable, hindering attempts to describe its basic structure and dynamics.  While it is well established that the jet is sensitive to a variety of forcing mechanisms, little is known about the jet?s time-varying structure and the dominant mechanisms responsible for this variability.  As a postdoctoral investigator, my research focused in part on constructing a description of the unperturbed structure of the shelfbreak jet in the Middle Atlantic Bight, diagnosing the dynamics of this mean structure, and then characterizing the mesoscale fluctuations about the mean.  This was accomplished using a collection of highly resolved shipboard acoustic Doppler current profile (ADCP) velocity and hydrographic occupations which were repeated over several years along a section crossing the shelfbreak.  The data was collected as part of the PRIMER experiment funded by the Office of Naval Research, an interdisciplinary program aimed at better understanding the physical oceanography and acoustics of the outer shelf.  My analysis provided the first-ever, high-resolution mean description of both the primary (alongstream) and secondary (cross-stream) structure of the shelfbreak jet. My results indicated that the mean jet is convergent throughout the entire water column in response to locally convergent topography.  Furthermore, diagnosing the circulation within a dynamic framework, I was able to demonstrate that converging isobaths can drive an ageostrophic circulation similar to that associated with open-ocean fronts. 

In order to address the time-varying character of the shelfbreak jet, I then examined the velocity records from the PRIMER moored array, consisting of instruments in the jet itself as well as over the adjacent continental slope.  Historical observations and models suggest that the perturbation of the shelfbreak jet may lead to potentially significant exchange between the shelf and slope regions.  I used the velocity timeseries from the moored array to characterize the mesoscale variability at the shelfbreak, concluding that the dominant velocity fluctuations at the outer shelfbreak are not caused by several of the forcing mechanisms traditionally thought to influence the jet.  These include local effects, such as wind driving and tidal rectification, as well as offshore forcing due to Gulf Stream rings, topographic Rossby waves, and the meandering of the Gulf Stream. Instead, I concluded that the dominant variability is probably caused by baroclinic instability of the jet.


Related Publications:

Fratantoni, P. S., R. S. Pickart, D. J. Torres, and A. Scotti, 2001. Mean structure and dynamics of the shelfbreak jet in the Middle Atlantic Bight during fall and winter. Journal of Physical Oceanography, 31, 2135?2156.

Fratantoni, P. S. & Pickart, R. S. (2003). Variability of the shelfbreak jet in the Middle Atlantic Bight: Internally or externally forced? Journal of Geophysical Research C: Oceans, 108, 35-1 - 35-16.

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