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.
