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Observations of an Along-shelf Jet Following Hurricane Edouard, 1996
(A work in progress in collaboration with H. Seim and S. Lentz
- June, 2003)
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Abstract
The second cruise of the CMO dye studies took place between September
4-18, 1996. On September 2, 1996, two days before the start of the
cruise, hurricane Edouard, which was tracking northward along the
east coast of the U.S., passed over the CMO mooring site. The
storm caused significant diapycnal mixing at the CMO site, which
in turn led to a reduction in stratification over much of the
water column. Despite such strong atmospheric forcing, however,
certain other aspects of the shelf circulation were relatively
unaffected by Edouard.
Of particular interest to the present study was a westward along-shelf
jet centered at approximately the 70 m isobath near the CMO site.
Shipboard ADCP observations taken by Barth et al. on September 1 (the
day before the storm), as well as a dye-release experiment performed
from September 7-11 and shipboard ADCP observations taken on September 8
(5-9 and 6 days after the storm, respectively) showed a localized
westward flow approximately 10 km in width, with maximum westward
velocities at its core in excess of 20 cm/s. As the jet was nearly
20 km inshore of the shelf-slope front, its relation to the front is unclear.
The following is a description of the observed jet in terms of its
hydrographic and velocity structure, plus some speculation regarding
its possible origin, associated dynamics, and eventual fate.
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Effects of Hurricane Edouard
The effects of hurricane Edouard on the New England shelf can be seen
in hydrographic sections made before and after the storm's passage. On
August 31 and September 1, 1-2 days before the storm's arrival at the
CMO site, the hydrographic sections shown in Figure 1 were made by
Barth et al. using Seasoar.*
The sections consisted of 3 legs, the first and second legs representing
a nearly continuous section (with a 1 hour interruption) on August 31,
and the third leg repeating the first on September 1. Particularly
noteworthy in the first and third legs is the existence of a warm surface
layer inshore of the shelf-slope front. The layer extended to
approximately 10 m depth, and was characterized by temperatures above
18 oC.
In contrast to the pre-hurricane sections, a cross-shelf transect taken
during the dye release cruise on September 5, 3 days after the hurricane
(Figure 2) shows a well mixed upper water column inshore of the front,
with temperatures < 15 oC over most of the water column.
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Figure 1. Hydrographic section taken by Barth et al. on August 31
- September 1, 1996, 1-2 days before hurricane Edouard.
(Images from S. Pierce's CMO Seasoar data report,
http://diana.oce.orst.edu/cmoweb/csr.)
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Figure 2. Hydrographic section taken on September 5, 3 days
after hurricane Edouard. (Note the change in colormap and scale compared
to Figure 1.)
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Pre-Edouard Velocity Measurements
Shipboard ADCP velocity measurements taken during the above Seasoar
transects showed a broad northwestward flow of between 10-30 cm/s over
much of the shelf, with a transition to southeastward flow of 10-20 cm/s
seaward of the shelf slope front (Figure 3). Of note in the last leg
of those sections (far right in Figure 3) is a localized core of
northwestward flow which has pinched off and is now bordered by an
eastward return flow on both its inshore and offshore sides. The data
show that this westward jet was centered at approximately
40.4 oN, was 10-15 km in width, and extended from the surface
to approximately 40 m depth. A plan view map of the same detided ADCP
measurements at a depth of 23 m is shown in Figure 4.
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Figure 3. ADCP section taken by Barth et al. on August 31
- September 1, 1996, 1-2 days before hurricane Edouard.
(Images from S. Pierce's CMO Seasoar data report,
http://diana.oce.orst.edu/cmoweb/csr.)
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Figure 4. Plan view of observed and subtidal velocity vectors
at 23 m depth taken by Barth et al. between August 31 - September 1,
1996, 1-2 days before hurricane Edouard. Red circle indicates the
region corresponding to the right-most panels in Figure 3. (Images
from S. Pierce's CMO Seasoar data report,
http://diana.oce.orst.edu/cmoweb/csr. Red circle added for the
sake of the present discussion.)
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Post-Edouard Velocity Measurements
Two ADCP transects made on September 8 also showed a westward jet with
approximately the same characteristics and in the same location as in the
September 1 transect (Figure 5). Again the jet was approximately 10-15 km in
width; however, this time it was centered slightly further on-shelf at
approximately 40.45 oN. More notably it now extended over
most of the water column. A plan view map of the residual barotropic velocity
(i.e., detided and demeaned) from the same ADCP measurements (Figure 6)
provide a second view of the jet's structure.**
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Figure 5. Raw (i.e., not detided) u-component of velocity
from two September 8 ADPC transects showing a westward-flowing
mid-shelf jet.
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Figure 6. (a) Residual barotropic velocity from September 1996
CMO dye cruise after removing tides and mean flow. Red vectors
correspond to the ADCP data shown in Figure 5. (b) u-component of
residual barotropic velocity as a function of latitude along ADCP
transects shown in Figure 5.
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Hydrography and Geostrophic Velocity
In an effort to better understand the dynamics of the jet, hydrographic
measurements were also made during the first leg of the September 8 ADCP
transect (see left panel of Figure 5). The resulting along-shelf
geostophic velocity is shown in Figure 7. The section shows good
agreement with the corresponding ADCP velocities, including a 10+ km
wide (nearly) barotropic westward flow, with a counter-flow to its
northern side. A small near-surface counter flow is also evident in
both sections left of the jet's center, although the magnitude differs
slightly in the two estimates. These results indicate that the jet is
geostrophically balanced to lowest order, and hence has the potential
of being a somewhat long-lived feature on the shelf. This would at
least in part explain its persistence from September 1 - September 8.
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Figure 7. Geostrophic velocity estimated from hydrographic
data collected during the first leg (left panel) of the ADCP
transect shown in Figure 5. Note the different colormap and
horizontal scale compared to that figure.
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Summary to Date
The above observations provide preliminary evidence of a 10-15 km wide
quasi-transient along-shelf jet centered at approximately 40.4 oN
over the New England shelf between September 1 and at least September 8, 1996.
The jet apparently began on September 1 as a baroclinic flow separation spun
off of a broad westward / northwestward flow along the shelf. The passage
of hurricane Edouard on September 2, 1996 apparently mixed the jet over much
of the water column. However, as evidenced by ADCP and hydrographic
observations on September 8, the jet remained otherwise in tact.
The existence and persistence of the jet raises a number of questions
regarding the dynamics of the shelf circulation, including its response
to hurricane Edouard. What was the relationship between the jet and the
mixing and restratification on the New England shelf before and after
the hurricane? What were the dynamics associated with the jet's origin
and its likely fate in terms of frictional or buoyancy spin-down?
What were the broader impacts of the hurricane in terms of mixing and
subsequent adjustment of the waters over the New England shelf?
We hope to address these questions in more detail in the near future.
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* See also S. Pierce's CMO Seasoar data report,
http://diana.oce.orst.edu/cmoweb,
for more information on the CMO Seasoar cruises.
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Project Background