July 29 Year day 209210

1
Secondary Circulation and Mixing in a Buoyant
Coastal Current
Secondary Circulation and Mixing in a Buoyant
Coastal Current
Robert J. Chant and Scott M. Glenn Inst. Of
Marine and Coastal Sciences, Rutgers University,
New Brunswick, N.J.
Abstract
Figure 6 Cross shore pressure difference in cm
between moorings 1 and 3 after mean has been
removed (upper panel). Comparison between
fluctuating cross-shore pressure gradient (black
line) and the fluctuating Coriolis acceleration
due to variations in the sectionally mean along
shore velocity (lower panel).
Shipboard and moored observations are
presented that characterize aspects the secondary
circulation and the rate of vertical mixing in
a buoyant coastal current that impinged on
Rutgers Universities Coastal Ocean Observatory
during July of 2000. While winds during the
event were light there was a dramatic response of
the current to the winds fluctuations and the
depth averaged fluctuations are in geostrophic
balance. An ensemble average of the down shelf
flow shows a vertical veering that is consistent
with a bottom Ekman layer that encompassed the
entire water column. A similar cross shore flow
structure is also observed in detided shipboard
ADCP data. Cross shelf flows are appreciably
stronger offshore where the water becomes
vertically stratified. The cross shore
baroclinic pressure gradient is an order of
magnitude smaller than the forcing of the
secondary flows by the vertical shear. By
assuming a balance between the shear forcing and
friction we estimate the vertical structure of
eddy viscosity.
Salinity and Velocity
Salinity and Velocity
Analysis on the previous panel indicates that
baroclinic forcing is weak relative to forcing of
the secondary flows by the vertical shear.
Therefore we assume that the shear forcing is
balanced primarily by friction. By applying the
system of equations below to the event mean flow
at mooring 2 we estimate the vertical structure
of shear stress and vertical eddy viscosity (Fig
14). Eddy viscosity is maximum in the surface and
bottom and is reaches a minimum value of 510-4
in the interior. Given a layer thickness of 10
meters this corresponds to a vertical mixing time
of about 2 days.
Along Shore Velocity and Salinity
Along Shore Velocity and st
cm/s
Cross Shore Velocity and st
Cross Shore Velocity and Salinity
Figure 9 Hourly cross shore flows from mooring 4
at 5 meters above bottom and 15 meters above
bottom.
Depth Averaged
Depth Dependent
Fig 11 Overlay of detided along-shore velocity
and salinity (upper left) along shore velocity
and density (upper right), cross shore velocity
and salinity (lower left) and cross shore
velocity and density (lower right).
Offshore
Figure 7 Low passed currents from four moorings
during passage of buoyant plume. Vectors change
color from red at the surface to black at the
bottom. Currents are orientated such that
offshore flows are to the right and down shelf
flows are downward. Along shore wind stress is
plotted in green line.
25 cm/s
Wind Stress
Shear Equation
Down Shelf
5
Moored data indicates that tidal period motion
undergoes appreciable variations in its vertical
structure (Fig 9). For example, tidal currents
are appreciably sheared on day 206 but are weakly
sheared following day 210. The vertical structure
of the tidal phase also undergoes appreciable
change. After day 210 tidal period motion at 5
and 15 mab occurs in phase, in contrast to tidal
period motion following day 200, when bottom
currents are phased advanced. A standard tidal
model composed of 4-5 constituents would not
capture these variations in the structure of the
tide. Subsequently we use a tidal model
containing only a semidiurnal and a diurnal
constituent obtained from moored data one day
around each shipboard survey. These constituents
are interpolated in the cross-shelf direction to
detide the shipboard ADCP data. Estimates of the
detided shipboard data for July 28 and July 29
are shown in Fig 10, along with filtered currents
from the four moorings. In general the agreement
between the low-passed moored data and the
detided shipboard data is good. The agreement is
best for for the transect to the north of the
mooring array, which shows a relatively weak
veering on July 28, and a strong
counter-clockwise rotation of the currents with
depth on July 29.
Figure 1 Regional map
On the northern most line the maximum down shelf
velocities exceed 60 cm/s in a coherent surface
jet located 10 km offshore (Fig 11). The jets
core overlies the stratified fluid where the 30
psu isohaline has detached from the bottom.
Downshelf flows are strongly sheared both
vertically and horizontally. Vertical shear is
strongest over the jet and weak in the
unstratified fluid near the shore. Relative
vorticity across the jet exceeds f/2.
Cross-shore flows are also the strongest in the
jet and are consistent with the expected vertical
structure in a bottom Ekman layer. Surface
currents flow shoreward at 5-10 cm/s offshore at
10 cm/s at depth. The offshore flow occurs just
in the stratified fluid beneath where the
isopycnals associated with the buoyant plume have
detached from the bottom.
4
Dyne/cm2
Mooring
10 km
3
2
1
2
3
Data set was collected in July, 2000 with Doppler
current meters at moorings 2,3,4,5 (Fig 2),
T-chains and pressure sensors at 2 4, salinity
sensor at LEO-15 (1) and ADCP/CTD shipboard
surveys north and south of the array.
4
2
5
Year Day
With the velocity and density data shown in
figure 11 we calculate two terms in the momentum
balance the drive cross shelf shears. The first
due to the cross shore density gradient (Px-
Px), where Px the is the vertical average
(figure 12 upper panel) and the forcing due to
the Coriolis effect acting on the vertical shear
in the along-channel flow, f(v-v) (figure 12
lower panel). While the magnitude of these two
terms is similar, increased density gradients are
only associated with small-scale undulations of
the pycnocline. When these terms are horizontally
averaged over the scale of the coastal jet,
between km 6-16, it is seen that the baroclinic
pressure gradient is 5-10 times weaker than the
shear forcing (figure 13).
Figure 2 Mooring deployment July 2000. Isobaths
are in meters. LEO-15s Node-A is at location 1
A coastal current impinged on the LEO-15 research
area just after day 200 and again during days
206-210 (Fig 4). During the second event both
cross-shore and along-shore wind stresses were
weak and bottom salinity drops below 30 psu (Fig
3). Low passed surface currents exceed 40 cm/s
(Figure 4). As the current impinges on the
region a bottom cold front moves offshore. The
front passes mooring 2 on day 206 and mooring 4
on day 207, during which time wind are weak. Off
shore advection is consistent with cross shore
flows that are directed off-shore at the bottom
and off shore at the surface (Fig 4).
Fluctuations in the depth averaged along shelf
flows are nearly in geostrophic balance (Fig 6).
Fluctuations in the cross shelf pressure gradient
correspond to approximately 2 mm/km. During the
passage of the buoyant plume the current vector
rotates counter clockwise with depth, suggestive
that the bottom Ekman layer occupies the entire
water column (figure 7) . However, the intensity
of this veering is extremely sensitive to wind
stress. Even the weak events with stresses less
than 0.3 dynes/cm2 , such near day 210,
profoundly affect the structure of the current.
The ensemble mean of the down shelf flows (Fig 8)
depicts a bottom Ekman layer encompassing the
entire water column. Estimates of the terms in
the depth averaged equation indicate that the
mean wind stress is near zero during this event,
and bottom stress is directed up the coast and
onshore. The residual term would be balanced by
a sea level that slopes upward to the north at .2
mm/km and off shore by 2 mm/km.
July 28 (Year day 208-209)
July 28 (Year day 208-209)
Figure 14 Estimates of the residual cross-shore
interior stresses (left panel) and vertical eddy
viscosity (right panel) based on
Px
Figure 10 Detided shipboard ADCP data and low
passed filtered moored ADCP data from July 28th
(upper panel) and July 29th (lower panel). Red,
green and blue vectors depicts currents at the
surface, mid-depth and bottom respectively.
Figure 3 Low passed wind stress and salinity at
LEO-15
1) Fluctuations in along-shelf flows are in
geostropic balance with the cross shelf pressure
gradient. 2) The forcing of secondary flow in a
buoyant coastal current by the vertical shear is
not balanced by baroclinic forcing. 3) Cross
shore movement of cold saline waters is
accomplised by secondary flows in coastal
current. 4) Cross shore flows are maximum in the
stratified fluid where isohalines on the
offshore side of coastal current detach from the
bottom. 5) The structure of the coastal current
is very responsive to even weak wind forcing, as
suggested by Yankovski and Garvine (JPO, 2000).
m/s210 -5
Along shore currents and temperature
Cross shore currents and temperature
July 29 (Year day 209-210)
July 29 (Year day 209-210)
2
m
Figure 8 Ensemble mean of currents and terms in
the depth averaged momentum equation over times
that the mean flow is down the shelf at speeds
greater than 10 cm/s
m/s210 -5
4
18
12
12
14
m/s210 -5
14
Fig 12 Cross shore baroclinic pressure gradient
(color) and density (contour) upper panel. Shear
forcing (color) and along-shelf flow (contours)
lower panel.
Fig 13. Mean of shear forcing (red line) and
baroclinic forcing (blue line) across jet (km
6-16)
Figure 4 Low passed cross -shore velocity and
temperature.Isotherms are contoured at 2o
intervals. Offshore velocities are negative.
Figure 3 Low passed along-shore velocity and
temperature. Isotherms are contoured at 2o
intervals
On the web at http//marine.rutgers.edu/cool/coolr
esults/agu2002