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EGU2016-11482
C. A. Collins, T. Margolina and T. A. Rago
Naval Postgraduate School, Oceanography Department, Monterey, CA, USA
Abstract
A number of programs have collected observations of deep
currents and water properties over the continental shelf off
Central California during the past three decades. Here we
summarize the results of these measurements. The oldest data
set included bimonthly observations off Point Sur (33°20’N) from
April 1988 to April 1991 using an acoustically tracked dropsonde
and a NBIS Mk III CTD. The poleward flow observed above 1000
m was weaker at depth but generally dominated the flow pattern.
Deep (~1000 m) currents were also observed off California
from August 1994 to September 2009 using current meters and
RAFOS floats. Current meter data were collected at nine
locations for time periods ranging from two months to 76 months.
A total of 144 months of float data were collected. Analysis of
current meter data included histograms, progressive vector
diagrams, stick plots, kinetic energy and rotary spectra, stick
plots, means and standard deviations. Float data were analyzed
using trajectories and calculating means, standard deviations,
and diffusivities.
For current meter data, semidiurnal tidal energy dominated the
kinetic energy spectrum, anticyclonic rotary motion exceeded
cyclonic motion, kinetic energy was typically an order of
magnitude greater than for diurnal frequencies, and kinetic
energy decreased about an order of magnitude as depth
increased by 1000 m. Mean speed for current meter (float) data
was 6.1 (4.0) cm s-1 and alongshore variability exceeded across
shore variability. Two floats were entrained in mesoscale eddies,
one cyclonic and the other anticyclonic; the eddies moved
westward at a speed of about one cm s-1. Seasonal variability
along the continental slope was marked by late summer or early
. warming; eddy kinetic energy was minimum in February, 3
fall
cm2 s-2.
Figure 6. S2 1200 m currents (upper)
Rotary spectra. (lower) Rotary coefficient.
Figure 2. Mean and standard deviation of velocities from RAFOS floats
deployed from 1994-2004. Pressures ranged from 1152 to 2205 dbar.
Data were binned in the geographical areas outlined in blue. Note that
different scales are used for the mean velocity and the variance ellipse.
Float trajectories are shown in grey.
Figure 4. Vector mean flow and variance ellipses for current meter results
using Aanderaa RCM 8 current meters. The current meters were deployed
about 20 m above the bottom. Ellipse color corresponds to the deployment
shown in the box to the right of the depth scale.
Summary
Vertical profiles over the slope indicated mean
poleward flow in the upper 1000 m but deeper flow
was not statistically greater than zero.
Float measurements (1000-2000 m depth) indicated a
divergence of alongshore flow. Mesoscale eddies
(both cyclonic and anticyclonic) were observed.
Some evidence for equatorward recirculation above
the deeper portion of the slope. Mean speeds for the
floats were 4.0 cm s-1 and alongshore variability
exceeded across shore variability.
Figure 1. Mean velocity profiles off Point Sur,
California (36.3°N) from acoustically tracked
dropsonde (Pegasus) measurements, 1988-1991.
Data were collected bimonthly at nine stations
along 36.3°N over the continental slope from 1000
to 3700 m deep.
Figure 3. Float 74 was entrapped in an anticyclonic eddy for 294
days at a pressure of 1541 dbars. The period of the eddy was 8.6
days, diameter 22.7 km, swirl velocity was 9.5 cm s-1 and kinetic
energy 45.6 cm2s-2, and Rossby number was 0.1. The eddy moved
toward 291°T at 1.0 cm s-1.
Correspondence: Curtis A. Collins, Department of Oceanography, Naval Postgraduate School, Monterey, California, USA;
([email protected])
Figure 5. Annual variability of currents (upper) and temperature (lower) at
1200 m (about 200 m above the bottom) at mooring S2 (black ellipse in
figure 4). Data were collected from 1998 to 2005 using an RCM8 current
meter. Temperature increase from mid April to mid June was associated
with acceleration of southward flow.
For current meter data, semidiurnal tidal energy
dominated the kinetic energy spectrum, anticyclonic
rotary motion exceeded cyclonic motion, kinetic
energy was typically an order of magnitude greater
than for diurnal frequencies, and kinetic energy
decreased about an order of magnitude as depth
increased by 1000 m. Mean speed for current meter
data was 6.1 cm s-1 and alongshore variability
exceeded across shore variability. Two floats were
entrained in mesoscale eddies, one cyclonic and the
other anticyclonic.
Seasonal variability along the
continental slope was marked by late summer or early
fall warming; eddy kinetic energy was minimum in
February, 3 cm2 s-2.
Acknowledgements. Financial support for data collection was provided by the U.S. Navy (Office of Naval Research, Oceanographer
of the Navy, Naval Postgraduate School, N45), Environmental Protection Agency and Monterey Bay Aquarium Research Institute.