1. Observations of Deep Flow along the Central California Coast
C. A. Collins, T. Margolina and T. A. Rago
EGU
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 fall warming; eddy kinetic energy was minimum in February, 3 cm2 s-2.
Figure 6. S m currents (upper) Rotary spectra. (lower) Rotary coefficient.
Figure 2. Mean and standard deviation of velocities from RAFOS floats deployed from 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 ( 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.
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. .
Figure 1. Mean velocity profiles off Point Sur, California (36.3°N) from acoustically tracked dropsonde (Pegasus) measurements, 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 The eddy moved toward 291°T at 1.0 cm s-1.
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.
Correspondence: Curtis A. Collins, Department of Oceanography, Naval Postgraduate School, Monterey, California, USA;
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.