Observing Surface Circulation

OBSERVING THE SURFACE CIRCULATION ALONG THE SOUTH-CENTRAL CALIFORNIA COAST USING HIGH FREQUENCY RADAR: CONSEQUENCES FOR LARVAL AND POLLUTANT DISPERSAL

Principal Investigators: Libe Washburn (UCSB), and Steven Gaines (UCSB)

Summary of Research

Four high frequency (HF) radars were operating in the Santa Barbara Channel for use in observing the surface circulation patterns in the region. In collaboration with Dr. Carter Ohlmann we conducted experiments which combine observations from drifters and HF radars to assess the accuracy of current velocities derived from HF radar.

Edwin Beckenbach's (Ph.D. June 2004) dissertation work was largely funded by this and a previous MMS research grant. The primary focus of his dissertation research is the surface circulation dynamics of the western Santa Barbara Channel. This work builds significantly on oceanographic research in the region conducted by the Center for Coastal Studies at the Scripps Institution of oceanography. A paper was published earlier this year (Beckenbach and Washburn, 2004) which describes a propagating wave-like phenomenon resembling a type of coastal trapped wave. Another focus of Beckenbach’s dissertation is the relationship between wind forcing and the two-dimensional surface circulation in the western Channel.

A second HF radar system was deployed in the eastern Santa Barbara Channel in May to September 2004. The first radar system in the eastern Channel was installed in 2003 at Reliant Energy’s Mandalay generating station. In addition to providing important data on the circulation of the eastern Channel, observations from this radar supported a juvenile fish recruitment study also funded by MMS (Milton Love, principal investigator).

2. Surface Circulation in the Santa Barbara Channel

As described by (Beckenbach and Washburn, 2004), we used a three-year time series of HF radar-derived currents to describe propagating rotary wave-like features in the western Santa Barbara Channel. This was a new discovery and our analysis suggested that the features are topographic Rossby modes, a resonance phenomenon setup by large scale coastal trapped waves which excite resonant oscillatory flows over the Santa Barbara Basin. Reviewers of our paper commented that this is the best oceanographic evidence to date documenting the existence these modes.

Another focus of our research is the Channel’s two-dimensional flow structure and its relationship to wind forcing. We find that the long term mean circulation is characterized by convergence over the Santa Barbara Basin, opposite the flow pattern predicted by the wind field (Fig. 1). This is important because it indicates the convergence is not wind driven. On shorter time scales a clear relationship between wind forcing and the surface flow is evident. Conditional averaging of the surface flow based on modes of the wind field shows that consistent circulation states correspond to the wind modes. This analysis also supports model predictions of (Oey et al., 2001) which relate the strength of the characteristic cyclonic flow to the along-channel wind gradient.

Figure 1. Four-year mean (1998-2001) surface divergence field in the western Santa Barbara Channel. Large blue area near the center of the color contours indicates convergence and downwelling-favorable flow. Smaller red areas along northern boundary of contours shows areas of divergence and upwelling-favorable flow. Color bar at right shows divergence scale where blue indicates convergence and red divergence. Black contours are lines of constant velocity potential such that the divergent part of the velocity field is at right angles to the contours. Dotted contours are negative, solid contours positive. The divergent part of the flow field points from lower to higher contours.

A new aspect of the Channel circulation that we are exploring is the recent discovery of small eddies over the inner shelf and mid-continental shelf. Moored nutrient measurements (funded by the Santa Barbara Channel Long Term Ecological Research program) show that these eddies transport nutrients from deep offshore waters onto the inner shelf. Continuing analysis of this project will further describe these features and their relationship to larval transport and nutrient transport to nearshore waters.

Figure 2. Surface circulation in the western Santa Barbara Channel on 8 June 2002 at 1600 GMT. Dark blue area of clockwise rotating arrows indicates location of small nearshore eddy off of Coal Oil Point, CA. These eddies transport cold, nutrient rich waters from offshore onto the continental shelf. Color scale indicates rotation rate compared with the earth’s rotation rate. Red areas indicate counter-clockwise rotation and blue areas indicate clockwise rotation. Arrow at lower right indicates flow speed of 0.3 m s^-1.

3. Comparisons between Lagrangian drifters and HF radars

We are also collaborating with Dr. Carter Ohlmann in a comparative study to determine how small scale spatial variability affects the accuracy of HF radar-derived current vectors. Our experimental approach is to repeatedly deploy patterns of drifters in selected radar coverage cells. In situ surface current vectors are computed from sequential drifter positions and compared with simultaneous surface current vectors obtained from the radars. Results from our continuing analysis indicate that differences between the drifter and radar velocity vectors are smallest when small scale spatial variance in the drifter derived velocities is smallest. This suggests that unresolved small scale velocity features are important in determining the accuracy of HF radar-derived currents. This work will progress more rapidly when Dr. Ohlmann returns from his year-long sabbatical at Scripps. This MMS grant also served as “seed funding” for a successful NSF grant which will allow us to expand the scope of this aspect of our research.

4. Larval fish recruitment and oceanographic observations

We currently have two radars deployed in the eastern Santa Barbara Channel. A private landowner in Summerland allowed us to erect a radar site on his coastal property for the period 1 May – 1 September 2004. This time period spanned the peak recruitment time for bocaccio, an important rock fish species which settles on oil production platforms. This site, along with our site at Reliant Energy’s Mandalay generating station, provided good coverage over much of the eastern Channel, including coverage over all of the oil production platforms in the area.

Data from the Mandalay and Summerland sites will be used to examine surface circulation features in the area. Inspection of current patterns to date reveals the presence of small scale eddies nearshore. This indicates that features we observed previously in the western Channel may important general features of the nearshore current field.

Our HF radar current data in the eastern channel will complement in situ observations now being obtained on oil production platforms Gina and Gail. Instrumentation now deployed on the platforms include acoustic Dopper current profilers (ADCP’s), conductivity-temperature-depth (CTD) sensors, and thermistors. Data from these instruments will be used to interpret data from twice-per-week SCUBA surveys to determine rates of settlement of rockfish species on these two platforms (research supported by MMS; principal investigator is Dr. Milton Love, of the UCSB Marine Science Institute). Our working hypothesis is that settlement of pelagic juvenile rockfish is related to identifiable near-surface current patterns. Our research group is working closely with Milton Love’s group in this research.

2. Status of the HF radar array

We are currently operating four HF radars in the Santa Barbara Channel at the following locations: 1) Summerland; 2) Refugio State Beach; 3) Coal Oil Point; and 4) Mandalay generating station near Oxnard. We are also maintaining a fifth site near Pt. Sal on Vandenberg Air Force Base. This site is not presently in operation because we moved its electronics boxes to other sites. However, its antennas, housing, and other equipment have been left in place in anticipation of the site being used in the future Southern California Coastal Current Observing System. Coverage from these radar sites now spans the Santa Barbara Channel and extends approximately from Gaviota to the eastern channel entrance.

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