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BOON research Projects > Oceanography
currents- the Cape Mendocino Eddy

Contact: John Largier Ph.D.

High-frequency radar (HF-Radar) units have been installed along the California Coast as part of the California Ocean Currents Monitoring Program (COCMP). These systems provide high-resolution maps of surface currents just offshore. Hourly estimates of the surface currents (speed and direction) are provided on a few-kilometer grid covering the region from the coast to the deeper ocean (between 60-120 kilometers depending on location).

Two HF-Radar stations have been mapping currents between Cape Mendocino and Point Arena since August 2008. The most striking feature is the “Cape Mendocino Eddy”, a seasonally recurring eddy that forms just offshore of Cape Mendocino in the early summer. It then migrates slowly southward, maintaining its position between Cape Mendocino and Point Arena into the fall. The eddy migrates offshore in the winter.

Large eddies in this region have been noted before. For example, Lagerloef [1991] used satellite maps of sea-surface temperature to document the occurrence of a seasonally recurring eddy far offshore of this same section of coast. The strength of the observations described herein is that we actually record the ocean surface currents (not just temperature) at high spatial and temporal resolution, and can measure much closer to the coast than permitted by satellites. The resolution of these measurements allows us to track changes in currents on an hourly basis, and to describe the influence of the eddy on the trajectories of nutrient-rich water parcels, which serve to supply the productive northern California ecosystem.
Researchers are currently studying the formation mechanisms of the eddy, its predictability, and its influence on the surrounding environment. A combination of HF-Radar observations and satellite measurements (sea-surface temperature, chlorophyll, winds, and larger-scale currents) are used to describe the eddy.

SatelliteAltimeterMovie.mov (Quicktime ƒ)

Purpose: To place the Cape Mendocino Eddy in a Longer-Term Context

This movie shows the surface currents estimated using satellite altimetry from September 1992 through June 2009 (satellite estimates are produced by Ssalto/Duacs and distributed by Aviso with support from Cnes). These geostrophic currents represent large-scale currents that are only due to sea-surface height variations. Smaller-scale motions (such as local wind-driven effects) are not included in the estimates.

  • legend black arrowBlack arrows are the geostrophic currents predicted from satellite altimetry.
  • legend magenta circleMagenta circles are the locations of the HF Radar stations.
  • legend blue diamondThe blue diamond is the location of NDBC buoy 46014 (where the wind is measured).

SSHDeviationsCapeMend.mov (Quicktime ƒ)

Purpose: To place the Cape Mendocino Eddy in a Longer-Term Context, and to provide a view near the coast during those years that the satellite estimated geostrophic currents had "coastal" gaps (2001-2006).

This movie shows the sea surface height anomalies from January 2000 through April 2010 (obtained from Coastwatch, originally produced by Aviso with support from Cnes). Deviations of the ocean surface are indicative of large-scale flow patterns, and can be used to estimate geostrophic currents. For example, the large positive anomaly (red) near the coast during August 2008 is associated with anticyclonic (clockwise) flow, and represents the Cape Mendocino Eddy.

More information is provided in the following documents:

This movie shows the surface currents measured by the HF-Radar in red, and "geostrophic" (large-scale) currents estimated using satellite altimetry in black (satellite estimates are produced by Ssalto/Duacs and distributed by Aviso with support from Cnes). Geostrophic currents represent large-scale currents that are only due to sea-surface height variations. Smaller-scale motions (such as local wind-driven effects) are not included in the estimates.

Symbols used in this movie: