Instrument Type: CODAR Ocean Sensors SeaSonde HF Radar System

Sea surface current data is measured with SeaSondes, compact HF radars designed by CODAR Ocean Sensors, Ltd. They use patented FMCW (frequency modulated continuous wave) signal processing and crossloop direction finding to measure ocean surface currents and wave parameters of the coastal ocean. Each station consists of a transmitter, receiver, 2 antennas, and data acquisition and processing computers. A transmitter broadcasts a frequency modulated radio frequency pulse at 2Hz (two pulses per second). The Doppler shifted return signal (sea echo) is detected with a compound cross loop/monopole receive antenna and the signal is processed into estimates of surface current speed and direction and wave heights, period and direction. BML's three 12 MHz stations have a spatial resolution of 2 km and create hourly maps of surface currents out to a distance of 30-50 km (20-30 miles) along a 65 km (40 mile) length of shore. Onshore wave directional spectral data are also produced. Total coverage area for the 3- radar array is approximately 2800 km2. BML's two 5 MHz stations have a spatial resolution of 5 km and create hourly maps of surface currents out to a distance of 90-200 km (55-125 miles). Onshore wave directional spectral data are also produced.

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Support for these observations has been provided by the California Coastal Conservancy.

Ocean Currents

Ocean currents are most commonly observed by several methods including drifters, radar and current profilers. Drifters are used to measure surface currents, which are mainly driven by wind and tide. When deployed, drifters are allowed to move freely with the current, logging position data to an internal GPS unit. Upon recovery, the path of the drifter can be plotted and information gained about the movement of the surface water.

Acoustic Doppler Current Profilers (ADCP’s) use sonar techniques and the Doppler Effect and are used to study currents throughout the water column. These measurements can be made at a stationary location, such as an oceanographic mooring, or from a sensor mounted on the hull of a vessel. ADCP’s can be installed on the seafloor in an upward looking orientation, or from a buoy or boat in a downward looking orientation. Transmitting sound waves into the water, the instrument listens for echoes scattered back off particles suspended in the water column. Assuming that these particles are moving with the ocean current, the Doppler shift of the return signal can be used to determine the magnitude and direction of motion. Below the surface layer, currents are mainly influenced by tides and the rotation of the Earth.

High Frequency Radar is used to measure the currents at the ocean surface, called sea surface currents. Similar to ADCPs a radar transmitter sends a signal out to sea and the conductive seawater surface returns a signal that measures the Doppler shift to determine the velocity and direction. High frequency radar instruments are deployed on shore and are generally operated in pairs.

Researchers are interested in ocean currents for several reasons: the flow of water affects the transport of nutrients and organic matter, as well as pollutants. Currents also affect the settlement locations of many larval species. The larvae of organisms such as mollusks and crustaceans are transported to habitat by the current. Fresh water run-off and the pollution it can carry also move with the current. The analysis of currents may have predictive value for the settlement of these organisms.

Upwelling is a seasonal current effect. Caused by strong offshore winds, upwelling refers to the movement of the normally deep, nutrient rich waters up toward the surface. Plankton blooms generally follow a period of upwelling. It is useful for researchers to know when upwelling occurs, so that residual effects such as changes in the food web can be studied.

The ocean is in constant motion and this movement of water helps to regulate global climate through the transfer of heat. Warm equatorial waters move toward the poles, cool and move back toward the equator in a circular motion called a gyre. Understanding this motion is important to researchers studying the Earth’s climate.

Learn more about the benefits of sea surface current observations

Sea surface current data and sensor specifications >