An eddy is a water current that runs contrary to the main current. The large "Sitka" and "Haida" eddies, named for the town of Sitka, Alaska, and the native name for the Queen Charlotte Islands, British Columbia, Canada, form along the Alaskan Panhandle and Canadian west coast each year and drift into deeper waters to the west.

The TOPEX/Poseidon satellite has tracked these and other eddies since the 1992-93 winter. Years with heavy El Ni�o winds appear to produce particularly large eddies that can last for several years and replenish nutrient-starved regions of the ocean. Observations of the Haida Eddy by the Canadian research vessel J.P. Tully show that the eddies move fresh water, iron and nitrates from land to sea.

"Our concern over the depletion of fish in this region makes altimeter measurements such as TOPEX/Poseidon data particularly important to understanding the formation and movement of these nutrient-rich eddies and how they influence salmon growth and other fisheries," said William Crawford of Fisheries and Oceans Canada at the Institute of Ocean Sciences.

He and colleague Frank Whitney have been using TOPEX/Poseidon images produced by the University of Colorado to track large-scale eddies along the Pacific Northwest. They observed unusually high Sitka and Haida eddies in the ocean during the severe El Ni�o of 1998. Both eddies were 30 centimeters (12 inches) higher than surrounding waters.

"These eddies, which brought higher nutrient levels and a local resurgence of phytoplankton, became two of the largest observed," Crawford said. Phytoplankton is the minute plant life found in bodies of water.

"With the subsidence of the Haida Eddy over the next year, we began to observe in the eddy a steady depletion of nutrients that are important to the food chain," added Crawford.

The eddies usually drift westward and disappear within two years in deep waters off the Gulf of Alaska. These rotating masses of water can average up to a few hundred kilometers in diameter, forming along the coast within the northbound coastal current, Crawford said, and a large eddy can contain up to 5,000 cubic kilometers of water, which is about the volume of Lake Michigan.

Salinity and temperature measurements from the Canadian ship J.P. Tully have indicated that the subsurface water is fresher and warmer in this region than surrounding waters.

Plans are under way to augment that data and to combine topographic measurements from space with new data on nutrient levels and fish abundance from ships to help fisheries predict annual food production.

Crawford and Whitney will use TOPEX/Poseidon observations in the Gulf of Alaska to determine the average seasonal height of the sea surface and help determine the northward flow of surface currents along the Pacific coasts.

NOAA Satellite data For Sea-Surface Temperatures

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