|Eddies in the Gulf of Alaska|
|It was in mid-September 1998 that Bill Crawford and Frank Whitney met over
coffee to discuss Whitneys cruise on the Coast Guard Vessel
John P. Tully into the Gulf of Alaska. While senior scientist on
board ship the previous three weeks, Whitney had found a huge, warm,
relatively fresh water mass 200 km wide and more than 1000 m deep, about 600 km
west of Vancouver Island.
"Do you see this feature in the TOPEX data?" Whitney asked. TOPEX/Poseidon can measure sea surface height accurate to within 2 centimeters. Crawford and colleague Josef Cherniawsky of the Institute of Ocean Sciences had been processing TOPEX/Poseidon data for several months to look for sea level rise in coastal waters, as part of the El Niño event the previous winter. The idea is that as its temperature increases, sea water expands, and TOPEX/Poseidon can measure the corresponding change in sea surface elevation. Although most water in the eddy is warmer than the surrounding ocean, the waters near the surface are either similar or even slightly cooler than surrounding seas. For this reason, satellites that sense ocean surface temperature seldom find these eddies.
An American-French program launched TOPEX/Poseidon in 1992, and released the first data from it in October that year. The satellite senses sea surface height along a 20-km-wide swath, on an orbital track that repeats about every ten days. Each ten-day sample is denoted a "cycle." "TOPEX" refers to the American dual-frequency radar sensor that is turned on for nine of the ten cycles. "Poseidon" is the French radar unit that samples on every tenth cycle. The satellite can measure sea surface topography accurately to within several centimeters.
Cherniawsky told Crawford that he found a new Web site that posts
near-real-time TOPEX images. Crawford signed on and entered the
latitude and longitude range of the Gulf of Alaska, and the cruise date,
and there it was: a red bulls-eye of water whose core rose 30 cm above
the surrounding ocean, at the same place and diameter as Franks
warm, relatively fresh water mass (see letter "A" in sea surface height image below). He had
just found a Web site that posts, for free, the most up-to-date,
accurate information on these eddies!
|Temperature (°C) in the Gulf of Alaska as measured in Aug-Sept 1998 from the Canadian Coast Guard Ship John P. Tully. The depressed isotherms near 600 km from the coast are at point A in the sea surface height image below. Canadian scientists have sampled ocean waters from Vancouver Island to Station P at 50°N, 145°W for more than 40 years. (Image courtesy Fisheries and Oceans Canada, Institute of Ocean Sciences)|
Robert Leben of the University of Colorado had posted the web site only three weeks before Cherniawsky looked. Leben wanted to enable the public to find their own eddies. He combined TOPEX/Poseidon altimetry data with similar observations by the ERS-2 satellite, launched by the European Space Agency. Leben then applied spatial filters to enhance the display of ocean eddies and suppress large-scale seasonal signals. He developed this tool for his own studies in the Gulf of Mexico, but by putting all the data on his web site, he provided a new "digital eye on the world." To see for yourself, visit his altimeter data viewer site.
Crawford used this web site to plot images of the eddy over the previous seven months, and continues to track the eddy. Its status in June 2000 was ambiguous, but a trace of it might be found at 45.5°N, 142°W. The satellite images revealed that the eddy formed in winter 1997/1998 along the West Coast of the Queen Charlotte Islands. He labeled it Haida-1998, after the First Nations of the region and its year of formation. Crawford and Cherniawsky have completed their own analysis of TOPEX/POSEIDON and ERS-2 data, beginning with processed data provided by Richard Ray and Brian Beckley of NASA Goddard Space Flight Center. Crawford and Cherniawsky have applied their own tidal constants near shore, and have found that the eddy first began to form off the west coast of the Queen Charlotte Islands in November 1997. (These tidal constants are provided by a detailed numerical model of tides in the Gulf of Alaska computed by the team of scientists at the Institute of Ocean Sciences led by Michael Foreman.) They also determined that some of these eddies might be the source of meanders and eddies in the Alaskan Stream (Crawford et al., 2000).
This team is presently using the same models to determine the average seasonal height of the sea surface along the Canadian margin of the Gulf (Cherniawsky et al., submitted; Foreman et al., submitted). Once combined with all satellite altimetry data, they will be able to determine absolute sea surface heights, and use them to compute northward flow of surface currents along our coast.
The Colorado web site showed Haida-1998 to be one of an annual supply of eddies that transport fresh water and nutrients into the Gulf from the Alaskan Panhandle and the Canadian West Coast. The unusually high elevation of the eddy core marks it as one of the largest eddies observed in this region. Haida eddies belong to a class of anticyclonic, coastal-generated eddies first noticed in water property data near Sitka, Alaska at 57 °N (Tabata, 1982), and later in satellite infrared measurements by Thomson and Gower (1998). Crawford and Whitney (1999) identified another region where eddies are typically generated between 51°N and 54°N, off the West Coast of the Queen Charlotte Islands. Over the years 1994 to 1999, they found that three to five large eddies formed along the Alaskan Panhandle and Canadian West Coast in any one winter.
These false-color images show contours of sea surface height from ERS-2 and TOPEX altimeters, as displayed on the Colorado Center for Astrodynamics Research Global Near Real-Time Altimeter Data Viewer web site.