|Inside an Eddy|
Whitneys salinity and temperature measurements in August 1998 showed the waters in Haida-1998 to be to be fresher and warmer than surrounding waters below 100-m depth (see previous page). Above 100 m in depth, both salinity and temperature in the eddy were slightly lower than in surrounding waters. Dynamic height calculations, which use seawater density profiles to determine how high the eddy surface "sits" above the surrounding ocean surface, reveal that sea surface in the core of the eddy was 30 cm higher than outside the eddy. This calculation matches the altimetry measurements from TOPEX/Poseidon. Nutrient levels in its thermocline were substantially higher than in surrounding waters. (Here, "thermocline" refers to the temperature gradient across the width and depth of the eddy.) The ocean water type of this eddy matches that found near the Queen Charlotte Islands in winter (53°N, 133°W).
In February and June 1999, Crawford sent the web-generated images to Whitney at sea on the John P. Tully to direct him to the eddys location for sampling. His measurements taken in September 98, February 99, and June 99 show the steady erosion of the nutrient excess in the eddy waters, and a three-fold enhancement of phytoplankton in the September 1998 samples around the perimeter of the eddy. Whitneys measurements demonstrate that the eddy provided nutrient to a nitrate-starved region of the Gulf of Alaska (Whitney, Wong, and Boyd 1998).
According to Crawford and Whitney, the eddies usually drift westward
and disappear within two years in deep waters in the Gulf of Alaska.
These rotating masses of water average up to a two hundred kilometers in
diameter, and a large eddy can contain up to 5,000 cubic kilometers of
water, which is about the volume of Lake Michigan.
The Canadian Coast Guard Offshore Research & Survey Ship, John P. Tully. (Image courtesy Canadian Coast Guard)
Crawford notes that in 1999, colleagues of theirs published a paper showing that Sitka and Haida eddies are frequently created in their computer simulations of wind-driven currents along this coast (Melsom 1999). The researchers believe it is baroclinic instability of the coastal flow that triggers the set up of eddies. ("Baroclinic instability" may occur in a flow in which there are density gradients along surfaces where the pressure is constant. Such instabilities are typically produced in rotating systems where there is ample potential energy being converted into kinetic energy.)
Based on calculations of dynamic heights of the 100-m surface relative to the 1000-m surface, using archived water property data, two of the highest-elevation eddies were Haida-1998, and Haida-1983, both generated in severe El Niño winters. This finding supports the calculations by Melsom et al. (1999), based on their numerical model.
So whats up with these eddies now? By mid-June 2000, new Haida and Sitka eddies had drifted away from shore, and the final remnants of Haida-1998 were merging into the surrounding seas, as shown on the previous page. The eddies of 1999 were weak and by June 2000, had either disappeared or were barely visible. Whitney is senior scientist on another cruise of the John P. Tully to sample Haida-2000 in June 2000. Its position shown in places it over Bowie Seamount, a potential Canadian Marine Protected Area. "We now have a combined eddy and seamount study, with too little time to sample both," says Whitney. The cruise was set up to examine nitrate and iron concentrations in the eddy, and to map their depletion in time and impacts on surrounding biota. He hopes to examine eddy water "upstream" of the seamount, and than run a quick survey over the seamount on the way home.
The first satellite images solved a previous mystery. Canadian scientists had wondered why Bowie Seamount biota could be so similar to coastal species, when no prevailing currents flowed from shore to the seamount. However, the track of Haida-1998, passing directly over Bowie Seamount, provided the missing link. The eddies had carried coastal species away from shore right to the seamount.
Cherniawsky, J.Y., M.G.G. Foreman and W.R. Crawford, Ocean Tides from TOPEX/ POSEIDON sea level data, submitted to Journal of Atmospheric and Oceanic Technology.
Crawford, W.R., J.Y. Cherniawsky and M.G.G. Foreman, 2000: Multi-year meanders and eddies in Alaskan Stream as observed by TOPEX/Poseidon altimeter, Geophysical Research Letters, 27(7), 1025-1028.
Crawford, W.R. and F. Whitney, 1999: Mesoscale eddies aswirl with data in Gulf of Alaska Ocean, EOS, Transactions of the American Geophysical Union, 80(33), 365, 370.
Foreman, M.G.G., W.R. Crawford, J.F.R. Gower, L. Cuypers and V.A. Ballantyne, 1998: Tidal correction of TOPEX/POSEIDON altimetry for seasonal sea surface elevation and current determination off the Pacific Coast of Canada. J. Geophys. Res. 103:(C12) 27,979-27,998.
Foreman, M.G.G. W.R. Crawford, J.Y. Cherniawsky, R.F. Henry, and M. Tarbotton,: A high-resolution assimilating tidal model for the Northeast Pacific Ocean, submitted to J. Geophys. Res.
Gower, J. F. R., and S. Tabata, 1993: Measurement of eddy motion in the northeast Pacific using the Geosat altimeter, in Satellite Remote Sensing of the Oceanic Environment, edited by I. S. F. Jones, Y. Sugimori and R. W. Stewart, pp 375-382, Seibutsu Kenkyusha, Tokyo.
Melsom, A., S. D. Meyers, H. E. Hurlburt, E. J. Metzger, J. J. O'Brien, 1999: ENSO effects on Gulf of Alaska eddies, Earth Inter., 3, pap. 001, (Available at http://EarthInteractions.org.)
Tabata, S., 1982: The anticyclonic, baroclinic eddy off Sitka, Alaska, in the Northeast Pacific Ocean, J. Phys. Oceanogr., 12, 1260-1282.
Thomson, R. E., and J. F. R. Gower, 1998: A basin-scale oceanic instability event in the Gulf of Alaska, J. Geophys. Res., 103, 3033-3040.
Whitney, F. A., C. S. Wong, and P. W. Boyd, 1998: Interannual variability in nitrate supply to surface waters of the Northeast Pacific Ocean, Mar. Ecol. Prog. Ser., 170, 15-23.
|This schematic shows an idealized eddy in the Gulf of Alaska. "Isotherms" are lines connecting points of equal temperature, as on a weather map. Warm, nutrient-rich coastal water spirals clockwise, forming the core of the eddy. Phytoplankton grow in the edges of the eddy near the ocean surface, nourished by the nutrient-rich eddy water. (Image by Robert Simmon)|