A new method was employed in the Bering Sea to locate an eddy during NOAA ship Miller Freeman cruise MF97-05, Leg 2, in May 1997. The ship was directed to the general vicinity of the eddy from maps of sea surface height anomalies. Once nearby, real-time CTD (conductivity temperature-depth) data processing and geostrophic velocity computation revealed the radial symmetry in the currents characteristic of an eddy.
Background
Eddies are a ubiquitous feature of the circulation in the southeastern Bering Sea. They are important because they may move encapsulated water and organisms from one region to another. Observations show that eddies form in deep water near the continental shelf break and propagate northwestward along the shelf break and westward into the Bering Sea basin. Eddies that impinge upon the continental shelf soon dissipate in the shallow water (<200 m) there. Eddie diameters range from 25 to 100 km. They propagate at a few cm/s, but rotational speeds can reach 80 cm/s making the currents associated with eddies the largest in the deep basin away from oceanic boundaries. Clockwise (anticyclonic) eddies are more numerous. The eddy formation mechanism is not well understood, but several factors probably contribute. Among these are (1) the inflow of Pacific Ocean water from the Alaskan Stream into the Bering Sea through numerous Aleutian Island passes, (2) the turning of the current from eastward north of the Aleutians to northwestward along the shelf break, and (3) the instability of eastern boundary currents.
Bering Sea eddies have been detected by the near-circular trajectories of satellite-tracked drifting buoys that were entrained within them or from radially symmetric geostrophic velocity cross sections inferred from water density as measured by CTD casts. In the past, drifters have entered the eddies either through chance entrainment or through placement in patches of walleye pollock (Theragra chalcogramma) fish larvae as detected with net tows.
New Eddy Detection
One of many "hills" in the sea surface was detected from sea surface height anomalies measured by TOPEX and ERS-2 satellite altimeters and computed by Dr. Robert Leben at the University of Colorado's Center for Astrodynamics Research. The 8-cm altimetric high representing a clockwise eddy is shown in orange in the SE corner of Figure 1 at 55.5° N, 169° W on May 12, 1997. PMEL's Dr. Phyllis Stabeno monitored the high's development and kept the ship appraised of the potential eddy's position via email. The cruise's Chief Scientist, Dr. Edward Cokelet, directed the ship on a transect across the high (Figure 2), computing the geostrophic velocity referred to 1500 dbar (Figure 3) cast by cast until satisified that he had crossed the roughly circular eddy's chord. A satellite-tracked buoy was placed in the high velocity core, and subsequent trajectories (red in Figure 2) revealed the circular motion as the eddy moved slowly toward the northwest throughout the month of May 1997.
Fig. 1. On the www at ftp://wind.colorado.edu/pub/bering/gif/gifs_05_97/rgdr.05_12_97.gif
