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Subarctic Water Moved South Along US West Coast in 2002
Reference
Barth, J.A.  2003.  Anomalous southward advection during 2002 in the northern California current: Evidence from Lagrangian surface drifters.  Geophysical Research Letters 30: 10.1029/2003GL017511.

What was done
During April, July and September of each year from 1998 to 2002, five drifters were released at sea in a cross-shelf line at about the latitude of Newport, Oregon [44.65°N] at distances of 18.5, 27.8, 46.3, 83.3 and 120.4 km offshore.  The drifters were tracked via satellite, and location fixes were obtained roughly 9-12 times per day.  The resulting data were then used to compute east-west and north-south drifter displacements and speeds over 15, 30 and 45 days.

What was learned
Barth reports that "equatorward velocities in the core of the upwelling jet of the northern California Current were found to be on average 0.05-0.06 m s-1 faster in spring and summer 2002 than the average computed over 1998-2002," noting further that "at this speed, anomalous water displacements of over a degree of latitude can occur in 20-25 days."

What it means
Barth says the southward displacement of subarctic water identified in his study is "a plausible explanation for the relatively cool halocline water observed off Oregon during the summer of 2002 (Freeland et al., 2003)," and that it is "consistent with the large-scale forcing events described by Murphree et al. (2003) and provide a quantitative estimate of their combined effect, an increase of near-surface equatorward flow of 0.05-0.06 m s-1."  He also notes that his results are consistent with anomalous southward and onshore flow into the northern California Current as measured by cross-track altimeter surface velocities (Strub et al., 2003), and that the drifter velocity near the center of the jet at 27.8 km offshore ranged up to 0.13 m s-1, which he says is consistent with "the 0.12 m s-1 anomalous equatorward velocity measured at a mid-shelf mooring located 18.5 km offshore of Newport, Oregon (Kosro, 2003)."  Last of all, Barth reports that "the observed cold halocline water is accompanied by high nutrients (Wheeler et al., 2003) which, when upwelled over the shelf, fuel an increase in the amount of primary productivity off the Pacific Northwest (Wheeler et al., 2003; Thomas et al., 2003)."

As to what these dramatic changes represent, Barth opines they are manifestations of interannual variability at the opposite extreme of "the increased poleward velocity and warmer temperatures observed in this same region during the fall and winter of El Niņo years."  Only time will tell if this interpretation is correct, or if the recent anomalous invasion of subarctic water is a harbinger of a more comprehensive and prolonged regime shift of the type described by Chavez et al. (2003).

References
Chavez, F.P., Ryan, J., Lluch-Cota, S.E. and Niquen C., M.  2003.  From anchovies to sardines and back: multidecadal change in the Pacific Ocean.  Science 299: 217-221.

Freeland, H.J., Gatien, G., Huyer, A. and Smith, R.L.  2003.  Cold halocline in the northern California Current: An invasion of subarctic water.  Geophysical Research Letters 30: 10.1029/2002GL016663.

Kosro, P.M.  2003.  Enhanced southward flow over the Oregon shelf in 2002: A conduit for subarctic water.  Geophysical Research Letters 30: 10.1029/2003GL017436.

Murphree, T., Bograd, S.J., Schwing, F.B. and Ford, B.  2002.  Large-scale atmosphere-ocean anomalies in the northeast pacific during 2002.  Geophysical Research Letters 30: 10.1029/2003GL0175303.

Strub, P.T., Thomas, A. and James, C.  2003.  Anomalous transports into the California Current as seen in cross-track altimeter surface velocities.  Geophysical Research Letters 30: 10.1029/2003GL017513.

Thomas, A.C., Strub, P.T., Brickley, P. and James, C.  2003.  Anomalous satellite-measured chlorophyll concentrations in the northern California Current in 2001-2002.  Geophysical Research Letters 30: 10.1029/2003GL017409.

Wheeler, P.A., Huyer, A. and Fleischbein, J.  2003.  Cold halocline, increased nutrients and higher productivity off Oregon in 2002.  Geophysical Research Letters 30: 10.1029/2003GL017395.


Reviewed 28 January 2004