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A high-resolution numerical Pacific ocean model from the Naval Research Laboratory is used to investigate the interannual variations of the coastal circulation along western North America. A multi-year simulation with realistic wind forcing produces changes in circulation and eddy activity. Interannual variability in the upper ocean circulation off California and in the Gulf of Alaska is linked to the El Niño/Southern Oscillation phenomenon in the tropical Pacific. One known mechanism for linking these distance latitudes is coastally trapped Kelvin waves which propagate 2-3 m/s poleward along the coast. The ocean model reproduces much of the observed coastal sealevel changes with typical linear correlations of 0.8.
We find El Niño events usually destabilize the Alaska Current, creating multiple strong anticyclonic eddies along the coast. These eddies then slowly propagate into the Gulf of Alaska where they survive for years. Eddy formation is not observed during El Viejo (La Niña) events, suggesting these events suppress eddy formation in the Gulf of Alaska. The opposite situation occurs at lower latitudes in the California Current system. Fewer eddies appear to form during El Niño events and a greater number of cyclonic eddies form during El Viejo events. The mechanism behind this eddy formation/suppression is believed to be enhanced/diminished vertical shear induced by the Kelvin waves on the mean circulation. The role of the mean circulation accounts for the different dynamical response at different latitudes. This represents a new and important understanding of variations in the surface circulation off California and higher latitudes at interannual timescales.
by S. D. Meyers 1, H. E. Hurlburt 2, A. Melsom 3, E. J. Metzger 2, and J. J. O'Brien 1
1 Center for Ocean-Atmospheric Predictions Studies
2 Naval Research Laboratory
3 University of Oslo, Norway
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