Seasonal and interannual changes in the North Atlantic subpolar gyre from Geosat and TOPEX/POSEIDON altimetry

Sea surface slopes from the altimetric satellites Geosat and TOPEX/POSEIDON are used to calculate eddy kinetic energy of the North Atlantic subpolar gyre (40-65 degrees N, 60 degrees-5 degrees W). Using two years of data from each satellite (December 1986 to December 1988 and October 1992 to September 1994, respectively), interannual differences in the North Atlantic Current (NAG) are revealed. In regions of strong currents the eddies are driven by baroclinic instability of the mean flow, are not seasonally varying, and may therefore be used as a surrogate for the mean flow itself. Wind stress curl fields for the same periods show that the northward and southward shifts in the current branches across the Mid-Atlantic Ridge are related to interannual differences in the winter wind stress curl pattern when the zero in wind stress curl is well defined and wind stress is at a maximum. Outside the NAG, the eddies are driven primarily by wind stress, indicated by a significant seasonality. Time series of eddy kinetic energy and wind stress are generated for the 4 years, and the magnitude, phase, and significance of annual and semiannual signals are determined. Interannual changes in the timing of the maximum eddy kinetic energy are associated with that of the maximum wind stress, with each being shifted by a month or two later or earlier. Throughout much of the northern North Atlantic a significant annual signal in eddy kinetic energy is observed, lagging the wind stress by, on average, 6 weeks. A negligible lag is found in the region of the East Greenland Current and Irminger Sea, whereas the greatest lag, about 2 months, is found in the Labrador Sea.