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°N, 60°-5°W). 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 NAC, 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 four years, and the magnitude, phase, and significance of annual and semiannual signals are determined. -from Authors