Time scales of mesoscale variability and their relationship with space scales in the North Atlantic

Abstract

A systematic study of characteristic time scales of mesoscale variability over the North Atlantic was done using two years of Geosat data. Time scales are first characterized by 10° latitude by 10° longitude bins. A more detailed description was obtained by globally mapping the Sea Level Anomaly temporal correlation after one cycle (17.05 days). The scales are shortest in areas of high mesoscale activity (Gulf Stream, North Atlantic Current) while relatively long time scales are observed over the Mid-Atlantic Ridge and in the eastern part of the basin. In general, time scales are not proportional to space scales. Propagation velocities obtained by dividing space scales by time scales appear to be minimal east of the Mid-Atlantic Ridge. Frequency-wavenumber spectral analysis complemented this statistical description of mesoscale variability. It shows that the dominant wavelengths of around 200 to 500 km (depending on latitude) are associated with long periods (\u3e150 days) in the eastern part of the basin, while near the Gulf Stream significant energy is found at shorter periods. Propagation velocities are generally westward. Pseudo-dispersion relations deduced from Geosat data suggest two distinct dynamic regimes, as in quasigeostrophic turbulence models: a turbulent regime for smaller scales, with proportionality between space and time scales, and an apparently more linear regime where an inverse dispersion relation is found in the eastern part of the basin. This latter characteristic is in agreement with quasigeostrophic models forced by fluctuating winds