On the wave number spectrum of oceanic mesoscale variability observed by the SEASAT altimeter

Abstract

Variations of sea surface height measured by the SEASAT altimeter in nearly repeat orbits during the last 24 days of the mission are used to compute the wave number spectrum of mesoscale variability in various parts of the oceans. The instrument noise level ( ∼150 cm2/cycles/km) has limited the resulting oceanic spectrum to wavelengths longer than 100 km. We found that the characteristics of the oceanic spectrum are dependent on the energy level of the mesoscale variability. In the high‐energy areas close to major current systems, most of the energy is contained at wavelengths longer than 250 km. At wavelengths shorter than 250 km the spectrum basically follows a k−5 (k is wave number) dependence. In the low‐energy areas remote from major current systems the spectrum follows a k−1 dependence at wave‐lengths from 100 to 1000 km. Based on the assumption of horizontal isotropy of mesoscale variability, scalar‐wave number spectra of sea surface height and geostrophic kinetic energy are also presented. Dynamical implications of these spectra are discussed. The effects of residual geoid and atmospheric water vapor on the computed oceanic spectra have been rigorously examined; it was found that the general characteristics of the oceanic spectra were not significantly affected by them.