On modulation instability in a system of jets, waves and eddies off California

Abstract

SSH altimetry observations for 1992 to 2009 off Central and Southern California are used to show that observed quasi-zonal jets were likely driven by near-resonance interactions between different scales of the flow. Quartet (modulational) instability dominated and caused non-local transfer of energy from waves and eddies to bi-annual oscillations and quasi-zonal jets. The total number of quartets induced off California was approximately 10 times larger than the number of existing triads, and quartet amplitudes in general were larger than triad amplitudes. The spectral centroid regularly shifted into the domain of low-order modes. Local “negative” viscosity probably did not generate a classical inverse cascade because the spectrum of SSHs did not demonstrate power behavior. Two types of quartets were identified: (a) quasi-zonal jets, annual and semi-annual Rossby waves and eddies, and (b) bi-annual oscillations, semiannual Rossby waves and eddies. For a case with bottom friction, quartet instability required the existence of a certain level of dissipativity in the flow.