Detection of Oceanic Quasi-Zonal Jets from Altimetry Observations

Abstract

Recent analyses of observations and ocean model outputs have revealed coherent low-frequency quasizonal jets in observed sea surface height (SSH) anomaly and model velocity fields. The jets were latent, that is, they were not detectable by eye, but revealed and selected by time-averaging procedures. Time-averaging procedures, when applied to fields that contain propagating features (eddies and waves), can create jetlike structures of nonphysical nature (artifacts). This paper suggests the application of three criteria to distinguish real jets from these artifacts, and demonstrates that quasi-zonal jets extracted from satellite altimetry observations off California were not artifacts. First, quasi-zonal jets off California were stronger than artifacts: the observed SSH for the jets reached 4–5 cm, which is considerably larger than SSH artifacts, which did not exceed 0.9–1.2 cm. Second, axes of the observed jets were not always oriented along the paths of propagating mesoscale features (waves and eddies). Observed jet axes rotated as late as 12 months after propagating mesoscale features changed their propagation direction. This behavior differed from that of artifacts, the axes of which should be oriented in the same direction as propagation paths of mesoscale features. Third, generation (amplification) of quasi-zonal jets was accompanied by phase synchronization or locking of flow time scales resulting from interactions between these scales. Because artifacts were a result of linear averaging procedures, they cannot exhibit such phase synchronization.