Long-range Radiation of Barotropic Rossby Waves from an Unstable Current

The world oceans are filled with mesoscale eddies, but we do not understand how these eddies are energized. The mesoscale eddy field surely gains (and loses) energy via many mechanisms, such as instability of the local currents, local direct forcing (e.g., wind), and wave-wave interactions. One mechanism that has been advanced in recent years is local instability of the large-scale flow. Another potentially important mechanism for energizing the eddy field is nonlocal instability of the large-scale flow, in which a few of the world’s most energetic currents radiate energy to the midocean gyres and other remote regions. Here, we provide an example to illustrate the potential importance of this process.

Analysis of sea-surface height (SSH) anomalies from satellite altimetry shows variability throughout the North Pacific that is coherent with Tropical Instability Waves. In the tropics (10N-20N) this variability has regular phase patterns that are consistent with barotropic Rossby waves having northward energy propagation (Farrar, J. Phys. Oceanogr., 2011). Further north, the phase patterns become confused and the variance decreases, but hot spots of coherent variability reemerge in the Gulf of Alaska and south of the Aleutian Islands. Ray-tracing calculations and comparisons with numerical simulations support the conclusion that this remote (and seemingly isolated) variability can indeed be attributed to barotropic Rossby waves generated near the equator and undergoing bathymetric refraction as they propagate northward. This sort of barotropic wave variability, coupled to mesoscale instabilities and occurring at similar space and time scales, contributes to the mesoscale variability observed in SSH.