Observations of long-distance radiation of barotropic Rossby waves from an unstable current system

The instabilities that form on an unstable current may be trapped in the vicinity of the current, or they may instead develop as wavelike disturbances that can radiate energy and momentum to distant locations. The instabilities may feedback to the larger-scale flow, both locally, in the vicinity of the unstable current, and nonlocally, at locations far from the unstable current. These nonlocal effects pose an important challenge for parameterization of the effects of current instabilities in climate models, but they have proven difficult to assess observationally.

We analyze sea-surface height (SSH) anomalies from satellite altimetry to show that the instability of the Pacific equatorial current system triggers oceanic Rossby waves that affect oceanic variability in distant regions. SSH variability throughout much of the North Pacific is coherent with the SSH signal of tropical instability waves (TIWs) that result from instabilities of the swift equatorial current system. This variability has regular phase patterns that are consistent with barotropic Rossby waves radiating energy away from the unstable equatorial currents, and the waves can be clearly seen to propagate from the equatorial region to at least 30°N. Comparisons with numerical simulations further support this interpretation. These remote, coherent signals exhibit remarkable 'patchiness' in their spatial patterns, which we interpret as a combined effect of wave interference and topographic refraction.