Wave, Vortex and Wave‐Vortex Dipole (Instability Wave): Three Flavors of the Intra‐Seasonal Variability of the North Equatorial Undercurrent

Abstract

Intra‐seasonal variations have been frequently observed in the North Equatorial Undercurrent (NEUC) jets, yet their dynamical nature remains elusive. Based on field observations and model results, we identify three flavors of the ISVs: Wave (accounting for 10%), wave‐vortex dipole (WVD) (20%) and vortex (70%). The wave flavor refers to a Rossby wave, the WVD flavor consists of a pair of counter‐rotating vortices, and the vortex flavor refers to a sub‐thermocline monopole eddy. To our knowledge, the WVD is identified for the first time and is found characterized by a unique dynamical feature: It manifests both as a second baroclinic mode‐like Rossby wave and as a pair of dipole‐like sub‐thermocline eddies. It is further identified as a second baroclinic mode‐like Rossby wave‐initiated instability wave in an equilibrium being fueled by baroclinic conversion. These results indicate that mesoscale instability waves are an important component of subthermocline intra‐seasonal and mesoscale variations.

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