Scattering of Equatorial Kelvin Waves by Tropical Instability Vortices

Tropical Instability Waves in the Equatorial Oceans generate coherent anticyclonic vortices known as Tropical Instability Vortices (TIVs) that can drive vigorous lateral mixing of tracers within their closed circulations. The potential vorticity (PV) in particular tends to be homogenized in the core of TIVs, plateauing at a value close to zero. We examine the impact of these TIV-induced changes in the equatorial PV field on the propagation and energetics of equatorial Kelvin waves. Linear equatorial wave theory suggests that the presence of expansive regions of near-zero PV can scatter Kelvin wave energy into Poincare waves due to a disruption of the geostrophic meridional force balance. The quantity of Kelvin wave energy scattered depends on the frequency of the incoming Kelvin wave and the zonal and meridional extent of the regions of mixed PV. The predictions of the linear equatorial wave theory are compared to the results of an equatorial ocean model constructed to contain a quasi-steady TIV field. By introducing Kelvin waves with known properties into the ocean model we study the full non-linear interaction between TIVs and Kelvin waves. The results have implications for the role of TIVs in the equatorial oceans and their potential influence on the initiation and spatial structure of El Nino events.