Improved Theoretical Estimates of the Zonal Propagation of Global Nonlinear Mesoscale Eddies

Abstract

Mesoscale eddies are essential for transport and mixing processes in the global ocean, with their characteristic westward propagation being a significant finding from the satellite altimetry era. Traditional predictions of their zonal propagation rely on the theoretical phase speed of long baroclinic Rossby waves; however, this approach is known to overestimate eddy speeds equatorward of approximately $35°$ latitudes. To address this issue, we incorporate local eddy wavelengths inferred from satellite-based eddy radii into the estimation of global eddy speeds, thereby significantly reducing the overestimation biases in mid-to low-latitude regions. This improvement is consistent with the observation that mesoscale eddies in these latitudes have length scales comparable to the local deformation scales and thus refrain from satisfying the long-wave approximation, whereas the long baroclinic Rossby wave phase speed remains useful for capturing the most energetic but less abundant eddies. The remaining discrepancies between the revised theoretical speeds and observations primarily stem from uncertainties in the background zonal flow, spatial variability of vertical modal structures (and the associated deformation radii), and estimation of local eddy length scales. These findings have important implications for understanding long-range mesoscale eddy propagation and eddy-driven mixing in the global ocean, which are anticipated to benefit future ocean model developments and enhance predictions of mesoscale eddy dynamics.