Baroclinic nonlinear saturation and secondary instability of current-undercurrent meanders

Undercurrents, which flow oppositely to the upper layer, are important in subsurface water transport and circulation connection. To understand Western Pacific Ocean undercurrent dynamics, we use an idealized model to study the nonlinear formation and secondary instability of jet meanders due to baroclinic instability. We develop a weakly nonlinear framework to track the perturbation evolution and along-stream mean flow. The instability of saturated meanders is analyzed with a three-dimensional Floquet-based secondary instability framework. The current and undercurrent interaction due to nonlinear and secondary instabilities is revealed. Implications for realistic oceanic flows are discussed.