Rossby wave-ocean interaction induced Low-frequency oscillation

Over the western North Pacific warm pool, the convection associated with the summer monsoon trough exhibits pronounced oscillation with a period of 20-40 days. Observational evidence is presented to show that when convection is active, enhanced surface winds and cloudiness reduce the downward radiation and buoyancy flux which cool the underlying ocean and lead to suppressed convection and weakening of the monsoon trough. This negative feedback can offsets unstable growth and induce a low-frequency oscillation. A theoretical model is advanced to demonstrate that the equatorial Rossby wave interacting with the ocean mixed layer through thermodynamic coupling produces a self-regulating oscillation in the presence of summer mean state. On the other hand, in the presence of winter background state, the coupled mode prefers an equilibrium state. The seasonal cycle, therefore, modulates the amplitude of the low frequency oscillation.