Typhoon Rammasun-Induced Near-Inertial Oscillations Observed in the Tropical Northwestern Pacific Ocean

Abstract

Wind-induced near-inertial oscillations (NIOs) have been known to propagate their energy downward and equatorward, yet few observations have confirmed this in tropical regions. Using measurements from a moored ADCP in the tropical northwestern Pacific, we report an energetic NIO event associated with Typhoon Rammasun in May 2008, when an anti-cyclonic warm eddy existed around the mooring site. Our analyses reveal that the anti-cyclonic eddy traps the NIO energy at two layers around 120 and 210 m where the buoyancy frequency show high values. The MO energy continuously decays at layers below its maximum at 210 m, and disappears at depths below the thermocline. During their propagation from 137 to 649 stretched-meter depths (equivalent to 100 - 430 m), NIOs shift their frequencies from 0.92f to 1.05f probably due to the effective f, which changes its magnitude from smaller to larger than local inertial frequency fin the anti-cyclonic eddy. In addition, their vertical energy propagation becomes faster from 0.17 to 0.64 mm s(-1). Decomposition of downward and upward MO energy propagation shows that the typhoon-induced NIOs remain 29% of their energy in the upper layer, and transfer 71% to the subsurface layers. Our results suggest that typhoon-induced NIOs interacting with meso-scale eddies can play an important role in providing the energy source available for ocean mixing in the tropical regions.