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 NIO 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 stretchedmeter 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 f in 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 NIO 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.
Highlights
Wind-induced near-inertial oscillations (NIOs) are ubiquitous in the open ocean
The meridional currents (Fig. 5d) are northward before May 16 and changes southward after May 16. These results indicate that typhoon-induced NIOs occurred during the period when the mooring site was located near the southern boundary of the anti-cyclonic eddy
This study presents an energetic NIO event in the tropical northwestern Pacific generated by Typhoon Rammasun in May 2008 using a moored acoustic Doppler current profiler (ADCP)
Summary
After being generated NIOs generally propagate equatorward, yet they propagate northward up to a ‘turning latitude’ and turn equatorward due to the β-effect (Anderson and Gill 1979; Alford 2003) They propagate downward with different characteristics depending on wind strength, thickness of the mixed layer, and buoyancy frequency (Gerkema and Shrira 2005; Park et al 2009; Byun et al 2010). Previous studies, including those mentioned above, are mostly about measurements of NIOs near the continental shelf or at mid and high latitudes in the open ocean (e.g., Alford 2003; Alford and Whitmont 2007), probably due to rare opportunities to observe them at low latitudes. We discuss NIO energy and period, both of which vary during vertical propagation, as well as the impact of the background currents on the dispersion of NIOs around the mooring site
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