Abstract
Three typhoon events were selected to examine the impact of energy transformation on near-inertial oscillations (NIOs) using observations from a subsurface mooring, which was deployed at 125° E and 18° N on 26 September 2014 and recovered on 11 January 2016. Almost 16 months of continuous observations were undertaken, and three energetic NIO events were recorded, all generated by passing typhoons. The peak frequencies of these NIOs, 0.91 times of the local inertial frequency f, were all lower than the local inertial frequency f. The estimated vertical group velocities (Cgz) of the three NIO events were 11.9, 7.4, and 23.0 m d−1, and were relatively small compared with observations from other oceans (i.e., 100 m d−1). The directions of the horizontal near-inertial currents changed four or five times between the depths of 40 and 800 m in all three NIO events, implying that typhoons in the northwest Pacific usually generate high-mode NIOs. The NIO currents were further decomposed by performing an empirical orthogonal function (EOF) analysis. The first and second EOF modes dominated the NIOs during each typhoon, accounting for more than 50% of the total variance. The peak frequencies of the first two EOF modes were less than f, but those of the third and fourth modes were higher than f. The frequencies of all the modes during non-typhoon periods were more than f. Our analysis indicates that the relatively small downward group velocity was caused by the frequent direction changes of the near-inertial currents with depth.
Highlights
Free-propagating internal waves in the ocean usually have frequencies lower than the buoyancy frequency N and higher than the inertial frequency f [1,2]
We found that the typhoons mostly controlled the first and second modes of the Near-inertial oscillations (NIOs) signals and that theStructure third and fourth modes were similar to NIO signals that were not affected by the
Three energetic typhoon-generated NIO events were examined based on subsurface mooring observations lasting over 16 months (473 days) in the northwest Pacific Ocean
Summary
Free-propagating internal waves in the ocean usually have frequencies lower than the buoyancy frequency N and higher than the inertial frequency f [1,2]. Near-inertial oscillations (NIOs) are internal waves with frequencies close to the local inertial frequency f. NIOs have been observed in all ocean basins and the entire ocean column [3,4,5]. The breaking of near-inertial waves can cause ocean mixing, which influences pollutant dispersal and marine productivity [3,6,7], maintains ocean thermohaline circulation, and modulates the climate [8]. Alford presented spatial maps of wind–NIO energy fluxes [8] between 50◦ S and 50◦ N, which showed that the energy supplied to NIOs by wind is of the same order of magnitude as that provided to baroclinic tides by their barotropic counterparts. NIOs are strongly intermittent or highly temporally variable [9] and have high wavenumber aspects [10,11,12]
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