Abstract
Based on in situ observations, six episodes of near-inertial internal waves (NIWs) were detected on the East China Sea (ECS) continental slope, and the mechanisms and characteristics of them were examined. The generation mechanisms of the observed NIWs included typhoon, wind burst, lateral propagation, and energy transfer from low-frequency flow. The depth-integrated near-inertial kinetic energy (NIKE) showed no significant seasonal variation, and the annual mean NIKE and near-inertial currents were 400 J/m2 and 3.50 cm/s, respectively. Downward propagation of NIKE was evident in the small wavenumber band according to the rotary vertical wavenumber spectra. The NIKE was subsurface-intensified, and the near-inertial vertical shear reached 0.01 s−1. The vertical phase speeds of the NIWs ranged from 5 to 19 m/h. The frequencies of the NIWs were mostly red-shifted, however, blue-shift also existed. One episode had both blue- and red-shifted frequencies vertically, and had both upward and downward propagating vertical phase speeds. The e-folding times of the observed NIWs ranged from 4 to 11 days, which were influenced by successive wind bursts and background vorticity. On the left-hand side of Kuroshio, the background vorticity is usually positive; however, the NIWs were almost red-shifted, which resulted from the Doppler shift of the Kuroshio.
Highlights
Internal waves around local inertial frequency (f ) are significant components of oceanic waves, and are called near-inertial internal waves (NIWs) or near-inertial oscillations (NIOs)
The time and depth ranges of six episodes of NIWs with depth-integrated nearinertial kinetic energy (NIKE) exceeding one standard deviation are marked by the white rectangles
The results suggest that the observed NIW episodes 3 and 6 are not induced by the Parametric subharmonic instability (PSI) mechanism
Summary
Internal waves around local inertial frequency (f ) are significant components of oceanic waves, and are called near-inertial internal waves (NIWs) or near-inertial oscillations (NIOs). The tidal forcing, loss of stability of large-scale circulation, and eddy-topography interactions are potential forcing mechanics of NIWs [7,8,9]. NIWs appear as a prominent peak in an internal wave spectrum, leading to intense vertical shear, and are thought to be a potential energy source for oceanic internal mixing to maintain the abyssal stratification [2,3,4,7]. NIWs generated by wind are observed both in the open ocean [7,10] and in the marginal seas [11,12,13,14,15,16]. Distributed mooring observations suggest that nearinertial kinetic energy (NIKE) is surface-intensified and shows seasonal cycle [18]. In the western North Atlantic Ocean, the NIKE exhibits a strong seasonal variation with a wintertime maximum, and is dominated by downward energy propagation [19]
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