Abstract
Near-inertial waves (NIWs) contain a pronounced portion of shear energy in the internal wave field and is of great importance to deep ocean mixing. However, accurate simulation of NIWs remains a challenge. Here we analyzed 3-year long mooring observation of velocity profiles over 80–800 m to study the responses of near-inertial downward shear to varying wind stress curls and sea level anomalies (SLAs). It is demonstrated that moderate (even weak) cyclone makes more contributions to enhanced shear below the pycnocline than very strong cyclone. Because very strong curl can stall the downward propagation of large shear. The large positive and negative SLAs cause the accumulation of large shear in the lower and upper parts of the pycnocline through inducing downwelling and upwelling motions, respectively. Time variation of near-inertial shear was strongly influenced by cases of large curls and interannual variation of SLA, and thus did not follow the seasonal variation of wind stress. Our analyses suggest that matched fields of wind stress curl and SLA, and well representing the ocean response to moderate cyclone are needed in simulating the role of NIWs on mixing.
Highlights
Near-inertial waves (NIWs) form a prominent peak in the frequency spectrum of ocean current and contain a pronounced portion of shear energy in the internal wave field[1]
The question about how much of the near-inertial energy input by wind is available for the deep ocean mixing is still being debated[8]
In comparing with near-inertial energy in negative and positive wavenumbers and in different depths observed by upward- and downward-looking ADCPs, the former situations were larger than the latter one
Summary
Near-inertial waves (NIWs) contain a pronounced portion of shear energy in the internal wave field and is of great importance to deep ocean mixing. At the depth above 170 m (upper dashed box in Fig. 7a), shear intensity is positively correlated with the level of forcing curl, and mean shear energy of the strong curl ensemble is larger than those of moderate and weak curl ensembles. 2) The large positive and negative SLA fields cause the accumulation of large shear in the lower and upper parts of the pycnocline through inducing downwelling and upwelling motions, respectively These conclusions give us some hints on how to improve the capability of simulating wind-induced near-inertial energy. During the whole observation period, intensities of all cyclones were not strong enough to reach the criteria of typhoon; this instead provides us an opportunity to study how the near-inertial shears respond to common cyclones, which are widely existing but drawn fewer attentions before Another good opportunity for this study is that the corresponding SLA fields can be nearly divided into positive and negative periods, and amplitudes of SLAs during two periods were almost the same. The structure, variability, and mechanism of upward energy propagating shear and their relationships with downward shear will be considered
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