Xiaodong Shang

Evolution of the semidiurnal (M2) internal tide on the continental slope of the northern South China Sea

Kinetic energy spectra from a site on the continental slope in the South China Sea reveal that significant peaks appear at some nonlinear interaction frequencies, namely M-3 (M-1 + M-2) and fM(1) (M-1 + f), where f is the inertial frequency, M-1 is the diurnal internal wave, and M-2 is the lunar semidiurnal internal tide. A possible generation mechanism of M-3 is explored. Analysis of bicoherence and shear spectra suggests that strong M-3 is indirectly associated with parametric subharmonic instability ( PSI) of M-2. In another word, under the effect of PSI the energy of M-2 is first transferred to M-1; then via other nonlinear coupling, some nonlinear waves (e.g. fM(1), M-3) are generated. Moreover, M-1 is also present at another site near the bottom of the continental slope. The shear spectra from these two sites show, for the first time, that M-1 can be significantly distinguished from lunar diurnal O-1 and lunisolar diurnal K-1.

Observations of parametric subharmonic instability‐induced near‐inertial waves equatorward of the critical diurnal latitude

Moored current observations of 75 days duration in the northeastern South China Sea (similar to 20 degrees N) suggest that parametric subharmonic instability (PSI) of semidiurnal (D(2)) internal tides can not only generate waves of frequencies close to D(2)/2, but also excite near-inertial waves whose frequencies are different from D(2)/2. Time series of shear amplitudes clearly show a 14-day cycle. Although near-inertial and near-diurnal motions dominate the shear, this cycle is in phase with the fortnightly spring-neap cycle of D(2)-waves. After separation of near-inertial and near-diurnal waves using band-pass filters, shear magnitudes for both motions still follow this 14-day cycle, rather than that of diurnal internal tides or variations of the local wind field. This strongly suggests that PSI equatorward of the critical latitude for D(2)/2 waves (similar to 29 degrees) not only transfers D(2)-energy to D(2)/2 waves, but also to high-mode near-inertial waves. Near-inertial waves (f) and another subharmonic (D(2)-f), together with D(2) waves, compose a PSI-triad following strong interaction. Citation: Xie, X.-H., X.-D. Shang, H. van Haren, G.-Y. Chen, and Y.-Z. Zhang (2011), Observations of parametric subharmonic instability-induced near-inertial waves equatorward of the critical diurnal latitude, Geophys. Res. Lett., 38, L05603, doi: 10.1029/2010GL046521.

Variations of diurnal and inertial spectral peaks near the bi‐diurnal critical latitude

Three sets of ADCP data obtained from the upper ocean are used to examine possible influence of parametric subharmonic instability (PSI) at phi(c) approximate to 13 - 15 degrees N. Both kinetic-energy and shear spectra at phi(c) reveal that significant peaks appear at subharmonic frequencies of diurnal internal tides. Especially, the 0.5 K-1 (subharmonic of K-1 tides) can be trapped poleward of its critical latitude (14.52 degrees N), significantly distinguished from wind generated near-inertial internal waves. Moreover, the enhanced 0.5 K-1 motions are more or less subject to a fortnightly spring-neap circle. Relative to a higher latitude (say 18 degrees N), diurnal motions at phi(c) are greatly weakened. In contrast, small vertical-scale motions in the inertial band and the shear induced by these motions are significantly enhanced. Likely, PSI mechanism plays an important role in these observations, whilst trapped sub-inertial waves (0.5 K-1) may be associated with the presence of negative sub-inertial eddies. Citation: Xie, X.-H., X.-D. Shang, G.-Y. Chen, and L. Sun (2009), Variations of diurnal and inertial spectral peaks near the bi-diurnal critical latitude, Geophys. Res. Lett., 36, L02606, doi: 10.1029/2008GL036383.

Work done by atmospheric winds on mesoscale ocean eddies

Mesoscale eddies are ubiquitous in the ocean and dominate the oceans kinetic energy. However, physical processes influencing ocean eddy energy remains poorly understood. Mesoscale ocean eddy-wind interaction potentially provides an energy flux into or out of the eddy field, but its effect on ocean eddies has not yet been determined. Here we examine work done by atmospheric winds on more than 1,200,000 mesoscale eddies identified from satellite altimetry data, and show that atmospheric winds significantly damp mesoscale ocean eddies, particularly in the energetic western boundary current regions and the Southern Ocean. Furthermore, the large-scale wind stress curl is found to on average systematically inject kinetic energy into anticyclonic (cyclonic) eddies in the subtropical (subpolar) gyres, while mechanically damps anticyclonic (cyclonic) eddies in the subpolar (subtropical) gyres.

Poleward propagation of parametric subharmonic instability‐induced inertial waves

This study presents two sets of current records obtained from the South China Sea and satellite altimeter data, and it suggests that near-inertial waves induced by parametric subharmonic instability (PSI) associated with internal tides can be transported poleward beyond their critical latitude phi(c) by background geostrophic flow (BGF). The two mooring locations were poleward of phi(c) (approximate to 14 degrees N) for diurnal subharmonics (0.5D(1); half diurnal frequency D-1); however, both of the current records revealed clear signals at 0.5D(1). The enhanced subinertial motion at 0.5D(1) exhibited a fortnightly spring-neap cycle but did not agree with that of D-1, indicating that it may not be generated via PSI associated with the local D-1. Observations from the altimeter data and a ray-tracing simulation suggested that these nonlocally generated 0.5D(1) waves may be excited near their phi(c), after which they propagated poleward under the role of the BGF to the observation site with a latitude higher than phi(c). The poleward propagation of near-inertial waves can produce elevated vertical shears; thus, it may play an important role in enhancing the local turbulent mixing.

Research on Cold Core Eddy Change and Phytoplankton Bloom Induced by Typhoons: Case Studies in the South China Sea

The effects of 8 typhoons which passed by coldcore eddy (CCE) areas in the South China Sea (SCS) from 1997 to 2009 were observed and evaluated. The changes in the preexisting CCE acted upon by typhoons were described by eddy kinetic energy (EKE) and eddy available gravitational potential energy (EAGPE). The mechanical energy of CCE was estimated from a two-layer reduced gravity model. Comparing with the scenario that typhoon passes by the region without CCEs, the preexisting CCE area plays an important role in the increase of chlorophyll-a (chl-a) concentration in the CCEs impacted by the typhoons. The preexisting chl-a in CCE is about 25%~45% (8%~25%) of postexisting chl-a in CCE for higher (slower) transit speed typhoons. If the EAGPE of CCE increases greatly after typhoon passing by with slow transit speed, so does the chl-a in the CCE area. The EKE (EAGPE) changes of the preexisting CCE are in the order of O(1014~1015 J). EKE and EAGPE of CCE are dominantly enhanced by typhoon with slow transit speed (<3 m/s) and the posttyphoon EAGPE is always larger than posttyphoon EKE for 8 cases. The maximum EAGPE change of the preexisting CCE reaches  J, which was induced by typhoon Hagibis.

The effect of an electric field on the nonlinear response of InAs/GaAs quantum dots

The refractive nonlinearities of InAs/GaAs quantum dots under a dc electric field at photon energies above its band gap energy have been studied using the reflection Z-scan technique. The effect of the dc electric field on the nonlinear response of InAs/GaAs quantum dots showed similar linear and quadratic electro-optic effects as in the linear response regime at low fields. This implies that the electro-optic effect in the nonlinear regime is analogous to the response in the linear regime for semiconductor quantum dots. Our experimental results show the potential for voltage tunability in InAs quantum dot-based nonlinear electro-optic devices.

An Expendable Microstructure Profiler for Deep Ocean Measurements

AbstractMeasurements of turbulence in the deep ocean, particularly close to the bottom, are extremely sparse because of the difficulty and operational risk of obtaining deep profiles near the seafloor. A newly developed expendable instrument—the VMP-X (Vertical Microstructure Profiler–Expendable)—carries two microstructure shear probes to measure the fluctuations of vertical shear into the dissipation range and can profile down to a depth of 6000 m. Data from nine VMP-X profiles in the western Pacific Ocean near 11.6°N over rough topography display bottom-intensified turbulence with dissipation rates increasing by two factors of 10 to 4 W kg−1 within 200 m above the bottom. In contrast, over smooth topography in the southern South China Sea near 11°N, three profiles show that turbulence in the bottom boundary layer increases only slightly, with dissipation rates reaching 1 W kg−1. The eddy diffusivity over rough topography reached to 5 m2 s−1. The average diffusivity over all depths was 0.3 and 0.9 m2 s−1...

Assessment of fine-scale parameterizations at low latitudes of the North Pacific

Fine-scale parameterizations based on shear and stratification are widely used to study the intensity and spatial distribution of turbulent diapycnal mixing in the ocean. Two well-known fine-scale parameterizations, Gregg–Henyey–Polzin (GHP) parameterization and MacKinnon–Gregg (MG) parameterization, are assessed with the full-depth microstructure data obtained in the North Pacific. The GHP parameterization commonly used in the open ocean succeeds in reproducing the dissipation rates over smooth topography but fails to predict the turbulence over rough topography. Failure of GHP parameterization over rough topography is attributed to the deviation of internal wave spectrum from the Garrett–Munk (GM) spectrum. The internal wave field over rough topography is characterized by energetic intermediate-scale and small-scale internal waves that are not described well by the GM model. The MG parameterization that is widely used in coastal environments is found to be successful in reproducing the dissipation rates over both smooth and rough topographies. The efficacy of GHP and MG parameterizations in evaluating the dissipation rates has been assessed. The result indicates that MG parameterization predicts the magnitude and variability of the dissipation rates better than the GHP parameterization.

Deep Sea Currents Driven by Breaking Internal Tides on the Continental Slope

Mooring data collected on the continental slope of the South China Sea show that along-slope deep sea bottom currents are generated when large spring internal tides (internal waves with tidal frequency) are observed, with the maximum velocity amplitude exceeding 0.15m/s. The observations are consistent with predictions that near-bottom breaking of internal waves can result in generation of along-slope flows when these waves obliquely approach the slope. A linear internal tide model in one horizontal dimension with realistic topography and stratification is used to show that the breaking of internal tides is likely due to near-critical reflection on the slope. Combining the mooring observations and the model simulation, an along-slope near-bottom transport of similar to 0.5Sv is estimated. Along-slope bottom flows caused by breaking internal waves potentially provide a significant way to deform continental slopes and affect deep water exchange between the marginal sea and open ocean.