Yongsheng Xu

Global Variability of the Wavenumber Spectrum of Oceanic Mesoscale Turbulence

AbstractThe wavenumber spectra of sea surface height from satellite altimeter observations have revealed complex spatial variability that cannot be explained by a universal theory of mesoscale turbulence. Near the edge of the core regions of high eddy energy, agreement is observed with the prediction of the surface quasigeostrophic (SQG) turbulence theory, which has fundamental differences from that of the traditional quasigeostrophic (QG) turbulence theory. In the core regions of high eddy energy, the spectra are consistent with frontogenesis that is not fully accounted for by the SQG theory. However, the observations in the vast ocean interior of low eddy energy exhibit substantial differences from the predictions of existing theories of oceanic mesoscale turbulence. The spectra in these regions may reflect the ocean’s response to short-scale atmospheric forcing and air–sea interaction. The observations presented in this paper serve as a test bed for new theories and ocean general circulation models.

The Effects of Altimeter Instrument Noise on the Estimation of the Wavenumber Spectrum of Sea Surface Height

The wavenumber spectrum of sea surface height (SSH) observed by satellite altimetry was analyzed by Xu and Fu. The spectral shape in the wavelength range of 70-250 km was approximated by a power law, representing a regime governed by geostrophic turbulence theories. The effects of altimeter instrument noise were assumed insignificant at wavelengths longer than 70 km. The authors reexamined the assumption in the study. Using nearly simultaneous observations made by Jason-1 and Jason-2 during their cross-calibration phase, this study found that the white noise level of altimetry measurement was best estimated from the spectral values at wavelengths from 25 to 35 km. After removing a white noise level based on such estimate from the SSH spectrum, the spectral slope values changed significantly over most of the oceans. A key finding is that the spectral slopes are generally steeper than k(-2) (k is wavenumber) poleward of the 20 degrees latitudes, where flatter spectral slopes in some regions have previously caused problems for dynamic interpretations. The new results indicate that the spectral slopes in the core regions of the major ocean current systems have values between the original geostrophic turbulence theory and the surface quasigeostrophic theory. The near k(-4) spectrum suggests that the sea surface height variability at these wavelengths in the high eddy energy regions might be governed by frontogenesis.

Variability of internal tides and near-inertial waves on the continental slope of the northwestern South China Sea

Structure and variability of internal tides (IT) and near-inertial waves (NIW) on the continental slope of the northwestern South China Sea were investigated, based on 9-month moored current observations from autumn to early summer in 2008 and 2009. The diurnal IT kinetic energy, dominant over that of semidiurnal tides, is found to exhibit apparent seasonal variability-strongest in summer and weakest in winter-whereas the semidiurnal variance remained nearly uniform throughout the observation period. Moreover, the diurnal IT were more coherent (i.e., phase-locked to the astronomical forcing) than the semidiurnal constituents. Coherent diurnal variance accounts for about 40% of diurnal motions, but semidiurnal tides contain a much smaller fraction (10%) of coherent motions. Further analysis demonstrates that the diurnal IT are dominated by the first mode, whereas the semidiurnal tides show a variable multimodal structure: the second mode is dominant in summer and comparable to the first mode in spring and autumn, but the first mode predominates in winter. Multimodal semidiurnal IT are more influenced by varying stratification structures and background currents and thus exhibit highly incoherent and intermittent behavior, which may wash out seasonal variability during their long propagation from the generation source. The observed NIW are seasonally independent and comparable to the semidiurnal motions. During the passage of Typhoon Hagupit, however, the NIW became the most energetic component of the inertia-gravity waveband motions. NIW energy and shear were significantly enhanced and exceeded tidal counterparts by a factor of 2 to 3 in the upper layer.

A method to study the impact of climate change on variability of river flow: an example from the Guadalupe River in Texas

This work introduced a method to study river flow variability in response to climate change by using remote sensing precipitation data, downscaled climate model outputs with bias corrections, and a land surface model. A meteorological forcing dataset representing future climate was constructed via the delta change method in which the modeled change was added to the present-day conditions. The delta change was conducted at a fine spatial and temporal scale to contain the signals of weather events, which exhibit substantial responses to climate change. An empirical transformation technique was further applied to the constructed forcing to ensure a realistic range. The meteorological forcing was then used to drive the land surface model to simulate the future river flow. The results show that preserving fine-scale processes in response to climate change is a necessity to assess climatic impacts on the variability of river flow events.

Fundamental‐mode basin oscillations in the Japan/East Sea

[4] In the real JES, energy dissipation exists, four straits connect to the open ocean and bottom topography is highly variable, so Rikiishi’s model may be expected to differ somewhat in period and structure from observations. We find no published report of direct observations of basin oscillations in the JES. In this paper we provide observational evidence that fundamental mode basin oscillations exist continually in the JES. The observed results are compared with Rikiishi’s model, and the forcing is investigated.

Dipping-Induced In-Plane Molecular Alignment of LB Films

Abstract Super-quadratic growth of SHG intensity with thickness could be realized due to the extra dipping-induced and epitaxy-enhanced in-plane polarization in the hemicyanine LB multilayers.

The impact of wave‐induced Coriolis‐Stokes forcing on satellite‐derived Ocean Surface Currents

Ocean surface currents estimated from the satellite data consist of two terms: Ekman currents from the wind stress and geostrophic currents from the sea surface height (SSH). But the classical Ekman model does not consider the wave effects. By taking the wave-induced Coriolis-Stokes forcing into account, the impact of waves (primarily the Stokes drift) on ocean surface currents is investigated and the wave-modified currents are formed. The products are validated by comparing with OSCAR currents and Lagrangian drifter velocity. The result shows that our products with the Stokes drift are better adapted to the in situ Lagrangian drifter currents. Especially in the Southern Ocean region (40 degrees S-65 degrees S), 90% (91%) of the zonal (meridional) currents have been improved compared with currents that do not include Stokes drift. The correlation (RMSE) in the Southern Ocean has also increased (decreased) from 0.78 (13) to 0.81 (10.99) for the zonal component and 0.76 (10.87) to 0.79 (10.09) for the meridional component. This finding provides the evidence that waves indeed play an important role in the ocean circulation, and need to be represented in numerical simulations of the global ocean circulation.

Penetration depth of diapycnal mixing generated by wind stress and flow over topography in the northwestern Pacific

The role of turbulent diapycnal mixing in the northwestern Pacific was estimated by employing a fine-scale parameterization method based on 6756 high-resolution CTD profiles spanning a period of 8 years from the Japan Oceanography Data Center (JODC) and the Kuroshio Extension System Study (KESS). The rate of turbulent mixing in the upper ocean within 300-1800 m depth displayed a distinct seasonal cycle, bearing a statistically significant correlation to wind-induced near-inertial energy flux (hereafter denoted by WNEF). Enhanced turbulent mixing was also found near the rough seafloor relative to that over smooth topography. Enhanced dissipation at surface and bottom was found to be able to penetrate the ocean interior up to 1800 m and 3300 m, respectively, with penetration depths varying with the WNEF and topographic roughness. Our study here provides evidence for the important role of near-inertial energy input from wind stress and the influence of bottom topography in maintaining mixing in the ocean interior.

De-Aliasing of Large-Scale High-Frequency Barotropic Signals from Satellite Altimetry in the Japan/East Sea

Abstract In the Japan/East Sea, energetic high-frequency large-scale barotropic motions are shown to lead to large aliasing errors in satellite altimetry observations. The combined aliasing from several neighboring and crossing tracks produces artificial mesoscale signals in altimeter-mapped products, significantly changing the map interpretation. The alias can be well suppressed by subtracting the large-scale barotropic motions observed by bottom pressure sensors. By using coastal tide gauge data in the Japan/East Sea, about 78% of the alias source variance can be removed, which offers an alternative way to suppress the alias for other time intervals without bottom pressure measurements.

Novel Fluorinated Liquid Crystals. Part VIII. The Synthesis and Mesomorphic Properties of 4“-n-Alkoxyphenyl 4”-[(4-n-Alkoxy-2, 3, 5, 6-tetrafluorophenyl)ethynyl]benzoates

Abstract Eighteen members of 4“-n-alkoxyphenyl 4”-[(4-n-alkoxy-2, 3, 5, 6-tetrafluorophenyl) ethynyl] benzoates have been synthesized. Polarizing microscopic textural observation shows that they exhibit enantiotropic nematic phase and smetic C phase.