Jiexin Xu

Comparative study of short-term diurnal tidal variability

Examination of the simultaneous temperature measurement from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument, on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, not only confirms the existence of the inversion layer but also reveals the global nature of the inversion, suggesting the presence of a transient planetary wave in the mesosphere. The large tidal variability, therefore, is probably a consequence of the interaction between the transient planetary wave and tides. This possibility is investigated by using the NCAR thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) and by comparing model results with the lidar, SABER, and TIMED Doppler Interferometer (TIDI) measurements. With a large transient planetary wave specified at the model lower boundary, the model is able to produce strong diurnal tidal variability comparable to that from the lidar observation, and the modeled temperature inversion is similar to that from the SABER measurement. The model results suggest that the planetary/tidal wave interaction excites nonmigrating tides and modulates the gravity modes and/or the rotational modes of the diurnal migrating tide. Among the nonmigrating tides, the diurnal zonally symmetric (S = 0) component is the strongest, and its interaction with the planetary wave leads to a strong diurnal eastward wave number 1 component.

Simulations of Internal Solitary Wave Interactions with Mesoscale Eddies in the Northeastern South China Sea

AbstractWith the combined analysis of synthetic aperture radar image and satellite altimeter data collected in the northeastern South China Sea (SCS), this study found one type of distorted phenomenon of internal solitary wave (ISW) with the long front caused by the oceanic mesoscale eddy. Motivated by these satellite observations, the authors carried out numerical experiments using the fully nonhydrostatic and nonlinear MITgcm to investigate the perturbation of ISWs by an isolated cyclonic or anticyclonic eddy. The results show that the ISW front is distorted by these oceanic eddies due to the retardation and acceleration effects at their two sides. The ISW energy along the front is focused onto (scattered from) the wave fragment where a concave (convex) pattern is formed, and the previously accumulated energy in the focusing region is gradually released after the ISW propagates away from the eddies. The ISW amplitude is modulated greatly by the eddies due to the energy redistribution along the front. Se...

Density stratification influences on generation of different modes internal solitary waves

An ideal tide-topography interaction model is utilized for studying the influence of density stratification (pycnocline depth d, thickness δ, and the density difference Δρa across the pycnocline) on nonlinear disintegration of the first (mode-1) and second (mode-2) baroclinic mode internal tides into internal solitary waves (ISWs). The solution methods include weakly nonlinear analysis and fully nonlinear simulation. It is found that as d increases, even though the energy flux into mode-1 internal tides is always larger than that into mode-2 ones at generation, mode-2 ISWs emerge and mode-1 ISWs are suppressed. As δ increases, the total energy conversion and the fluxes into both mode-1 and mode-2 tides all increase first and then decrease. During propagation, a thick pycnocline is actually not favorable for the emergence of mode-2 ISWs, and the simulated well-developed mode-2 ISWs for a pycnocline of intermediate thickness are due to the tide generation process. As Δρa increases, the total conversion and the fluxes into both mode-1 and mode-2 tides all increase almost linearly. Even though the flux into mode-1 tides is always larger than that into mode-2 ones at generation, mode-1 tides cannot disintegrate but mode-2 ISWs develop very well. During propagation, Δρa has no influence on the generation of ISWs. The present work systematically investigates the influence of density stratification on formation of ISWs by considering both internal tide generation and propagation processes.

The effect of a seasonal stratification variation on the load exerted by internal solitary waves on a cylindrical pile

On the basis of Morison’s empirical formula and modal separation method in estimating the force and torque exerted by internal solitary waves (ISWs) on a cylindrical pile, it is found that the loads exerted by the ISWs change largely in different seasons at the same site of the continental shelf in the South China Sea (SCS) even under the condition that the amplitudes of ISWs are the same. Thus, the effect of a seasonal water stratification variation on the force and torque exerted by the ISWs is investigated, and a three-parameter stratification model is employed. It is shown that the loads exerted by the ISWs depend largely on the water stratification. The stronger the water stratification, the larger the force and the torque; when the depth where the maximum thermocline appears is deepened, the force decreases but the torque increases; when the width of the thermocline is narrowed, the force increases but the torque decreases. The seasonal variation of the force and the torque exerted by the ISWs in four seasons in the SCS is thus explained.

Distortion and broadening of internal solitary wavefront in the northeastern South China Sea deep basin

Internal solitary waves (ISWs) with peculiar fronts are frequently observed in the world ocean by satellite images, though with quite few explanations. In this study a distorted and broadening ISW front across the northeastern South China Sea deep basin is presented by using synthetic aperture radar (SAR) image. To illustrate this peculiar front, a nonlinear refraction model is developed to simulate and evaluate the effects of realistic bottom topography, current, and stratification on its transformation. Simulated results in realistic oceanic environments show good agreements with this SAR-observed front. Based on separate and comparative results in different background environments, we demonstrate that the distortion is actually caused by the strong mesoscale currents at periphery of an anticyclonic eddy. Moreover, the broadening is due to the difference in change of wave half width at different rays, which is associated with the different transformation of ISWs across variable bottom topography in the deep basin.

Numerical simulation of the Kuroshio intrusion into the South China Sea by a passive tracer

Owing to lack of observational data and accurate definition, it is difficult to distinguish the Kuroshio intrusion water from the Pacific Ocean into the South China Sea (SCS). By using a passive tracer to identify the Kuroshio water based on an observation-validated three-dimensional numerical model MITgcm, the spatio-temporal variation of the Kuroshio intrusion water into the SCS has been investigated. Our result shows the Kuroshio intrusion is of distinct seasonal variation in both horizontal and vertical directions. In winter, the intruding Kuroshio water reaches the farthest, almost occupying the area from 18°N to 23°N and 114°E to 121°E, with a small branch flowing towards the Taiwan Strait. The intrusion region of the Kuroshio water decreases with depth gradually. However, in summer, the Kuroshio water is confined to the east of 118°E without any branch reaching the Taiwan Strait; meanwhile the intrusion region of the Kuroshio water increases from the surface to the depth about 205 m, then it decreases with depth. The estimated annual mean of Kuroshio Intrusion Transport (KIT) via the Luzon Strait is westward to the SCS in an amount of–3.86×106 m3/s, which is larger than the annual mean of Luzon Strait Transport (LST) of–3.15×106 m3/s. The KIT above 250 m accounts for 60%–80% of the LST throughout the entire water column. By analyzing interannual variation of the Kuroshio intrusion from the year 2003 to 2012, we find that the Kuroshio branch flowing into the Taiwan Strait is the weaker in winter of La Niña years than those in El Niño and normal years, which may be attributed to the wind stress curl off the southeast China then. Furthermore, the KIT correlates the Niño 3.4 index from 2003 to 2012 with a correlation coefficient of 0.41, which is lower than that of the LST with the Niño 3.4 index, i.e., 0.78.

Effect of background parabolic current on characteristics and energetics of internal solitary waves by numerical simulation

Based on modifications of the observed background parabolic current in upper layer of the northeastern South China Sea (SCS), the effects of eight kinds of background currents on the characteristics and energy conversion of internal solitary waves (ISWs) are investigated by an Internal Gravity Wave (IGW) model. It is found that, although the background current has little effect on the number of the generated ISWs, it reduces the resulted phase speed of ISW. When the background parabolic current appears with its lower boundary near or above the main thermocline, the ISW amplitude and the depth of the isopycnal undergoing maximum displacement increase; when the background parabolic current curvature is reduced, the ISW amplitude and the ratio of baroclinic to barotropic energy reduce, whilst the phase speed of ISW, the baroclinic energy, and the ratio of baroclinic kinetic energy (KE) to available potential energy (APE) increase; when the lower boundary of background parabolic current extends down to the seabed and the background current curvature is reduced, the ISW amplitude and phase speed decrease, whilst the barotropic kinetic energy, the baroclinic energy and the ratio of KE to APE increase. At a whole depth, when the lower background current curvature is reduced and the upper current curvature is increased, the ISW amplitude, and phase speed, the ratio of baroclinic to barotropic energy, the baroclinic energy, and the ratio of KE to APE all increase.

Spatiotemporal Variations of Mesoscale Eddies in the Sulu Sea

Mesoscale eddies have been observed in the Sulu Sea, but their characteristics have not been well described. This study investigates the eddy population in the Sulu Sea using 22 years of satellite altimeter data with high spatiotemporal resolution. On average, there are approximately 1.6 eddies observed in the Sulu Sea each day and 1.8 eddy tracks generated each month. Two of the main eddy genesis regions are west of Negros Island and the Zamboanga Peninsula. The mean radius, lifespan and propagation speed of the eddies are 76.6 km, 32 days and 4.5 cm/s, respectively. The eddy radius and amplitude are generally large in the central Sulu Sea but small on its margin. The mean eddy kinetic energy and vorticity generally monotonically decrease from south to north, consistent with the distributions of background current kinetic energy. Over the seasonal cycle, there are more cyclonic eddies during boreal winter, and they tend to have a larger amplitude and radius than the other 3 seasons, while there are more anti-cyclonic eddies during boreal summer, and they tend to have a larger amplitude and radius than the other 3 seasons. The instability of the mean current and the island gap wind jets are the two key eddy genesis mechanisms in the Sulu Sea.

Generation of internal solitary waves over a large sill: From Knight Inlet to Luzon Strait

A fully nonlinear, nonhydrostatic numerical model is utilized to investigate the generation of Internal Solitary Waves (ISWs) upstream of the Knight Inlet sill. While an upstream hydraulic jump initiates the ISW generation and both hydraulic jump and upstream influence contribute to the generation, it is found that upstream influence is dominant and the hydraulic jump is not necessary for the ultimate generation of ISWs. Decreasing the tidal forcing or upstream sill width may render the flow subcritical (i.e., the hydraulic jump disappears) and ISWs can be generated by nonlinear steepening of long wave disturbances induced by upstream influence. Increasing the tidal forcing or upstream sill width may generate a hydraulic jump blocking strong upstream propagating disturbances. The jump subsequently becomes a turbulent bore and later disperses into a train of ISWs as the tide relaxes. Further increase in the tidal forcing may sweep the turbulent bore downstream and a train of ISWs is emitted upstream toward the end of waning tide. By reducing the stratification strength by 1 order of magnitude, the near-sill flow is in the transcritical regime and ISWs are resonantly generated over the lee side slope. Connections to the internal tide release mechanism at Luzon Strait and to the unsteady lee wave model are also discussed. The present work provides some more insights into the ISW generation process at Knight Inlet and the connection between the generation mechanism at Knight Inlet and that at Luzon Strait is identified.

ENERGY BUDGET OF INTERNAL SOLITARY WAVES IN SOUTH CHINA SEA FROM CONTINUOUSLY STRATIFIED NONHYDROSTATIC MODEL

Abstract An in-house fully nonlinear, nonhydrostatic numerical code is utilized for simulations of internal solitary waves (ISWs) generated by tidal flow over a Gaussian sill topography. A complete, rigorous theoretical framework is then adopted for the energetics analysis of these ISWs. It is found that ISWs contain most of the baroclinic energy in the internal wave field. The nonhydrostatic energy flux is in opposite direction to the hydrostatic and nonlinear fluxes. For fixed tidal excursion parameter (ε), the nonlinear portion of the total baroclinic flux decreases as the slope parameter (γ) increases. In addition, the ISW energy, energy flux and barotropic-to-baroclinic energy conversion rate all peak when the bottom topography is critical (γ = 1). For typical ISWs generated in the Luzon Strait, 87.9% of the total barotropic input energy is converted into baroclinic energy, and the other part of the energy is used for barotropic dissipation. 63.7% of the converted baroclinic energy is radiated far away and the remaining part of the baroclinic energy is dissipated through local mixing. When compared to the energy budget of linear internal tides, the percentages of the energy that are converted and radiated decrease, or equivalently, the percentages of the energy that are used for barotropic and baroclinic dissipations increase. Thus, the emergence of ISWs can effectively enhance both barotropic and baroclinic dissipations.