Zhenhua Xu

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.

Analysis of seasonal variation of water masses in East China Sea

Seasonal variations of water masses in the East China Sea (ECS) and adjacent areas are investigated, based on historical data of temperature and salinity (T-S). Dynamic and thermodynamic mechanisms that affect seasonal variations of some dominant water masses are discussed, with reference to meteorological data. In the ECS above depth 600 m, there are eight water masses in summer but only five in winter. Among these, Kuroshio Surface Water (KSW), Kuroshio Intermediate Water (KIW), ECS Surface Water (ECSSW), Continental Coastal Water (CCW), and Yellow Sea Surface Water (YSSW) exist throughout the year. Kuroshio Subsurface Water (KSSW), ECS Deep Water (ECSDW), and Yellow Sea Bottom Water (YSBW) are all seasonal water masses, occurring from May through October. The CCW, ECSSW and KSW all have significant seasonal variations, both in their horizontal and vertical extents and their T-S properties. Wind stress, the Kuroshio and its branch currents, and coastal currents are dynamic factors for seasonal variation in spatial extent of the CCW, KSW, and ECSSW, whereas sea surface heat and freshwater fluxes are thermodynamic factors for seasonal variations of T-S properties and thickness of these water masses. In addition, the CCW is affected by river runoff and ECSSW by the CCW and KSW.

Dissipative Nonlinear Schrodinger Equation for Envelope Solitary Rossby Waves with Dissipation Effect in Stratified Fluids and Its Solution

We solve the so-called dissipative nonlinear Schrodinger equation by means of multiple scales analysis and perturbation method to describe envelope solitary Rossby waves with dissipation effect in stratified fluids. By analyzing the evolution of amplitude of envelope solitary Rossby waves, it is found that the shear of basic flow, Brunt-Vaisala frequency, and beta effect are important factors to form the envelope solitary Rossby waves. By employing trial function method, the asymptotic solution of dissipative nonlinear Schrodinger equation is derived. Based on the solution, the effect of dissipation on the evolution of envelope solitary Rossby wave is also discussed. The results show that the dissipation causes a slow decrease of amplitude of envelope solitary Rossby waves and a slow increase of width, while it has no effect on the propagation velocity. That is quite different from the KdV-type solitary waves. It is notable that dissipation has certain influence on the carrier frequency.

Depression and elevation internal solitary waves in a two-layer fluid and their forces on cylindrical piles

Both large amplitude depression and elevation internal solitary waves (ISWs) were observed on the continental shelf of the northwest South China Sea (SCS) during the Wenchang Internal Wave Experiment. In this study, we investigate the characteristics of depression and elevation ISWs based on comparisons between observational results and internal wave theories. It is suggested that the large amplitude depression wave is better represented by the extended Korteweg-de Vries (EKdV) theory than by the KdV model, whereas the large amplitude elevation wave is in better agreement with the KdV equation than with the EKdV theory. Wave-induced forces on a supposed small-diameter cylindrical pile by depression and elevation waves are also estimated using the internal wave theory and Morison formula. The wave-induced force by elevation ISWs is rarely reported in the literature. It is found that the force induced by the elevation wave differs significantly from that by the depression wave, and the elevation wave generally produces greater force on the pile in the lower water column than the depression wave. These results show that ISWs in the study area can present a serious threat to ocean engineering structures, and should not be ignored in the design of oil platforms and ocean operations.

Spatiotemporal variations of the surface Kuroshio east of Taiwan Island derived from satellite altimetry data

Spatiotemporal variation of the surface Kuroshio east of Taiwan Island is investigated by quantitatively analyzing 23-year (1993–2015) sea surface Absolute Dynamic Topography data. The annual mean state of the Kuroshio is shown, with a flow width ~136 km and surface transport ~7.75×104m2/s. The corresponding standard deviations of these are 28 km and 2.14×104m2/s. Results of power spectrum analysis indicate that the primary periods of Kuroshio surface transport east of Taiwan Island are 1 and 2.8 years, respectively. Spatially, the Kuroshio surface transport southeast of Taiwan Island has greater variability than that to its northeast. That transport showed strong seasonality, with a maximum 8.8×104m2/s in summer and minimum 7.5×10 4 m 2 /s in winter, which was mainly caused by local monsoon winds. A linear long-term upward trend of Kuroshio surface transport during 1993–2015 was found, during which the mean, southeast, and northeast of the Kuroshio east of Taiwan Island increased by 0.30×10 4, 0.22×10 4 and 0.36×10 4 m 2 /s, respectively. Correlation and composite analysis show that the Philippines-Taiwan Oscillation (PTO) is important in the interannual variability of the Kuroshio. PTO-induced relative intensity of anticyclonic and cyclonic eddies is the dominant influence on the interannual variability of the Kuroshio east of Taiwan Island.

Seasonal variation of the Taiwan Warm Current Water and its underlying mechanism

Based on the historical observed data and the modeling results, this paper investigated the seasonal variations in the Taiwan Warm Current Water (TWCW) using a cluster analysis method and examined the contributions of the Kuroshio onshore intrusion and the Taiwan Strait Warm Current (TSWC) to the TWCW on seasonal time scales. The TWCW has obviously seasonal variation in its horizontal distribution, T-S characteristics and volume. The volume of TWCW is maximum (13 746 km3) in winter and minimum (11 397 km3) in autumn. As to the contributions to the TWCW, the TSWC is greatest in summer and smallest in winter, while the Kuroshio onshore intrusion northeast of Taiwan Island is strongest in winter and weakest in summer. By comparison, the Kuroshio onshore intrusion make greater contributions to the Taiwan Warm Current Surface Water (TWCSW) than the TSWC for most of the year, except for in the summertime (from June to August), while the Kuroshio Subsurface Water (KSSW) dominate the Taiwan Warm Current Deep Water (TWCDW). The analysis results demonstrate that the local monsoon winds is the dominant factor controlling the seasonal variation in the TWCW volume via Ekman dynamics, while the surface heat flux can play a secondary role via the joint effect of baroclinicity and relief.

Variability of Internal Solitary Waves in the Northwest South China Sea

Internal waves are waves that travel within the interior of the water column. Its existence is owned to the stratified density structure between two or continuous layers of fluids (Apel et al., 1987). Internal solitary waves (ISWs) are nonlinear internal waves, which are frequently observed all over the world oceans, where strong tides and stratification occur over varying topography features (Apel et al., 1985; Colosi et al., 2001; Osborne & Burch, 1980). They typically occur in packets at tidal intervals, suggesting that they mainly originate from the tide-topography interactions over variable topography (Gerkema et al., 1995). Depending on the different environmental conditions, there are two main mechanisms for the generation of ISWs: lee-wave mechanism (Maxworthy, 1979) and nonlinear internal tide mechanism (Lee & Beardsley, 1974). The lee-wave mechanism states that the lee-wave is formed by the ebb tide and released when the tide changes from ebb tide to flood tide, and evolves into a rank-ordered internal solitary wave (ISW) packet. By the nonlinear internal tide mechanism, internal tides spawn ISWs in three steps: initial generation of a front due to topographic blocking, nonlinear steepening of the front, and formation of a rank-order ISW packet under effects of nonlinearity and dispersion (Helfrich & Melville, 2006; Zhao & Alford, 2006). Internal solitary waves are important for many practical reasons. As they are commonly observed wherever strong tides and stratification occur next to the irregular topography, thus they are often prominent features seen in optical and radar satellite imagery of coastal waters. They can propagate over several hundred kilometers and transport both mass and momentum. They can also induce considerable velocity shears that can impose unexpectedly large stresses on offshore oil-drilling rigs and lead to turbulence and mixing. In addition, the mixing often introduces bottom nutrients into the water column, thereby fertilizing the local region and modifying the biology system therein (Jackson, 2004). ISWs in the South China Sea (SCS) have been observed at a variety of locations from Luzon Strait to the continental shelf (Cai et al., 2002; Lien et al., 2005; Liu et al., 2004; Ramp et al., 2004; Xu et al., 2010; Zhao et al., 2004). Until recently, considerable effort has been focused on the study of ISWs in the northeastern SCS (Farmer et al., 2009; Moore et al., 2007; Ramp et al., 2004). In contrast, Due to the shortage of high-quality data sets, studies of the ISWs in the northwestern SCS are quite limited and nonlinear internal waves in this region have been

Low‐Level Jets Over the Bohai Sea and Yellow Sea: Climatology, Variability, and the Relationship With Regional Atmospheric Circulations

The present study reveals climate features of low-level jets (LLJs) over the Bohai Sea and Yellow Sea (BYS) based on a 35-year (1979-2013) high-resolution (7km) atmospheric hindcast. The regional climate model COSMO-CLM driven by the ERA-Interim reanalysis data set was used to obtain the hindcast. Through comparison with observations, the hindcast was proved to robustly reproduce the climatology, the diurnal cycle, the variability of wind profiles, and specific LLJ cases. LLJs over the BYS feature a strong diurnal cycle, intra-annual, and interannual variability but weak decadal variability. LLJs are more frequent in April, May, and June (LLJ season) and less frequent in winter over the Bohai Sea and western coastal areas of the Yellow Sea, which is due to the intra-annual variations of large-scale circulation and local land-sea thermal contrast. In the LLJ season, the heights of jet cores are generally lower than 500m above sea level. The maximum wind speed of LLJs is mostly in the range of 10-16m/s, and prevailing wind directions are southerly and southwesterly. The LLJs are of the nocturnal type, with the highest occurrence frequency at approximately 2300 local time. Furthermore, a low-frequency link between anomalies of LLJ occurrence and regional large-scale barotropic circulation was identified using canonical correlation analysis and associated correlation patterns. Pressure systems over the East Asia-northwest Pacific region are significantly correlated with the variations of LLJ occurrence over the BYS in terms of the intra-annual and interannual variability.

Long‐Range Radiation and Interference Pattern of Multisource M2 Internal Tides in the Philippine Sea

Long-range radiation and interference of M-2 internal tides from multiple sources in the Philippine Sea are examined by driving a high-resolution numerical model. The M-2 internal tides are effectively generated around the boundary area, which includes the Luzon Strait, Ryukyu Island chain, Bonin Ridge, Mariana Arc, and Izu Ridge, favoring the occurrence of complex interference patterns. The local sources (mainly Daito Islands and Palau Ridge) inside the basin contribute to a small portion (5%) of total energy but enhance the geographical inhomogeneity of the baroclinic field. The mode-1 and mode-2 M-2 tidal beams from boundary sources radiate a long distance into the basin but exhibit different interference-modulated geography variations. A 2-D line source model characterizing interference can reproduce the general baroclinic field. Two notable interference cases are investigated: (1) the superposition of internal tides from Luzon Strait and Miyako Strait bifurcates into several southeastward beams, consistent with previous numerical simulations and altimeter measurements, and (2) the interference between Tokara Strait and Bonin Ridge exhibits a multiscale spatial pattern, which is modulated by the local generated energy and bathymetry features. Energetic dissipation occurs both near the boundary sources and in the basin. A locally dissipated fraction q of 0.4 is estimated at the Luzon Strait and Bonin Ridge with continuous bathymetry features, while q of 0.6 is estimated at the Ryukyu Island chain and Mariana Arc with discrete topographic variability. A lower locally dissipated fraction indicates a stronger energy flux radiating into the basin, where enhanced dissipation coincides closely with the interference-modulated flux field.

Present Climate Evaluation and Added Value Analysis of Dynamically Downscaled Simulations of CORDEX—East Asia

AbstractIn this study, we investigate the skills of the regional climate model Consortium for Small-Scale Modeling in Climate Mode (CCLM) in reproducing historical climatic features and their added...