Pore pressure observation: pressure response of probe penetration and tides, Abstract Excess pore water pressure is an important parameter that can be used to analyze certain physical characteristics of sediment. In this paper, the excess pore water pressure of subseafloor sediment and its variation with tidal movement was measured following the installation of a wharf in Qingdao, China by using a fiber Bragg grating (FBG) piezometer. The results indicated that this FBG piezometer is effective in the field. The measured variation of excess pore water pressure after installation is largely explained by the dissipation of excess pore water pressure. The dissipation rate can be used to estimate the horizontal consolidation coefficient, which ranged from 1.3×10-6 m2/s to 8.1×10-6 m2/s. The measured values during tidal phases are associated with the variability of tidal pressure on the seafloor and can be used to estimate the compressibility and the permeability of the sediment during tidal movement. The volume compression coefficient estimated from tidal oscillation was approximately 2.0×10-11 Pa-1, which was consistent with the data from the laboratory test. The findings of this paper can provide useful information for in situ investigations of subseafloor sediment.

Late onsets of tropical cyclones in the decaying years of super El Niño events Abstract The 2015/2016 El Niño event reached the threshold of super El Niño event, and was comparable to the super events in 1982/1983 and 1997/1998. Interestingly, the tropical cyclones (TCs) were found to have very late onsets in the decaying years of the super El Niño events. This study discusses the causes of late TC onsets related with atmospheric circulation, disturbance sources and trigger mechanisms. The analysis shows that the western North Pacific subtropical high (WNPSH) from January-June during the decaying years of the super El Niño events were stronger than the climatic mean, which resulted in a relatively stable atmospheric state by inhibiting deep convection. As a disturbance source, the April-June intertropical convergence zone (ITCZ) during the decaying years of the super El Niño events were significantly weaker than its climatic mean. The cross-equatorial flow and monsoon trough, as important TC generation triggers, were weaker from April-June during the decaying years of the super El Niño events, which further reduced the probability of TC generation. As for the late TC onsets, the role of atmospheric circulation anomalies (i.e., subtropical-high, the ITCZ, cross-equatorial flow, and monsoon trough) were more important. The cross-equatorial flow may take as predictor of TC onsets in the decaying years of the super El Niño events.

Effects of hurricane forward speed and approach angle on storm surges: an idealized numerical experiment Abstract The effects of hurricane forward speed (V) and approach angle (θ) on storm surge are important and a systematic investigation covering possible and continuous ranges of these parameters has not been done before. Here we present such a study with a numerical experiment using the Finite Volume Community Ocean Model (FVCOM). The hurricane track is simplified as a straight line, such that V and θ fully define the motion of the hurricane. The maximum surge is contributed by both free waves and a forced storm surge wave moving with the hurricane. Among the free waves, Kelvin-type waves can only propagate in the down-coast direction. Simulations show that those waves can only have a significant positive storm surge when the hurricane velocity has a down-coast component. The optimal values of V and θ that maximize the storm surge in an idealized semi-circular ocean basin are functions of the bathymetry. For a constant bathymetry, the maximum surge occurs when the hurricane approaches the coast from the normal direction when the free wave generation is minimal; for a stepped bathymetry, the maximum surge occurs at a certain acute approach angle which maximizes the duration of persistent wind forcing; a step-like bathymetry with a sloped shelf is similar to the stepped bathymetry, with the added possibility of landfall resonance when the free and forced waves are moving at about the same velocity. For other cases, the storm surge is smaller, given other parameters (hurricane size, maximum wind speed, etc.) unchanged.

Analytical optimization on GNSS buoy array for underwater positioning Abstract Global navigation satellite system (GNSS)/acoustic positioning precision is determined by the positioning geometry and the ranging precision; thus optimizing GNSS buoys array is meaningful to improve the positioning accuracy and reliability. An analytical method is proposed for optimizing the GNSS buoys array with regard to the cutoff angle constraints for underwater acoustic observations. For the practical limitation of coplanarity of GNSS buoys and the cutoff angle, an algorithm is proposed to analytically minimize the position dilution of precision (PDOP). The proposed method is validated to give complete solutions of PDOP minimization with five GNSS buoys. At last, in order to search a best configuration among the PDOP solution set, we propose a search algorithm to get the solution with the smallest geometric dilution of precision (GDOP). It indicates that within a given region, the GDOP minimization at the center of a region is equivalent to the PDOP mean minimization over the region. The relation between the positioning accuracy and the positioning geometry with five known points is illustrated in an experiment performed in South China Sea.

The third-order asymptotic solutions in the Lagrangian description for interfacial internal waves in a three layer fluid system Abstract In this paper, we discuss the interfacial internal waves with a rigid boundary in a three-layer fluid system, where the density of the upper layer fluid is smaller than that of the lower layer. With the Lagrangian matching conditions at the interfaces, the first-order solutions, the second-order solutions and the third-order asymptotic solutions for the interfacial internal waves are obtained in the Lagrangian description using the perturbation method, and the mass transport velocity, the wave frequency, the mean level and the particle trajectory are also given. The results show that the discontinuities across the interfaces appear for the mass transport velocity, wave frequency and mean level, but we find that these discontinuities may disappear if the water depth ratio and the density ratio of the three layer fluids satisfy certain conditions.

Sedimentary geochemical proxies for methane seepage at Site C14 in the Qiongdongnan Basin in the northern South China Sea Abstract Recent studies have shown that specific geochemical characteristics of sediments can be used to reconstruct past methane seepage events. In this work, the correlation between the Sr/Ca and Mg/Ca ratios of sediment samples is analyzed and the sulfate concentration profile in Site C14 from cold-seep sediments in the Qiongdongnan Basin in northern South China Sea is obtained. The results confirmed that, sulfate at 0-247 cm below sea floor (Unit I) is mainly consumed by organic matter sulfate reduction (OSR), while sulfate at 247-655 cm (Unit II) is consumed by both the OSR and the anaerobic oxidation of methane (AOM). In addition, the bottom sediment layer is affected by weak methane seepage. The Mo and U enrichment factors also exhibit similar trends in their respective depth profiles. The responses of trace elements, including Co/Al, Ni/Al, Cr/Al and Zn/Al ratios to methane seepage allowed the study of depositional conditions and methane seepage events. Based on the results, it is speculated that the depositional conditions of Unit II changed with depth from moderate conditions of sulfidic and oxic conditions to locally anoxic conditions, and finally to suboxic conditions due to methane fluid leakage. The stable isotope values of chromium-reducible sulfide produced by AOM and those of sulfide formed by OSR in the early diagenetic environment suffered serious depletion of 34S. This was probably due to weak methane leakage, which caused the slower upward diffusion and the effect of early diagenesis on the samples. It is necessary to consider the effects of depositional environments and diagenesis on these geochemical parameters.

Wave prediction in a port using a fully nonlinear Boussinesq wave model Abstract A wave forecasting system using FUNWAVE-TVD which is based on the fully nonlinear Boussinesq equations by Chen (2006) was developed to provide an accurate wave prediction in the Port of Busan, South Korea. This system is linked to the Korea Operational Oceanographic System (KOOS) developed by Park et al. (2015). The computational domain covers a region of 9.6 km×7.0 km with a grid size of 2 m in both directions, which is sufficient to resolve short waves and dominant sea states. The total number of grid points exceeds 16 millions, making the model computational expensive. To provide real-time forecasting, an interpolation method, which is based on pre-calculated results of FUNWAVE-TVD and SWAN forecasting results at the FUNWAVE-TVD offshore boundary, was used. A total of 45 cases were pre-calculated, which took 71 days on 924 computational cores of a Linux cluster system. Wind wave generation and propagation from the deep water were computed using the SWAN in KOOS. SWAN results provided a boundary condition for the FUNWAVE-TVD forecasting system. To verify the model, wave observations were conducted at three locations inside the port in a time period of more than 7 months. A model/model comparison between FUNWAVE-TVD and SWAN was also carried out. It is found that, FUNWAVE-TVD improves the forecasting results significantly compared to SWAN which underestimates wave heights in sheltered areas due to incorrect physical mechanism of wave diffraction, as well as large wave heights caused by wave reflections inside the port.

Enhancing the observing capacity for the surface ocean by the use of Volunteer Observing Ship Abstract Knowledge of the surface ocean dynamics and the underlying controlling mechanisms is critical to understand the natural variability of the ocean and to predict its future response to climate change. In this paper, we highlight the potential use of Volunteer Observing Ship (VOS), as carrier for automatic underway measuring system and as platform for sample collection, to enhance the observing capacity for the surface ocean. We review the concept, history, present status and future development of the VOS-based in situ surface ocean observation. The successes of various VOS projects demonstrate that, along with the rapid advancing sensor techniques, VOS is able to improve the temporal resolution and spatial coverage of the surface ocean observation in a highly cost-effective manner. A sustained and efficient marine monitoring system in the future should integrate the advantages of various observing platforms including VOS.

Investigation of the oil-seawater mixed flow under an electromagnetic field Abstract The electromagnetic separation method is a new approach to treat ship-based marine oily wastewater, in which oil droplets are dispersed in seawater (oil-seawater mixed flow). In order to clarify the separation process and determine the separation characteristics, the flow field and volume fraction of the oil droplets of the oil-seawater mixed flow under an applied electromagnetic field with different operating conditions were investigated by 2D numerical simulations with the Eulerian model. The results show that: (1) the downward Lorentz force causes seawater to flow downwards and the oil droplets to move upwards due to the electromagnetic separation force in the effective section of the separation channel; (2) the volume fraction of the oil droplets at the top of the outlet section increases with the current density, magnetic field, and the diameter of the oil droplet and decreases with the inlet velocity of the oily seawater. The results provide useful guidance for the design of electromagnetic separation devices of the oil-seawater mixed flow.

Sensitivity of WRF simulated typhoon track and intensity over the South China Sea to horizontal and vertical resolutions Abstract To determine the grid resolutions of the WRF model in the typhoon simulation, some sensitivity analysis of horizontal and vertical resolutions in different conditions has been carried out. Different horizontal resolutions (5, 10, 20, 30 km), nesting grids (15 and 5 km), different vertical resolutions (35-layers, 28-layers, 20-layers) and different top maximum pressures (1 000, 2 000, 3 500, 5 000 Pa) had been used in the mesoscale numerical model WRF to simulate the Typhoon Kai-tak. The simulation results of typhoon track, wind speed and sea level pressure at different horizontal and vertical resolutions have been compared and analyzed. The horizontal and vertical resolutions of the model have limited effect on the simulation effect of the typhoon track. Different horizontal and vertical resolutions have obvious effects on typhoon strength (defined by wind speed) and intensity (defined by sea level pressure, SLP), especially for sea level pressure. The typhoon intensity simulated by the high-resolution model is closer to the real situation and the nesting grids can improve computational accuracy and efficiency. The simulation results affected by vertical resolution using 35-layers is better than the simulation results using 20-layers and 28-layers simulations. Through comparison and analysis, the horizontal and vertical resolutions of WRF model are finally determined as follows: the two-way nesting grid of 15 and 5 km is comprehensively determined, and the vertical layers is 35-layers, the top maximum pressure is 2 000 Pa.