Ocean Dynamics最新文献

The strongly nonlinear Miyata-Choi-Camassa model, a two-layer internal-wave model that includes the free-surface effect (MCC-FS model), has shown excellent performance on simulating large-amplitude internal waves. In this study, we are interested in assessing whether the MCC-FS model can be applied to study problems involving surface waves, and how the model performs. For this purpose, we apply the two-layer MCC-FS model to a time-varying bottom to simulate surface water-waves by setting the same densities of the upper- and lower-fluid layers. Although densities of the upper and lower fluid layers are the same, it is found that the depth ratio of the two layers plays a remarkable role in the solution. By analyzing the linear dispersion relations of the MCC-FS model with three different depth ratios ((varvec{h}_{varvec{1}}/varvec{h}_{varvec{2}}=varvec{1}/varvec{9}), (varvec{3}/varvec{7}) and (varvec{5}/varvec{5}), where (varvec{h}_{varvec{1}}) and (varvec{h}_{varvec{2}}) are the depths of the upper- and lower-fluid layers, respectively), we determine that the MCC-FS model with the depth ratio (varvec{3}/varvec{7}) has the better performance on simulating surface water-waves. Under this configuration, we apply the MCC-FS model to simulate the surface solitary waves on a flat bottom, the solitary wave propagating over a submerged shelf and the surface water-waves generated by a bottom disturbance. By comparing with the laboratory measurements, the accuracy of the results provided by the MCC-FS model is validated.

Submesoscale processes (SMPs) play profound roles in energy cascade, air-sea heat flux, and marine ecosystem, the variability of which significantly regulates regional and global climate. The southeastern tropical Indian Ocean (SETIO) has abundant and complex dynamic processes, yet the dynamics of SMPs in this region remain unclear. Based on the outputs of two high-resolution models, the seasonality and potential mechanisms of SMPs in the SETIO, as well as accompanying submesoscale vertical heat transport (SVHT) are investigated in this study. The SMPs and SVHT are much stronger during the southeast monsoon period (June-October). Mixed layer instability (MLI) dominates the generation of SMPs, while frontogenesis only plays a minor role. The enhanced horizontal density gradients partly resulting from strengthened large-mesoscale flow strain, coupled with a deeper mixed layer induced by surface cooling and strong southeast monsoon, account for the stronger MLI in the southeast monsoon period. Besides, symmetric instability (SI) also contributes to the generation of SMPs by extracting kinetic energy from the geostrophic flows. Upward SVHT in medium- and high resolution ROMS simulations surpasses that in low-resolution ROMS simulation by a factor of three during the austral winter and is significantly stronger than mesoscale vertical heat transport. These results confirm the importance of SMPs in upper-layer vertical heat transport, and SMPs resolving models can represent the vertical heat transport much better. Our findings could deepen our understanding on multiscale dynamic processes and vertical heat transport in the SETIO.

The paper presents the reconstruction of sea levels in the North Sea and Baltic Sea using Kalman filter approach. Based on the statistical characteristics of one year of daily maps of sea level from the Geesthacht COAstal model SysTem (GCOAST) and daily data at tide gauges along the coastline of two basins, the method can reconstruct effectively and accurately the multidecadal sea level anomalies. The high accuracy reconstruction data were then used to investigate the interannual variability in both basins and to estimate the difference between outflows and inflows (net flux) through the Danish Straits. The highest mean, standard deviation, and extreme values of sea level anomalies appear in winter and are well reproduced in different regions, such as the German Bight, the Southern North Sea, the Bothnian Bay, the Gulfs of Finland and Riga. The sea level variability is highly correlated with the mean sea level pressure and the zonal wind, particularly in the German Bight and in the winter months. The contributions of river runoff and net precipitation on the net flux are significant in the spring. The local wind has a greater influence on the net flux than the remote drivers.

Tidal asymmetry in estuaries and other tidally dominated coastal systems is commonly evaluated to assess system states or their development. Based on different methods, local states are classified as either flood or ebb dominant. An increasing number of descriptors for deriving tidal asymmetry in recent years calls for a comparison and discussion of their sensitivity on input data and its quality. We compared tidal asymmetry from water level and current velocity using various descriptors that deduce from harmonic, ratio, and skewness methods. Computed from one-year measurements at different stations along the Ems estuary, their comparability was enabled by a new approach of scaling. Our results on the variation of sampling intervals demonstrated a highly site-specific sensitivity of the descriptors that led up to changes in the asymmetry direction in tidal duration asymmetry and phase lag. The slack water asymmetry appeared most sensitive to the studied parameter settings. As expected, variability of tidal asymmetry reduced with an increasing number of analyzed tides. At the same time, uncertainty from the asymmetry during spring or neap phases compared to spring-neap periods remained in all analyzed descriptors. Hence, the characterization of the estuary in terms of flood- or ebb-dominance depends critically on the quality and extent of the input data. For all parameter settings, the impact of river discharge on tidal asymmetry was pronounced but varied depending on the location in the estuary. The actual characterization of the effect of asymmetry, e.g., on sediment transport, is not conducted in this study. We propose that this requires a more comprehensive dataset, such as depth and cross-sectional variability of currents and sediment concentrations.

Wind gusts play an important role in the modulation of wind stress and have remarkable influence on surface waves. The response of wind stress and surface wave to wind gusts is investigated under wind-wave and swell dominated conditions through direct flux measurements on two coastal towers in the Bohai Sea and South China Sea. Observations show that wind stress significantly increases under gusty conditions compared to non-gusty conditions whether dominated by wind-waves or swells. The stronger the gusts, the greater the increase in wind stress. However, surface waves show different responses to the gusts under wind-wave and swell dominated conditions. When wind-waves dominate, the stronger the gust, the higher the significant wave height, while when swells dominate, the relatively lower the significant wave height. The separation of wind-wave and swell spectra indicates gusts can increase the wind-wave energy and decrease the swell energy. The weakening of swell and the strengthening of wind-wave may lead to an increase in wind stress.

We have analyzed the relationship between wind variability and sea level anomalies (SLA) on the Southwestern Atlantic Continental Shelf, focusing on sub-annual temporal scales. For this, we tested the capability of gridded altimetry to represent wind-driven SLA and compared results using an oceanographic model and tide gauge data. The present study used coherence analysis to analyze frequencies for which SLA and wind stress are coherent. The altimetry-SLA were found to have less energy below the three-month period compared to the model SLA. The coherence of along-shore wind stress and altimetry SLA was only significant for > 50 days (d), while the model SLA showed significant agreement in all periods considered, 20 d to annual. We further showed that geostrophic velocities on the continental shelf agreed significantly with SLA for > 50 d. As a result of an Empirical Orthogonal Function (EOF) analysis, we found that the second mode is highly coherent with the along-shore wind stress and accounts for 18.1% and 10.7% of variability in the model and altimetry sea level anomalies, respectively.

The study analyzes the impact of various wave-induced processes on relative dispersion and diffusivities in the North Sea using OpenDrift, a Lagrangian particle-drift model driven by a fully coupled NEMO-WAM model. The coupled model parameterizations include sea state-dependent momentum flux, energy flux, and wave-induced mixing. The study demonstrates that Eulerian currents, influenced by the interaction between the ocean and wave models, significantly enhance particle transport. Experiments conducted using drifter clusters obtained during an RV Heincke excursion further confirm the impact of wind-wave coupling. The analysis includes a comparison of results from experiments with and without wave coupling. The impact of diffusion in the Lagrangian model on relative dispersion is investigated, with the conclusion that diffusion is essential for achieving precise simulations. Furthermore, the incorporation of wind-wave-driven mixing parameters, including sea state-dependent momentum flux, energy flux, and wave-induced mixing, into the hydrodynamic model leads to elevated levels of relative dispersion and diffusivity.

Motivated by the phenomenon of Scotian Shelf Crossover events, the problem of a shelf flow that is interrupted by a strait is considered. Laboratory experiments in a rotating tank with barotropic and baroclinic flow over flat and sloping shelves confirm that the flow is steered by the bathymetric contours and mainly circumnavigates the gulf. In order to jump across the strait, as suggested by earlier theories, the flow must have unrealistically high Rossby numbers. However, the near bottom friction relaxes the bathymetric constraint and causes the formation of a peculiar jet crossing the strait diagonally. For the dissipation values such that a half of the transport goes around the gulf and half crosses the strait diagonally, the diagonal crossover jet becomes most evident. Numerical solutions for realistic values of the frictional parameter reproduce the results of the laboratory experiments and consideration of the actual Gulf of Maine bathymetry reproduces patterns similar to those observed by drift trajectories and in the satellite derived sea surface temperature fields.

The anticyclonic eddies (ACEs) shed from the Kuroshio intrusion often interact with internal waves (IWs). Those interactions are complex but play an important role in the biogeochemical effects in the northern South China Sea (SCS). However, previous studies on these interactions have mainly focused on physical processes, while biological responses to their interactions have been unclear. In this study, results of two field cruises with focus on two sets of high-frequency time-series observations over the continental slope of the SCS during the summers of 2016 and 2017 revealed that phytoplankton in the euphotic zone were affected by both IWs and ACEs shed from the Kuroshio Current. Distinct surface distributions of phytoplankton were attributable to different sources of the water, Kuroshio intrusions and the plume from the Pearl River. However, similarities of phytoplankton biomass and community composition in the subsurface chlorophyll maximum layer in both years could be explained by the similar upward transport of nutrients induced by combinations of small-amplitude IWs and mature ACEs in 2016 and large-amplitude IWs in 2017. The distinct vertical distributions of the phytoplankton community in both years were attributable to the different responses of phytoplankton groups to the direct effects of isopycnal uplifting, bottom-up controls, and top-down controls. Our results showed that the ecological effects of the interactions between IWs and ACEs shed from the Kuroshio water were complex, and those complex effects further influenced the structure of this marginal sea ecosystem.

First-ever measurements of the turbulent kinetic energy (TKE) dissipation rate in the northeastern Strait of Magellan (Segunda Angostura region) taken in March 2019 are reported here. At the time of microstructure measurements, the magnitude of the reversing tidal current ranged between 0.8 and 1.2 ms⁻¹. The probability distribution of the TKE dissipation rate in the water interior above the bottom boundary layer was lognormal with a high median value ({varepsilon }_{med}^{MS}=1.2times {10}^{-6}) Wkg⁻¹. Strong vertical shear, (left(1-2right)times {10}^{-2}) s⁻¹, in the weakly stratified water interior ensued a sub-critical gradient Richardson number (Ri<{10}^{-1}-{10}^{-2}). In the bottom boundary layer (BBL), the vertical shear and the TKE dissipation rate both decreased exponentially with the distance from the seafloor (xi), leading to a turbulent regime with an eddy viscosity ({K}_{M}sim {10}^{-3}) m²/s, which varied with time and location, while being independent of the vertical coordinate in the upper part of BBL (for (xi >sim 2) meters above the bottom).