Ocean Dynamics最新文献

An adjoint-free four-dimensional variational (a4dVar) data assimilation (DA) is implemented in an operational ocean forecast system based on an eddy-resolving ocean general circulation model for the Northwestern Pacific. Validation of the system against independent observations demonstrates that fitting the model to time-dependent satellite altimetry during a 10-day DA window leads to substantial skill improvements in the succeeding 60-day hindcast. The a4dVar corrects representation of the Kuroshio path variation south of Japan by adjusting the dynamical balance between amplitude/wavelength of the meander and flow advection. A larger ensemble spread tends to reduce the skill in representing the observed sea surface height anomaly, suggesting that it is possible to use the ensemble information for quantifying the forecast error. The ensemble information is also utilized for modification of the background error covariance (BEC), which improves the accuracy of temperature and salinity distributions. The modified BEC yields the skill decline of the Kuroshio path variation during the 60-day hindcast period, and the ensemble sensitivity analysis shows that changes in the dynamical balance caused by the ensemble BEC result in such skill deterioration.

Natural modes characterized by amplification diagrams with extremely narrow peaks are frequently observed in harbor oscillations, referred to as extreme modes. This study presents a detailed numerical investigation into extreme modes. A new definition of extreme modes is established based on the modal structure instead of solely relying on the frequency–response diagram. Extreme modes arise when they primarily oscillate between the harbor walls, and one or more nodal lines intersect the harbor entrance. Subsequently, the variations exhibited by the extreme mode in response to varying geometry and wave parameters are explored, with specific consideration given to the influences of wave direction, partial reflection, and topography. The creation of an extra entrance in the oscillatory direction of the extreme mode may cause it to disappear. Moreover, the extreme mode can be significantly mitigated by slightly reducing the wave reflection between harbor walls. Changing the entrance position can adjust the modal structure and prevent nodal lines from crossing the entrance, thereby averting excessively high amplification factors. Under symmetrical conditions, extreme modes may not be activated, but slight asymmetries in wave direction, topography, or harbor layout can trigger them. The extreme modes exhibit heightened sensitivity to variations in entrance width and length compared to ordinary modes. A reduction in entrance width or an extension of the entrance length can notably intensify the extreme mode.

The present study investigates the mixed layer variations near mesoscale eddies in the Bay of Bengal (BoB) using satellite altimeter and Argo data. Furthermore, the factors responsible for sea surface variations near mesoscale eddies are analyzed using the mixed layer heat and salinity budgets estimated from Argo profiles. In the diagnostic mixed layer heat budget analysis, the entrainment term is parametrized based on the presence and absence of the barrier layer. The role of inversion and barrier layers on eddy-induced temperature variations is also examined near eddy locations. Results showed that anti-cyclonic eddies deepen mixed layer depth (MLD) and barrier layer thickness (BLT). Whereas, near cyclonic eddies shallower MLD and BLT is evident. However, MLD and BLT variations near mesoscale eddies are prominent during monsoon and winter seasons, respectively. Heat budget analysis near eddy locations depicts that surface heat fluxes and vertical entrainment are the primary factors responsible for temperature variations near mesoscale eddies. Similarly, the salinity budget analysis near eddy locations reveals that horizontal advection (stirring effect) is the predominant processes responsible for the salinity variations. The outcome of the present study is believed to be useful in validating and improving the eddy-resolving ocean models.

Landfast ice is near-motionless sea ice attached to the coast. Despite its potential for modifying sea ice and ocean properties, most state-of-the-art sea ice models poorly represent landfast ice. Here, we examine two crucial processes responsible for the formation and stabilization of landfast ice, namely sea ice tensile strength and seabed–ice keel interactions. We investigate the impact of these processes on the Arctic sea ice cover and halocline layer using the global coupled ocean–sea ice model NEMO-LIM3. We show that including seabed–ice keel stress improves the seasonality and spatial distribution of the landfast ice cover in the Laptev and East Siberian Seas. This improved landfast ice representation sets the position of flaw polynyas to new locations approximately above the continental shelf break. The impact of sea ice tensile strength on the stability of the Arctic halocline layer is far more effective. Incorporating this process in the model yields a thicker, more consolidated, and less mobile Arctic sea ice pack that further decouples the ocean and atmosphere. As a result, the available potential energy of the Arctic halocline is decreased (increased) by (sim )30kJ/m(^2) ((sim )30kJ/m(^2)) in the Amerasian (Eurasian) compared to the reference simulation excluding sea ice tensile strength and seabed–ice keel stress. Our findings highlight the need to better understand landfast ice physical processes conjointly with the subsequent influences on the ocean and sea ice states.

Coastal flooding within Great Lakes communities poses severe threats to ecosystem and economic sustainability. Accurate and efficient flood predictions could provide critical advanced warnings and improve local resilience. Three types of modeling approaches, including the Bathtub Method (BTM), Extended Hydrodynamic model (EXT), and Total Water Level (TWL) approach, were evaluated for a flood event in the Great Lakes. These studied modeling approaches have successfully replicated water levels at four nearshore gauge stations in the lake, indicating a reliable starting point for coastal flood simulations. Comparisons were made between simulations of maximum flood extent using different methods in three typical high flooding risk areas, including an open-bay area, along coasts of drowned-river-mouth (estuaries) lakes, and a section of shoreline with heavy infrastructural facilities. In addition, aerial photos from news reports and synthetic aperture radar (SAR) data were analyzed in this study to provide observed information for the studied flooding events. According to the results, BTM and EXT were consistent in simulating flood extents for various types of coastal areas, while the TWL was limited in predicting flood propagation into inland areas, particularly in the coasts of river-mouth lakes. Despite slightly overestimating the flood extent in disconnected low-lying areas, the BTM can still serve as a cost-effective tool to provide preliminary flood simulations for the Great Lakes region. We further discuss operational perspectives of using BTM, EXT, and TWL for coastal flood modeling. The results of this study could be used to improve the guidance of coastal management by determining efficient and accurate approaches for coastal flood predictions.

Abstract

Atmospheric cold fronts quasi-periodically produce storm surges and generate significant subtidal oscillations of water levels and water transport in coastal environments. Yet, it is unclear how these weather events—at regional scales —control the hydrodynamics in delta-dominated coasts. Here, we used a numerical model (SCHISM) to simulate the inundation/drying and water circulations generated by varying winds associated with cold front passages across the Atchafalaya Bay in the northern Gulf of Mexico, specifically the Wax Lake Delta (WLD) region. Water transport induced by winds through major channels of the WLD and that between the adjacent Vermilion Bay and inner shelf were quantified to evaluate the impact of cold fronts. Results show that significant wetting/drying conditions are highly correlated with wind direction and strength. Southerly/easterly winds tend to cause water set-up, thus inundating the delta region, while northerly and westerly winds cause water set-down, draining the bay into the continental shelf. As a result, up to 60% of the delta area (~ 50 km²) can become exposed land under northerly winds. The interconnectivity of the delta channel system is also highly dependent on the wind direction and magnitude: up to 37% of the total transport is through the shallow waters outside of the channels during cold fronts. In contrast, the water levels and velocity variations in the delta region were negatively correlated with the alongshore wind, a result of Ekman transport. At the delta head, where freshwater flows like a strong jet into the region, the velocity is marginally correlated with the wind but mostly correlated with seasonal river discharge variability of river discharge. At the transitional zone between the bay and coastal ocean, water level and surface flows are dominated by tidal forcing in contrast to the delta lobes area where wind regulates the flooding area extension. The bottom velocity and water levels at sites along the Atchafalaya Bay mouth negatively correlate with onshore wind, indicating bottom return flows against the wind. Our study offers a glimpse of how a combination of atmospheric and hydrodynamic forces operate in a young delta, where the coast is experiencing the highest sea level rise in North America.

In this research, we utilize AVISO altimetry data, the GLORYS12V1 product, and the META3.2 DT Atlas to investigate the Benguela upwelling. By combining these three datasets, we explore the propagation of mesoscale eddies generated within the upwelling zone and examine the dispersion of particles originating from the upwelling zone. The geographical scope of our analysis is confined to the region between 10–36°S and 0–20°E. We employ Lagrangian analysis and the AMEDA approach to study the eddies formed in the upwelling zone. The diverse methods applied enable us to track the movement of upwelling fluid elements in the specified area. The identification of the upwelling zone relies on temperature and salinity gradients in the coastal region. The primary focus of this study revolves around mesoscale eddies emerging in the upwelling zone. We scrutinize the trajectories of cyclones and anticyclones as they propagate westward from the upwelling zone, highlighting variations in the number of upwelling-origin particles within these eddies. We observe distinctions in the locations of upwelling cells between cyclones and anticyclones. Our results indicate that among mesoscale eddies generated in the upwelling zone cyclones predominate. We show that Lagrangian particles, leaving the upwelling zone, propagate throughout the area under consideration. For these particles, we can determine the travel time from the upwelling zone from 1 to 365 days and distances of 500 km for cyclones and 300 km for anticyclones. We found that cyclones are more stable structures with a longer lifetime and with a longer distance traveled in contrast to anticyclones. We believe this is a distinctive feature of the eddies with upwelling origins in comparison with other mesoscale eddies in the area. Finally, we analyze the change of water properties inside the eddies after they leave the upwelling zone and show a significant renewal of vortex cores occurring after 1–2 months of their life.

Abstract

The Odra estuary in the southern Baltic Sea comprises the Odra (Szczecin) Lagoon, the Pomeranian Bay and a number of other shallow water areas and channels. Known for its abundance of fish, eutrophication in the Odra Lagoon is a pressing issue for science and environmental management representing a global problem: What determines the seasonal variability of nitrogen and nitrogen turnover in shallow water areas, and how does seasonal variability change due to climatic changes such as warming and sea level rise? How do such changes affect nutrient exports to the regional ocean? This study employs a high-resolution unstructured model system to investigate physical-biogeochemical interactions, nitrogen turnover, and conditions leading to nitrogen export to the Baltic Sea within the Odra estuary. The research comprises hindcast and a climatic experiment with modified water level and temperature inputs. The model reproduces the thermohaline dynamics of brackish shallow water areas, phytoplankton blooms and the variability of inorganic nitrogen. The simulations identify the dynamic partitioning of the Odra Lagoon into the highly eutrophic, lake-like Small Lagoon and more frequently flushed, zooplankton-rich Great Lagoon. Although the two years of the hindcast simulation feature very different boundary conditions in terms of river forcing, comparable patterns of seasonal nitrogen export emerge. In a climate change experiment with increased sea levels and global temperatures, the system appears sensitive, but remains stable with regard to nutrient transport and is therefore predictable. The climate change experiment reveals enhanced primary producer biomass concentrations, suggesting heightened eutrophication. While in the shallow waters of Odra Lagoon oxygen concentration remains relatively stable, oxygen depletion intensifies as the lagoon outflows enter the Pomeranian Bay. This phenomenon is linked to increased denitrification within the stratified Odra plume. Deeper, meandering channels, such as Swina, demonstrate resilience to oxygen reduction, influenced by sea level rise and enhanced currents. Based on the temporal-spatial high-resolution coupled, validated simulations, it is possible to develop tailor-made management solutions without having to run expensive and complicated observation campaigns in the shallow waters with complex topography.

The Amazon Continental Shelf (ACS) is a shallow region (< 100 m), with a maximum width of 330 km, which encloses the northern portion of the Brazilian continental shelf and has great ecological and climatic importance on a global scale. Although important scientific efforts have been made to understand the hydrodynamics of the ACS and the dispersion of the Amazon River plume, there are still few studies that address surface oceanic water intrusion into the ACS. This study describes the existence of preferential surface oceanic water intrusion pathways into the ACS along 3 sectors: Maranhão (MA shelf), Pará (PA shelf) and Amapá (AP shelf). The analysis is based on: (i) 306 surface drifter trajectories along 1344 km of the ACS (provided by the Global Drifter Program) and (ii) 20 years of Lagrangian simulations (with Parcels model forced by currents from the reanalysis GLORYS). The results show that the MA shelf sector is the main pathway for surface oceanic water intrusions into the ACS, corresponding to 56% of the intrusions, followed by PA shelf (43%) and AP shelf (1%). During the austral summer, intrusions occur with a higher frequency in PA and AP shelf. The MA shelf shows weak seasonality in the intrusions. The temporal variability of particle intrusion rates into the ACS is directly related to the variability of the trade winds, and the meso-scale circulation associated with the North Brazil Current and the North Equatorial Countercurrent.

The bidirectional mass exchange between the Marmara Sea and the Aegean Sea provides one part of the critical hydrodynamic links between the Black Sea and the Mediterranean Sea. In this study, we examined exchange in the Dardanelles based on a 3-D numerical model simulation covering an 11-year period under realistic atmospheric forcing. The model includes the Black Sea, the Marmara Sea, and a part of the Aegean Sea to include the remote effects of basin dynamics. The main features as one-, two-, and three-layered flow structures are successfully reproduced by the model in comparison to earlier observations. It is found that the strait is subject to submaximal exchange by only one control near the Nara Pass. According to long-term modeling results, most variability occurs on synoptic time scales, and wind stress has a dominant role in those variations. The seasonal and interannual variability of exchange flow is relatively low and displays a close relationship with freshwater input to the Black Sea.