Impact of shelf valleys on the spread of surface-trapped river plumes

Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 51(1), (2021): 247-266, https://doi.org/10.1175/JPO-D-20-0098.1.This study focuses on mechanisms of shelf valley bathymetry affecting the spread of riverine freshwater in the nearshore region. In the context of Changjiang River, a numerical model is used with different no-tide idealized configurations to simulate development of unforced river plumes over a sloping bottom, with and without a shelf valley off the estuary mouth. All simulated freshwater plumes are surface-trapped with continuously growing bulges near the estuary mouth and narrow coastal currents downstream. The simulations indicate that a shelf valley tends to compress the bulge along the direction of the valley long axis and modify the incident angle of the bulge flow impinging toward the coast, which then affects the strength of the coastal current. The bulge compression results from geostrophic adjustment and isobath-following tendency of the depth-averaged flow in the bulge region. Generally, the resulting change in the direction of the bulge impinging flow enhances down-shelf momentum advection and freshwater delivery into the coastal current. Sensitivity simulations with altered river discharges Q, Coriolis parameter, shelf bottom slope, valley geometry, and ambient stratification show that enhancement of down-shelf freshwater transport in the coastal current, ΔQc, increases with increasing valley depth within the bulge region and decreasing slope Burger number of the ambient shelf. Assuming potential vorticity conservation, a scaling formula of ΔQc/Q is developed, and it agrees well with results of the sensitivity simulations. Mechanisms of valley influences on unforced river plumes revealed here will help future studies of topographic influence on river plumes under more realistic conditions.This work is conducted by Canbo Xiao and Weifeng (Gordon) Zhang during CX’s one-year visit at Woods Hole Oceanographic Institution (WHOI) in 2018–19. CX was supported by China Scholarship Council

The Mixed Layer Salinity Budget in the Northern South China Sea: A Modeling Study

The seasonal variation in mixed layer salinity (MLS) plays a crucial role in global ocean circulation and hydrological cycle. The salinity budget of the mixed layer is important to understand the mechanism of the variation, but in the South China Sea (SCS), the details in the budget are missing due to insufficient observations. Here, we employed an eddy-resolving (horizontal grid resolution ~10 km) SCS circulation model to quantify the key physical processes in the seasonal cycling of MLS in the northern South China Sea (NSCS). Built on the success of the realistic numerical simulation for 2008–2018, the model reproduced the primary features of the observed seasonal MLS, wherein fresher waters are present in the region during the summer monsoon and salty waters appear along the slope during the winter monsoon. According to the salinity budget that was calculated during model execution, the term for air–sea freshwater flux and meridional advection represent the primary freshwater input in the summer and winter, respectively, while vertical processes including vertical mixing and entrainment form the major balancing terms in the budget. In different regions of the NSCS, vertical mixing can play a dominant role in the vertical processes, but the associated seasonality is different for regions of strong internal wave influence and regions of strong horizontal advection influence. In the winter, the intrusion and spreading of western Pacific water over the NSCS could modify the MLS structure and cause larger vertical entrainment than mixing in regions where the effect of mixing decreases with the slackening of the seasonal internal wave activities. Overall, the analysis of the ML salinity budget reveals that vertical mixing, together with vertical entrainment, is vital to maintaining the seasonal variation in MLS of the NSCS

Review of Artificial Downwelling for Mitigating Hypoxia in Coastal Waters

Hypoxia is becoming a serious problem in coastal waters in many parts of the world. Artificial downwelling, which is one of the geoengineering-based adaptation options, was suggested as an effective means of mitigating hypoxia in coastal waters. Artificial downwelling powered by green energy, such as solar, wind, wave, or tidal energy, can develop a compensatory downward flow on a kilometer scale, which favors below-pycnocline ventilation and thus mitigates hypoxia in bottom water. In this paper, we review and assess the technical, numerical, and experimental aspects of artificial downwelling all over the world, as well as its potential environmental effects. Some basic principles are presented, and assessment and advice are provided for each category. Some suggestions for further field-based research on artificial downwelling, especially for long-term field research, are also given

Framework to Extract Extreme Phytoplankton Bloom Events with Remote Sensing Datasets: A Case Study

The chlorophyll-a concentration (CHL) is an essential climate variable. Extremes of CHL events directly reflect the condition of marine ecosystems. Here, we applied the statistical framework for defining marine heatwaves to study the extremes of winter CHL blooms off the Luzon Strait (termed as LZB), northeastern South China Sea (SCS), from a set of remote sensing data. The application was enabled by a recent gap-free CHL dataset, the SCSDCT data. We present the basic properties and the long-term trends of these LZB events, which had become fewer but stronger in recent years. We further statistically analyze the LZB events’ controlling factors, including the submesoscale activity quantified by a heterogeneous index or surface temperature gradients. It was revealed that the submesoscale activity was also a vital modulating factor of the bloom events in addition to the well-understood wind and upwelling controls. This modulation can be explained by the stratification introduced by submesoscale mixed-layer instabilities. In the winter, the intensified winter monsoon provides a background front and well-mixed upper layer with replenished nutrients. During the wind relaxation, submesoscale baroclinic instabilities developed, leading to rapid stratification and scattered submesoscale fronts. Such a scenario is favorable for the winter blooms. For the first time, this study identifies the bloom events in a typical marginal sea and highlights the linkage between these events and submesoscale activity. Furthermore, the method used to identify extreme blooms opens up the possibility for understanding trends of multiple marine extreme events under climate change

Experimental Study on the Effects of a Vertical Jet Impinging on Soft Bottom Sediments

Artificial downwelling, which is an ecological engineering method, potentially alleviates bottom hypoxia by bringing oxygen-rich surface water down below the pycnocline. However, the downward flow is likely to disturb sediments (or induce sediment resuspension) when reaching the bottom and then have unwanted side effects on the local ecosystem. To evaluate this, our paper presents a theoretical model and experimental data for the sediment resuspension caused by artificial downwelling. The theoretical model considers the critical conditions for sediment resuspension and the scour volume with the downwelling flow disturbing sediment. Experiments with altered downwelling flow speeds, discharge positions relative to the bottom, and particle sizes of sediment were conducted in a water tank, and the results were consistent with our theoretical model. The results show that the critical Froude number (hereinafter Fr) for sediment resuspension is 0.5. The prevention of sediment resuspension requires the downwelling flow speed and the discharge position to be adjusted so that Fr < 0.5; otherwise a portion of sediment is released into the water and its volume can be predicted by the derived formulation based on the Shields theory. Furthermore, sediment resuspension has side effects, such as a water turbidity increase and phosphorus release, the magnitudes of which are discussed with respect to engineering parameters. Further study will focus on field experiments of artificial downwelling and its environmental impacts

Nutrient Removal from Chinese Coastal Waters by Large-Scale Seaweed Aquaculture Using Artificial Upwelling

Ecological engineering by artificial upwelling for enhancing seaweed growth and consequently increasing nutrient removal from seawater has proved promising in combating intense coastal eutrophication. However, a key issue needs to be answered: how much economic and ecological benefit could this engineering bring if it were to be implemented in national aquaculture areas. This study estimated the promoting effect of nutrient concentration change induced by artificial upwelling on kelp growth using a model simulation based on the temperature, light intensity, and nutrient concentration data from three bays in Shandong Province, China— Aoshan Bay, Jiaozhou Bay, and Sanggou Bay. Our results indicate that ecological engineering by artificial upwelling can increase the average yield of kelp by 55 g per plant. Furthermore, based on the current existing kelp aquaculture area of China and the aquaculture density of 12 plants/m2, we inferred that this ecological engineering could increase the natural kelp yield by 291,956 t and the removal of nitrogen (N) and phosphorus (P) nutrients by 4875–6422 t and 730–1080 t, respectively

Numerical Studies on the Suitable Position of Artificial Upwelling in a Semi-Enclosed Bay

Ecological engineering by artificial upwelling is considered a promising way to improve water quality. Artificial upwelling could lift nutrient-rich bottom water to the surface, enhance seaweed growth and consequently increase nutrient removal from seawater. However, one of the major obstacles of the engineering application is to determine the suitable position of ecological engineering, which is critical for artificial upwelling’s performance. In this paper, potential artificial upwelling positions in a semi-closed bay are simulated by using the unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM). The results show that the upwelling position with relative small tidal current and close to corner will be helpful to increasing nutrient concentration of surface water, and be appropriate to build the ecological engineering. With proper design of the ecological engineering, it is possible to have a noticeable impact in semi-closed bay. Thus, artificial upwelling has the potential to succeed as a promising way to alleviate the eutrophication