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11th Warnemunde Turbulence Days on Linking Ocean Mixing and Circulation at Various Scales What: Forty-five participants from 10 countries met to discuss contemporary issues of marine turbulence with a focus on the linkage between mixing and overturning circulation on all scales (https://www.io-warnemuende.de/wtd-2023.html). When: 17-20 September 2023 Where: Rostock, Germany KEYWORDS: Eddies; Inertia-gravity waves; Meridional overturning circulation; Mixing; Ocean circulation; Turbulence, 1. Introduction Walter Munk, in his famous abyssal recipes, showed more than half a century ago that the strength of the global overturning circulation is closely linked to diapycnal mixing. [...]

When flow passes over topographic features such as headlands and islands, island wakes can arise at the lee side of the flow. Island wakes are associated with enhanced biological productivity, increased mixing, and water mass transformation. While previous studies have mainly focused on the dynamical and biological effects of island wakes in the open ocean, here we focus on a large tidally‐dominated estuary with numerous islands, aiming to investigate the impact of such wakes on the offshore transport of river plumes. To this end, we use numerical simulations with unprecedented grid resolution in the plume region and around the islands. Our study area is the Pearl River Estuary, a region where satellite images indicate that oscillating wakes occur in the lee and far downstream of the islands. We show that submesoscale island wakes are ubiquitous in the plume‐influenced region and can affect a large area around the islands as the tidal flow reverses. These strong vorticity tails correspond well with the horizontal patterns of salinity gradients and salinity mixing. Sensitivity experiments show that these flow disturbances will largely decrease after the hypothetical "removal" of the islands. Analysis based on an isohaline coordinate framework shows that the isohaline surface area is limited by the presence of islands. It is proven that this "limiting" effect of islands on the plume extension is related to the salinity mixing and the associated diahaline water exchange. Plain Language Summary: It matters how far the fresh water from the river can reach because it usually carries nutrients and sediments into the ocean. When this thin layer of fresh water encounters islands in the coastal area, water exchange between fresher and saltier water will occur. Here, we use a numerical model in the Pearl River Estuary to test the effects of islands on the rate of water exchange and the extent of freshwater coverage. We found that the extent of freshwater coverage is limited due to the presence of islands. This is because the islands accelerate the water exchange not only in the neighboring water around the islands but also in the island wakes, which extend several island diameters away from the islands. In an alternative numerical experiment in which all the small islands are "removed," the flow disturbances will be largely decreased. Since the total estuarine mixing is controlled by the upstream runoff, the area covered by the isohaline surface needs to be increased to maintain the same amount of the total mixing in the "No Island" case. Key Points: High‐resolution simulations indicate that mixing around islands in river plumes is limiting offshore freshwater transportA large part of the mixing is related to submesoscale island wakesIsohaline analysis suggests that after the removal of the islands, the same amount of mixing will be redistributed on the isohaline surface [ABSTRACT FROM AUTHOR]

Salt mixing enables the transport of water between the inflow and outflow layers of estuarine circulation and therefore closes the circulation by driving a diahaline exchange flow. A recently derived universal law links the salt mixing inside an estuarine volume bounded by an isohaline surface to freshwater discharge: it states that on long-term average, the area-integrated mixing across the bounding isohaline is directly proportional to the freshwater discharge entering the estuary. However, even though numerous studies predict that periods of extreme discharge will become more frequent with climate change, the direct impact of such periods on estuarine mixing and circulation has yet to be investigated. Therefore, this numerical modeling study focuses on salinity mixing and diahaline exchange flows during a low-discharge and an extreme high-discharge period. To this end, we apply a realistic numerical setup of the Elbe estuary in northern Germany, using curvilinear coordinates that follow the navigational channel. This is the first time the direct relationship between diahaline exchange flow and salt mixing as well as the spatial distribution of the diahaline exchange flow is shown in a realistic tidal setup. The spatial distribution is highly correlated with the local mixing gradient for salinity, such that inflow occurs near the bottom at the upstream end of the isohaline. Meanwhile, outflow occurs near the surface at its downstream end. Last, increased vertical stratification occurs within the estuary during the high-discharge period, while estuarine-wide mixing strongly converges to the universal law for averaging periods of the discharge event time scale. Significance Statement: Inside estuaries, such as river mouths, terrestrial freshwater is mixed with salty ocean water. This is accompanied by an estuarine circulation with inflow of saltwater into the estuary and outflow of brackish water toward the ocean. Here, we aim to better understand how salt mixing and estuarine circulation in a tidal estuary react to periods of extreme freshwater discharge. We find that even during extremely high or low discharge, salt mixing follows the freshwater discharge on time scales as short as days, and that estuarine circulation patterns are largely explained by the local distribution of mixing. As extreme runoff events are likely to occur more often with climate change, these findings may help to understand the dynamics inside future estuaries. [ABSTRACT FROM AUTHOR]