Search

Observations from the global ocean have long confirmed the ubiquity of thermohaline inversions in the upper ocean, often accompanied by a clear signal in biogeochemical properties. Their emergence has been linked to different processes such as double diffusion, mesoscale stirring, frontal subduction, and the recently discussed submesoscale features. This study uses the central Baltic Sea as a natural laboratory to explore the formation of salinity inversions in the thermocline region during summer. We use realistic high‐resolution simulations complemented by field observations to identify the dominant generation mechanism and potential hotspots of their emergence. We propose that the strongly stratified thermocline can host distinct salinity minima during summer conditions resulting primarily from the interaction between lateral surface salinity gradients and wind‐induced differential advection. Since this is a generic mechanism, such salinity inversions can likely constitute a typical feature of the upper ocean in regions with distinct thermoclines and shallow mixed layers. Plain Language Summary: The upper ocean is characterized by a well‐mixed surface layer, below which temperature decreases rapidly with depth, forming the so‐called thermocline region. A corresponding salinity increase with depth is typically anticipated for stable density stratification to occur. Temperature and salinity inversions can, however, emerge in the upper ocean. Such thermohaline inversions have been observed in different regions of the world's oceans, and various mechanisms have been proposed to explain their generation. Here, the central basin of the Baltic Sea is used as a natural laboratory to explore the formation of distinct salinity minima in the thermocline region during summer conditions. Using high‐resolution numerical simulations and measurements from a field campaign, we show that inversions are abundant and can emerge throughout the entire basin. They increase with increasing wind speeds and concentrate mainly in regions with strong lateral salinity differences. We propose that thermocline salinity minima can occur during summer when the wind transports saltier water over less saline surface waters. This is a generic mechanism that can therefore be responsible for the formation of the salinity inversions observed worldwide in areas with distinct thermoclines and shallow mixed layers. Key Points: Observations collected in the central Baltic Sea during summer indicate patches of distinct salinity minima in the thermocline regionRealistic high‐resolution simulations are used to explore the origin of the salinity minima and to identify the hotspots of their genesisLateral surface salinity gradients interacting with wind‐induced differential advection are shown to generate most of the inversions [ABSTRACT FROM AUTHOR]

Recently discovered, ocean submesoscales have attracted considerable attention due to their ability to change the upper ocean stratification, affect vertical transport, and induce a downscale cascade of energy toward dissipation. In this paper, we highlight the effect of submesoscale fronts and filaments on surface layer properties and dynamics during storm events, extending previous idealized simulations toward real‐ocean applications. Here, we use the Baltic Sea as a natural laboratory to study the rich submesoscale activity of this system with the help of realistic high‐resolution numerical simulations. These simulations reveal a wealth of cold submesoscale filaments with sharp lateral buoyancy gradients, strong surface convergence, and high vertical velocities. Highly heterogeneous Mixed Layer Depth (MLD) modulations are associated with these features, maintaining locally reduced MLDs even during storm events due to vigorous submesoscale restratification. The interaction of near‐surface turbulence and submesoscale restratification results in strong and highly efficient mixing inside the submesoscale fronts. Focusing on a typical filament, it is shown that frontogenesis is associated with the convergent cross‐front circulation arising from a turbulent thermal wind balance, consistent with previous studies. We show that the sharp fronts generated by this process may become symmetrically unstable, and show enhanced (modeled) energy dissipation rates that are in good quantitative agreement with existing theoretical estimates. Plain Language Summary: Recent studies have revealed that horizontal density differences on scales of 0.1–10 km (submesoscales) are ubiquitous in the surface mixed layer and play an essential role in its evolution. Submesoscale features can restratify, that is, bring warm water above cold water and shallow the depth of the mixed layer, which along with their high vertical velocities, makes them particularly relevant for biology. In this study, we highlight the effect of submesoscales on the restratification process and near‐surface turbulence during the lifetime of storms, thereby extending previous idealized simulations toward real‐ocean applications. We analyze a realistic high‐resolution numerical simulation focusing on the central Baltic Sea, an area where observations from a field campaign confirm that it constitutes a hotspot of submesoscale activity, hence forming an ideal natural laboratory. The simulation reveals a strong lateral temperature gradient that persists during autumn. The basin is found to be rich in submesoscale features that create shallow and heterogeneous Mixed Layer Depths (MLDs). The MLD presents high temporal variability, with the submesoscales being able to maintain shallow MLDs during storms and induce rapid restratification when the wind subsides. Submesoscale restratification tends to counteract the destratification effect caused by atmospheric forcing, yielding stably stratified patches. Key Points: Using realistic simulations a strong surface front, characterized by intense submesoscale activity, was identified in the central Baltic SeaSubmesoscales maintain shallow mixed layers during storms and induce rapid restratification afterward creating stably stratified patchesThe interaction of near‐surface turbulence and submesoscale restratification results in highly efficient mixing inside submesoscale fronts [ABSTRACT FROM AUTHOR]

Coastal zones, impacted by major rivers, comprise distinct environments, such as river plumes and bottom boundary layers (BBL). These environments are characterized by high nutrient concentrations and high microbial activities and thus offer favourable conditions for nitrification, a key process in the coastal nitrogen cycle. Because nitrification provides substrates for both primary production and denitrification, elucidation of its magnitude and regulation is crucial for understanding the nitrogen cycle in coastal zones. During three research cruises covering three seasons, the enhancement of nitrification rates and their regulation by environmental variables, including salinity, temperature, oxygen, and inorganic and organic nitrogen were investigated in river plume and BBL of the Vistula Estuary (Bay of Gdansk, Southern Baltic Sea). Nitrification rates were not enhanced in the river plume (39 ± 38 nmol L −1  d −1 ) compared to coastal surface water (45 ± 18 nmol −1  d −1 ) but the relationship to salinity and particulate organic nitrogen changed, suggesting different regulatory mechanisms along the salinity gradient. Nitrification rates in the BBL covered a range from 1 to 227 nmol L −1 d −1 and did not differ seasonally. NH 4 + turnover was dominated by assimilation into biomass in summer and by nitrification in winter and spring. In summer, rates were only slightly enhanced in the BBL and clearly related to particulate organic nitrogen and carbon concentrations, indicating particle attachment of nitrifiers and close coupling to organic matter degradation. The lack of correlations between nitrification and environmental variables in winter and spring suggested other regulatory mechanisms than in summer. Short-term changes, including the oxygenation of anoxic deep offshore water and particle resuspension clearly enhanced nitrification and further highlighted the variable mechanisms regulating nitrification in the Bay of Gdansk. Although nitrification rates did not greatly differ between seasons or water layers, the variability in regulatory mechanisms and the seasonal switch in NH 4 + recycling are likely to have implications on coastal N-turnover and hence on the filter function of coastal waters. [ABSTRACT FROM AUTHOR]

The Knudsen theorem for estuarine exchange flow, based on mass conservation of water and salt, and its generalization with resolution in salinity coordinates, the Total Exchange Flow (TEF) analysis framework, are reviewed here. The former had been developed, and applied to quantify exchange flow between the North Sea and the Baltic Sea, more than a century ago. In this paper, the underlying short research paper published in 1900 by Martin Knudsen, originally written in German, is translated into English. Reaching far beyond the Knudsen relation in particular, the extensive scientific achievements of Martin Knudsen on the salinity of seawater are reviewed. Using the Knudsen theorem and the TEF analysis framework, validated multi-decadal (years 1949–2013) model simulations are analyzed in terms of exchange flow through straits of the Western Baltic Sea. When comparing the model results to the original findings of Martin Knudsen, it is impressive to see how relevant his analysis of ratios of averaged inflowing and outflowing water masses still is today, given that they were based on just a few salinity observations. The model-based long-term Knudsen and TEF analyses of exchange flow in the Western Baltic Sea reproduces the Major Baltic Inflows (MBIs) that have occurred since the 1950s. For the complex inflow years 2002/2003, with two baroclinic summer inflows and one barotropic winter inflow in between, the strong underestimation of the exchange flow by Eulerian analysis as compared to TEF analysis is demonstrated. [ABSTRACT FROM AUTHOR]

Abstract: The warm and saline deep inflow into the Eastern Gotland Basin (EGB), which lasted from the end of November 1997 until the beginning of May 1998, was the first of a series of warm deep water inflows. Temporal fluctuations of the deep circulation were monitored at 170 m depth by two current meter moorings deployed at the north-eastern and south-western rim. The three-dimensional structure of stratification parameters was inferred from the comparison of two high-resolution hydrographic surveys carried out before and during the end of the 1997/1998 inflow event. The results indicate that near-bottom isopycnals were lifted more than 50 m during the inflow up to a depth of 170–180 m. Since the basin is enclosed at 170 m depth this implies a complete deep water renewal. Detected warm-water intrusions are shown to enhance the temperature variability on isopycnal surfaces at intermediate depth, creating strong inverse temperature gradients below the halocline in an environment with stable stratification due to salinity. Corresponding Turner angles suggest that diffusive convection may significantly contribute to diapycnal mixing in this region. In contrast, for the deepest near-bottom layers only marginally affected by intrusions, basin-scale budgets of heat and salinity suggest comparable diffusivities (κ s =1.3–2.1×10−5 m2/s for salt and κ s =1.6–2.8×10−5 m2/s for potential temperature), implying that double-diffusion does not play a major role in mixing there. For the intermediate layer between 90 and 170 m, where the budget method could not be adopted, we applied an idealized model for the decay of temperature variance associated with the intrusions. The results indicated that the intrusions decay within a few weeks, corresponding to vertical diffusivities of a few times 10−5 m2/s. [Copyright &y& Elsevier]

Abstract: Dissolved reactive Mn (dMnreact) has been determined at the redoxclines of two anoxic deeps from the Baltic Sea (Landsort Deep and Gotland Basin) and two seasonally anoxic freshwater lakes (Lake Dagow and Fuchskuhle, Germany). This dMnreact fraction is rapidly oxidised under oxygen atmosphere and is assumed to consist predominantly of Mn(III). There is a distinct increase of dMnreact from the outer regions towards the central part of the Landsort Deep. Although the presence of MnOx particles in the Gotland Basin is evidence of ongoing oxidation of reduced Mn species, almost no dMnreact was detected. Since completely different processes of Mn oxidation appear rather unlikely, we suggest oceanographic properties are responsible. Lateral currents and intrusions in the Gotland Basin seem to prevent the formation of a stable suboxic zone, a prerequisite necessary for accumulation of dMnreact. Such perturbations supply trace amounts of O2 and H2S, causing either immediate oxidation/reduction of dMnreact or deterioration of its stabilising ligands. dMnreact has also been determined in Lake Dagow with values significantly exceeding the level of the Landsort Deep due to stable stratification of this lake. In contrast, H2S appearance throughout the entire water column and a pH <5 prevent accumulation of dMnreact in Lake Fuchskuhle. [Copyright &y& Elsevier]

Abstract: In January/February 2004 a ship cruise into the Arkona Sea (Western Baltic) happened to start just prior to a medium-size inflow event and to last until a few days after the inflow. The bottom layer of saline inflow water in the southern Sound has a current speed of up to 1m/s. The current bifurcates at Kriegers Shoal, where only the smaller branch follows the contour lines of the bathymetry at the western rim of the basin, while the main flow proceeds on the steepest path north of Kriegers Shoal. The pycnocline tilt due to geostrophy matches with current observations in both branches. Old bottom water in the basin is lifted up by the denser inflow and pushed forward to Bornholm Gate. Inflow from the Darss Sill at first consists of low-salinity Baltic surface water and a small amount of higher salinity water at the bottom. After a few days the water over the sill is mixed and moves east, establishing a strong subsurface current at the southern rim of the Arkona Basin. Neither the inflow through the Sound nor the water masses from the Darss Sill reached the Bornholm Gate in the east of the Arkona Sea before the end of the survey. The saline outflow observed there consists of bottom water that had been present in the Arkona Sea prior to the new inflow. Overflow water spreading on the sea bottom is modified by entrainment from the low salinity water body above. Best guesses of the entrainment rate, somewhat tentative due to salinity variations at the entrance, are in line with formulae from the literature. [Copyright &y& Elsevier]

Abstract: A modelling system for coupled physical–biogeochemical simulations in the water column is presented here. The physical model component allows for a number of different statistical turbulence closure schemes, ranging from simple algebraic closures to two-equation turbulence models with algebraic second-moment closures. The biogeochemical module consists of models which are based on a number of state variables represented by their ensemble averaged concentrations. Specific biogeochemical models may range from simple NPZ (nutrient–phytoplankton–zooplankton) to complex ecosystem models. Recently developed modified Patankar solvers for ordinary differential equations allow for stable discretisations of the production and destruction terms guaranteeing conservative and non-negative solutions. The increased stability of these new solvers over explicit solvers is demonstrated for a plankton spring bloom simulation. The model system is applied to marine ecosystem dynamics the Northern North Sea and the Central Gotland Sea. Two different biogeochemical models are applied, a conservative nitrogen-based model to the North Sea, and a more complex model including an oxygen equation to the Baltic Sea, allowing for the reproduction of chemical processes under anoxic conditions. For both applications, earlier model results obtained with slightly different model setups could be basically reproduced. It became however clear that the choice for ecosystem model parameters such as maximum phytoplankton growth rates does strongly depend on the physical model parameters (such as turbulence closure models or external forcing). [Copyright &y& Elsevier]