ALBERT

Description: In this work we present new insights into the Baltic Sea overturning circulation by adopting an isohaline water mass transformation framework. Relations between local diahaline volume and diffusive salt flux, and local diahaline mixing, are verified in a realistic model simulation, and presented as Baltic Sea maps. In this way, hot spots for mixing are visualized, and the impact of boundary mixing is studied. Moreover, a unique quantification of physical (due to turbulence parameterization) and numerical mixing (due to discretization errors from the numerical advection schemes) offers to estimate the associated contributions to the simulated overturning circulation. Finally, we demonstrate that integration of the different new local diagnostics confirms existing bulk theories.

Description: The Baltic Sea and North Sea are two highly productive, connected marginal seas in Northern Europe. Both are strongly influenced by inputs of terrestrial carbon but differ fundamentally in character. The Baltic Sea is a wind-driven, brackish water system that is almost completely enclosed by land and has residence times on the order of decades. In contrast, the North Sea is a tidally-driven, marine system, on the edge of the North Atlantic with residence times on the order of months. Episodic deep inflows of salty, oxygenated North Sea water penetrate the deep basins of the Baltic Sea, providing temporary oxygen supply to otherwise persistent hypoxic zones, while brackish, surface Baltic Sea water drains into the North Sea carrying with it a net export of carbon. Both systems have densely populated and intensively used coastlines, exposed to climate change and ever-increasing anthropogenic pressures. To understand the net carbon uptake behaviour of the coupled system, and how this might change in response to perturbations in atmospheric and river forcing, we reconstruct the seasonal and inter-annual variability in carbon pools and fluxes in the Baltic – North Sea continuum from 1993 to 2019 using a three-dimensional biophysical model. To our knowledge, this is the first hindcast of bulk biogeochemical and physical properties which explicitly models the flux of carbon between the seas and accounts for the observed increase in atmospheric CO〈sub〉2〈/sub〉 from 360 to 420 ppm in recent decades.

Description: As already predicted by the Knudsen relations, estuarine dynamics strongly depend on the freshwater runoff entering an estuary. At the same time, salt mixing can be understood as the driver of estuarine circulation. It enables the transport of water between inflow and outflow layers and therefore closes the estuarine circulation by driving a diahaline exchange flow. A recently derived universal law links salt mixing to freshwater runoff for an estuarine volume bounded by an isohaline surface: it states that on long-term average, the area-integrated mixing across the bounding isohaline surface is directly proportional to the freshwater runoff entering the estuary. However, even though numerous studies predict that periods of extreme runoff will become more frequent with climate change, the direct impact of such periods on estuarine mixing and circulation has not yet been investigated. In this study, we therefore focus on salt mixing and diahaline exchange flow during a low-runoff and an extremely high-runoff period. As a representation of typical mesotidal estuaries, we used a realistic numerical setup of the Elbe estuary in northern Germany. We find that the spatial distribution of diahaline exchange flow is directly linked to the local mixing gradient. Additionally, during the high-runoff period, increased vertical stratification occurs within the estuary, even though estuary-wide mixing scales with the freshwater runoff as predicted by the universal law. Our findings indicate that stratification occurs due to a locally limited mixing intensity, which requires an increased isohaline surface to ensure that the area-integrated mixing still follows the universal law.