Baltic Earth Working Group on Small-Scale Processes

In the ocean, the main forcing components defining the general circulation and stratification are the fluxes of momentum, heat and water through the sea surface and mostly their planetary-scale differences. Similarly, the main physical forcing components for the Baltic Sea system are the atmospheric forcing, exchange of heat energy and freshwater through the sea surface, and input of freshwater from rivers and saltier water through the Danish Straits. The input of energy at a large scale is balanced by kinetic energy dissipation at a metre-centimetre scale.

The presence and role of mesoscale eddies, converting the potential energy of sloping isopycnals into kinetic energy and transferring energy from large to smaller scales, are long recognized. However, the pathways for energy transfer from mesoscale to smaller scales and eventually dissipation have yet to be clarified. At the same time, the presence of submesoscale features/variability in the Baltic Sea has been shown by many studies based on modelling, remote sensing and in-situ data. Coastal/offshore fronts, upwellings/downwellings, and mesoscale eddies and fronts with large horizontal buoyancy gradients are the regions where the variability and processes at smaller scales emerge.

Observations of submesoscale processes are challenging due to their small spatial (100 m – 2-5 km) and temporal (hours to days) scales. While the measurements are challenging, more and more powerful modelling systems, allowing fine enough grid spacing and setups for experiments with varying background conditions and parametrization, are used to reveal the nature and large-scale impacts of submesoscale processes. The models reaching the relevant resolution need appropriate in-situ data for their validation.

We aim to characterize submesoscale variability and its connection to the variability/processes at other relevant temporal and special scales – from 3D mixing to mesoscale and basin-wide variability and processes. The topics include the connectivity of the variability at different scales (including biogeochemical signal dynamics), the impact of submesoscales in the development of vertical stratification and coastal-offshore exchanges, the role of different scales in shaping productivity and air-sea exchange of substances (including greenhouse gases). All these topics are relevant for understanding the large-scale impacts of processes and variability at smaller scales. Recent studies have suggested that misrepresenting submesoscale dynamics could be one source of uncertainties in future climate predictions.

The main objective of the working group is to summarize the knowledge on the submesoscale dynamics and their large-scale impact (or internal variability of the Baltic Sea system and its predictability) and suggest ways forward.

The duration of this working group could be 3-6 years, depending on the ambition (to stop after the reviews of knowledge gaps and potential actions are published or to implement the plans as well).