Baltic Earth Working Group on Small-scale (sub-mesoscale) processes and their impacts on large-scale Baltic Sea circulation

• How do sub-mesoscale processes influence vertical mixing, stratification, and coastal–offshore
  exchanges in the Baltic Sea?
• How is variability at sub-mesoscales connected to mesoscale and basin-scale processes, and how does
  this multi-scale interaction affect circulation, productivity, and biogeochemistry?
• Can models with sufficiently high spatial resolution accurately represent sub-mesoscale processes, and
  what measurements are required for a model-data comparison?

In the global ocean, the large-scale circulation and stratification are primarily driven by the fluxes of momentum, heat, and freshwater through the sea surface, along with their planetary-scale gradients. Similar processes govern the Baltic Sea system, where atmospheric forcing, heat and freshwater exchange across the sea surface, river input, and episodic inflow of saline water through the Danish straits shape the overall circulation. Energy introduced at large scales ultimately cascades toward progressively smaller scales, where it is dissipated at metre- to centimetre-scale turbulence. The presence and importance of mesoscale eddies in transferring potential energy from sloping isopycnals into kinetic energy, and in mediating energy transfer from large to smaller scales, have been recognised for decades. However, the pathways through which energy flows from mesoscale to sub-mesoscale processes – and finally to dissipation – remain poorly understood. At the same time, numerous studies based on modelling, satellite remote sensing, and in-situ observations have documented the prevalence of sub-mesoscale features in the Baltic Sea. These features often occur in regions of strong horizontal buoyancy gradients, including coastal and offshore fronts, up- and downwelling, and along mesoscale eddies and frontal zones. Observing sub-mesoscale processes remains challenging due to their small spatial scales (from 100 m to 2 km) and short temporal scales (from hours to days). Nevertheless, increasingly powerful modelling systems with sufficiently fine grid resolution, together with improved parameterisations and diverse experimental configurations, have greatly enhanced our ability to investigate the nature and large-scale effects of sub-mesoscale processes. Such models, however, require high-quality in-situ observations for validation. RT2 aims to characterise sub-mesoscale variability and its connections to processes at other spatial and temporal scales – from basin-scale circulation and mesoscale dynamics to three-dimensional mixing and biogeochemical variability. Key topics include the influence of sub-mesoscale processes on vertical stratification, coastal–offshore exchanges, primary productivity, and air–sea exchange of climatically relevant substances such as greenhouse gases. These issues are crucial for understanding the broader impacts of small-scale variability on the Baltic Sea system. Emerging evidence suggests that inadequate representation of sub-mesoscale dynamics may be a significant source of uncertainty in ocean circulation simulations. The main objective of RT2 is to summarise and synthesise current knowledge on sub-mesoscale dynamics and their large-scale impacts – including their role in the internal variability and predictability of the Baltic Sea system– and to outline priority research directions for the coming years.

Potential activities include providing an overview of the current state of knowledge on sub-mesoscale variability and its impacts on large-scale circulation patterns, including associated biogeochemical fields. Work may also involve identifying knowledge gaps and research needs, with the broader aim of improving the predictability of the Baltic Sea system. Additional efforts may focus on compiling an inventory of relevant observational infrastructure, modelling platforms, and available datasets – including high-resolution monitoring programmes – within the Baltic Sea research community. Further activities may include suggesting topics, programmes,
or project concepts to advance understanding of sub-mesoscale dynamics and their large-scale impacts, and identifying ways to implement the proposed actions, including mobilising the research community where appropriate.