Baltic Earth Working Group on Sea Level Dynamics in the Baltic Sea Region

Sea level dynamics in the Baltic Sea are influenced by meteorological, astronomical, hydrological and geological factors. As a semi-enclosed sea, the dynamics of Baltic Sea sea level at different time scales range from minutes to millennia. At very short time scales variations are associated with e.g. waves, currents, seiches, or meteo-tsunamis. Although of small amplitude, tides may also sometimes play a role in Baltic Sea sea level dynamics. Factors that are relevant at increasingly longer time scales include, among others, variations in wind and sea level pressure, river runoff, presence of sea ice and spill-over of water masses from the North Atlantic or expansion of the water column by thermosteric effects, land uplift/subsidence and sediment accumulation. Also the contribution from short-term events (e.g. their frequency or intensity) may vary over longer time scales.

The effects of such drivers on Baltic Sea sea-level can be regionally and temporally quite heterogeneous. Good understanding of Baltic Sea sea-level dynamics, their mechanisms of variability, and the assessment of variability and change over the past are prerequisite for both, improved forecasting on short to medium time scales and for reducing uncertainty associated with projections of future sea-level rise caused by anthropogenic climate change.

The Baltic Sea coastal sea-level is one of the best and longest sampled by tide gauges. Several high-resolution satellite products provide the means to analyze absolute (as opposed to relative coastal sea-level) dynamics in the sea interior, albeit for shorter periods. Increasingly also model reconstructions and projections become available for both, mean and extreme sea level variability.

There is still incomplete knowledge on the dynamics of short term sea level variations, in particular on their climatology and their long-term variability and change. Moreover, a more systematic assessment of interaction between variations at different time scales and their impact on extreme sea levels is still missing. Also, an improved understanding of the connections between Baltic Sea, North Sea and North Atlantic sea level and of the link between Baltic Sea sea level and large scale atmospheric circulation patterns such as the North Atlantic Oscillation is needed. While it is known that there is a strong influence of such atmospheric circulation patterns on winter time Baltic Sea sea level variability, it is not yet known why this influence was less pronounced in the early 20th century.

The quantification of contributions from climatic factors responsible for the observed and projected long-term trends and multi-decadal variations in Baltic Sea sea level through the 20th and 21st centuries need to be improved as well. This comprises, for example, multi-decadal trends in the wind regimes, salinity, and glacial isostatic adjustment or sea-ice cover, together with changes in ocean dynamics in the North Atlantic that may have an influence on sea-level rise. Quantification of these contributions provides an essential prerequisite for improving detection and attribution of anthropogenic signals in Baltic Sea sea-level rise and variability. In particular, research on the attribution of observed changes to causes and on the plausibility and consistency between observed and expected changes is needed.

There is substantial and increasing need in information about future sea level changes on time scales from seasons to decades. This refers to both, mean and extreme sea levels. Any progress to better address these issues for the Baltic Sea will be a substantial step forward. Although the data basis for the analysis of Baltic Sea sea-level is very favorable, a systematic comparison of tide-gauges and high resolution satellite products is just starting. In addition, more high-resolution ocean and atmosphere-ocean regional simulations of the Baltic Sea are becoming available. Consistent analysis of all data sets is needed and will help to benchmark the skill of the various products, in particular of regional climate models at multi-decadal timescales, and will help to quantify the relative importance of the different factors contributing to Baltic Sea sea level dynamics.

A review of the existing knowledge and challenges in Baltic Sea sea-level research is envisaged. The network of researchers in Baltic Sea sea level research should be maintained and fostered, by organising workshops or sessions at international conferences, and options for joint international research should be reviewed, also together with other regional sea level groups, in particular with the North Sea community. Common products to be developed could be a (meta-) database of Baltic Sea sea-level data and assessments of past, ongoing and future variability and change of the regional sea level.