Baltic Sea Physics

The Baltic is considered one of the most thoroughly studied seas in the world, one for which there are long series of measurement data. That may very well be true, but does it mean we know enough about our large inland sea? Hardly. In fact, we lack fundamental understanding of several important processes in the Baltic Sea. Generally speaking, oceanographic measurement values are also very difficult to interpret. Data is collected by research vessels at different times and different places. The measurements vary sharply in quality from one time to the next, from one place to the next. Let’s take a closer look at the curve over the mean salinity of the Baltic (Figure 1) and its variation during the 20th century. Calculating mean salinity is no easy task. Available data are unevenly distributed in time and space. No measurements were taken during the two world wars, so the reconstructed curve is based on estimates and we have no idea how precise those estimates are. We need more in-depth studies here.


(Click image to enlarge)

Click image to enlarge


Figure 1. Estimated mean salinity of the Baltic Sea and its variations during the 20th century. Mean salinity is calculated as a mean over the entire Baltic and at all depths. From (Winsor et al., 2001, 2003).


It’s worth pointing out that an uncertain curve is better than none at all, and mean salinity data give us valuable knowledge about the Baltic Sea. If nothing else, the diagram shows that everyone who interpreted the declining salinity of the 1980s and 1990s as a dead-certain sign of global and regional climate change were jumping to conclusions. The curve has dropped sharply before. The difference in mean salinity on a time-scale of 30 years is small. To determine whether the Baltic Sea climate is intact, more brackish, or more marine, we need high-quality measurement information covering a period of more than 30 years. The mean salinity curve is currently used in various climate and water balance studies. It has made possible an initial estimate on how well we can calculate freshwater inflow into the Baltic. But estimating the uncertainty in the curve will be an important and difficult future research task.


It has become popular among Baltic researchers to relate their measurement data to something called NAO. A strong connection between NAO and the Baltic Sea climate is shown quite frequently. NAO determines, for instance, whether the winter will be mild or severe. NAO, or North Atlantic Oscillation, is given as a measurement of differences in pressure that describe air currents over the North Atlantic. But it is a blunt measurement and clearly overestimated as an instrument for describing the large-scale atmospheric current – how and where the winds blow. And the causes of variations in NAO are still unclear. What we do know is that atmosphere circulation over the North Atlantic has a powerful impact on the Baltic Sea climate. But we do not know what causes changes in these air currents. Another central field of research is expanding here.


An inventory of what we know and do not know about how the Baltic Sea functions under current climate conditions was recently performed within the framework of the BALTEX program, the Baltic Sea Experiment (Omstedt et al., 2004, The summary indicates a strong need for research.


The research world has gained access to new databases in the last 10 years. The building of the new databases is an extraordinarily meaningful contribution to research, but they are unfortunately imperfect. Necessary documentation and information about data quality are often lacking. We still cannot go to the database and get answers to elementary questions about weather and wind. How much does it rain and how much does the wind blow on the Baltic? Several key processes in the sea are also poorly understood. These will form the base for several different investigations within the research group.



  • Omstedt, A., Elken, J., Lehmann, A., and J., Piechura (2004). Knowledge of the Baltic Sea Physics gained during the BALTEX and related programmes. Progress in Oceanography 63, 1-28.
  • Winsor, P., J. Rodhe, and A. Omstedt (2003). Erratum: Baltic Sea ocean climate: an analysis of 100 yr of hydrographic data with focus on the freshwater budget, Climate Research, 25, 183.
  • Winsor, P., J. Rodhe, and A. Omstedt (2001). Baltic Sea ocean climate: an analysis of 100 yr of hydrographic data with focus on the freshwater budget, Clim. Res., 18, 5-15, 2001.

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