Much of what was summarized about the North Sea dataset (Loewe, 2022) carries over to the Baltic Sea setting. To make the current text a stand-alone resource that summary is reproduced here mutatis mutandis. Despite all equivalence, there is an important difference as to gale classification arising from relocating the analysis grid that is addressed in the following as well.

Sea level pressure is a fundamental weather and climate element and the very basis of everyday weather maps. Daily sea level pressure distributions provide information on the influence of high and low pressure systems, air flow, weather activity, and, hence, synoptic conditions.

Using sea level pressure distributions from the NCEP/NCAR Reanalysis 1 (Kalnay et al., 1996) and a simplified variant of the weather-typing scheme by Jenkinson and Collison (1977) atmospheric circulation over the Baltic Sea has been classified as to pattern and intensity on a daily basis starting in 1948. A full account of the original weather-typing scheme for the North Sea can be found in Loewe et al. (2005), while the variant scheme has been detailed in Loewe et al. (2006). The original 16-point analysis grid devised for the North Sea was shifted 5 degrees to the North and 15 degrees to the East to accommodate the Baltic Sea. Though formally valid at its central point (60°N, 20°E), results are representative of the Baltic Sea region between 55°N-65°N and 15°E-25°E.

The modified scheme allows for six weather types, namely four directional (NE=Northeast, SE, SW, NW) and two rotational types (C=cyclonic and A=anticyclonic). The strength of the atmospheric circulation is classified by way of a peak-over-threshold technique, employing Coriolis-adjusted thresholds for the gale index G* of 29.9, 38.7, and 47.2 hPa for gale (G), severe gale (SG), and very severe gale (VSG), respectively. These thresholds are elevated by the Coriolis frequency ratio f(60N)/f(55N) (i.e. sin60°/sin55°) over those used with the North Sea dataset (Loewe, 2022) to ensure that gales are identified at an identical geostrophic wind and vorticity scale in either region. G* is a composite measure of gradient and Laplacian of the pressure field at each grid’s central point. Coriolis-adjustment accounts for the fact that the strength of geostrophic flow and vorticity of which G* is indicative also depends on latitude according to Coriolis frequency. Note also that previously given exceedance probabilities of 10, 2, and 1/3.65 % apply to the North Sea thresholds for the period 1971-2000, only. For the same period of reference empirical exceedance probabilities for the Baltic Sea are at 6.4, 1.0, and 0.5/3.65 %.

Technically, the set of weather-typing and gale-classification rules is implemented as a lean FORTRAN code (lwtbssim.f), internally known as "Simple Lamb weather-typing scheme for the Baltic Sea v1". The processing run was done on a Linux server under Debian 10 (Buster).

Both, weather types and gale days, form a catalogue of more than 70 annual calendars since 1948 that is presented and continuously updated to the present day at https://www.bsh.de/DE/DATEN/Klima-und-Meer/Wetterlagen-Stuerme/wetterlagen-und-stuerme_node.html. (A corresponding English page is currently being devised at https://www.bsh.de/EN/DATA/Climate-and-Sea/Weather-and-Gales/weather-and-gales_node.html .) This catalogue concisely documents synoptic conditions in the Baltic Sea region. Possible benefits are manifold. Special events and episodes in regional-scale atmospheric circulation are easily looked up and traced. Beyond that, the dataset is well suited for frequency, trend, persistence, transition, and extreme-value statistics.