The wind forcing at a 10 m level was derived from geostrophic winds as recommended by Bumke and Hasse (1989): the geostrophic wind speed was multiplied by 0.
This feature apparently stems from the better ability of the WAM model and adjusted geostrophic wind fields to reproduce the extreme events.
The most popular way consists in the use of geostrophic wind
fields that are adjusted to the 10 m level by means of a simplified procedure in which the geostrophic wind
speed (usually retrieved from the Swedish Meteorological and Hydrological Institute database) was multiplied by 0.
g = [delta]], where the velocity is equal to the geostrophic wind
The wave model was forced with wind data corresponding to an elevation of 10 m above the sea surface, constructed from the Swedish Meteorological and Hydrological Institute (SMHI) geostrophic wind
Figure 6 shows the 500/1000 thickness advection by geostrophic wind of 1000 hpa level in the selected case studies to represent.
Absolute vorticity advection by geostrophic wind in 500 hpa level and 1000/500 thickness advection by geostrophic wind of 1000 hpa level can interpret development or decline of the system pressure.
In particular, the abrupt change in the air flow, evaluated from geostrophic wind fields (Soomere and Raamet, 2014), signals a major change in the air pressure and upper-level wind system over the southern Baltic Sea in 1988.
The identified major regime shifts in the average air flow speed around 1988 could be related to the abrupt change in the geostrophic wind vector over the southern Baltic Proper.
The authors are most grateful to the SMHI, especially to Dr Barry Broman, for providing help in retrieving the geostrophic wind data, to the EMHI for historical wave data and weather maps provided by Ivo Saaremae, and to Inga Zaitseva-Parnaste for providing digitized wave data from Vilsandi, Pakri, and Narva-Joesuu.
The adjusted geostrophic wind data lead to about the same accuracy of reproduction of the frequency of different wind conditions.
The time series of the significant wave height and peak period were extracted from the long-term simulations of wave fields for 1970-2007 with a temporal resolution of 1 h for the entire Baltic Sea using the third-generation spectral wave model WAM  driven by properly adjusted geostrophic winds