Baltic Sea

discovered that most of the variance for short periods originates from internal forcing but may
induce some variance of longer periods. In other words, periods of shorter than 1 month in the
sea level are due to internal forcing. Furthermore Samuelsson and Stigebrandt (1996)
detected the Baltic Sea oscillates like a closed basin for short periods. From a kinematical point
of view these oscillations reect the rst natural seiche mode, with a maximal variability at the
extreme ends in the north and south and minimum in the Baltic Proper (a half-wave-length
oscillator). Among other things, this is the reason to use records of the Swedish coastal station
Landsort to describe changes in the lling level of the Baltic Proper. For longer periods the
internal forced oscillations are kinematically similar to an open bay with increasing amplitudes
from the mouth and inwards. Samuelsson and Stigebrandt (1996) explain the shift in
kinematics of internally forced oscillation with the limited transport capacity of the straits in
the mouth for "high frequency" motion.
Turning back to the changes in deep currents of the Stolpe Channel, signicant peaks in periods
found in the along-slope and across-slope current components of ADCPs SFN, SFS (Fig. 5.10),
as well as in the current meter moorings SE and NE deployed in the EGB (Fig. 5.11, Fig. 5.12
in the appendix) are all shorter than one month. An exception is mooring SW (Fig. 5.13
in the appendix) with a peak around 40 days in the across-slope components. Hence, these
time series of deep currents point to the conclusion that internal forcing could be the main
steering mechanism and therefore investigations, especially in the Stolpe Channel, are focused
on internal forcing, i.e. wind and density changes.
Accordingly daily eastward currents of ADCPs SFN and SFS (Fig. 4.12) were observed to be
pulse-like, with each pulse-like event lasting for about 2 - 4 days, as already discussed in chapter
4.3.3. Here, however, the role of changes in the regional wind conditions will be analysed in
more detail to understand the response of deep current uctuations above topographic slopes
of the eastern Stolpe Channel. Hypothetically, it is expected that such deep currents follow
roughly bathymetric contours due to the conservation of their potential vorticity.
Hence, to obtain the along-slope (Fig. 4.27 B, C) and across-slope components of the currents,
u and v were rotated anti-clockwise by 28 ◦ to follow the 75 m isobath. These current
components were then compared with the regional wind velocity along the main axis of the
Baltic Proper, denoted 38 (line between red dots in Fig. 4.28). 38 results from
averaging the records of the meteorological stations Arkona Buoy in the Arkona Basin (AB)
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