46
K. Myrberg and A. Lehmann
Fig. 2.5 Inertial oscillations
in the Baltic Sea in August
17–24, 1933 according to
Gustafsson and Kullenberg
(1936). The circular diagram
shows the current velocity on
August 21. From Leppäranta
and Myrberg (2009)
baroclinic mean circulation and the mean wind-driven circulation. Which one is
more important is difficult to determine in the case of such a strongly non-linear
system. The predominant component depends on the case studied and on the time
scale under investigation.
2.3.4 Inertial Oscillations
In physical oceanography inertial oscillations refer to the circular motions of water
in which the inertial and Coriolis acceleration balance each other. This situation occurs often, e.g., when wind ceases to drive a surface current. The observation of inertial oscillations in the Gotland Sea (Fig. 2.5) by Gustafsson and Kullenberg (1936)
was the first time when this phenomenon was documented in physical oceanography. Their study was based on current measurements collected during the summer
period in 1931–1933. Later on it became clear that the inertial motion is one of the
dominating features in the dynamics of the Baltic Sea.
Energy spectra of Baltic Sea current velocities show that the motions with the
inertial period of 13.2–14.5 hours are, on average, the strongest motions with a
more or less fixed period. At periods between 10 and 12 hours there are indications
of interaction between tidal waves and inertial oscillations. Also relatively intense
motions with shorter periods (8, 7.5, 6.5 and 4 hours) have been found.
When the external forcing ceases, the rotational effects take over and the inertial
oscillation becomes visible. Mathematical analysis of this situation is simple to perform using complex variables. Denote the velocity as q = u + iv, where i =
√ −1.
K. Myrberg and A. Lehmann
Fig. 2.5 Inertial oscillations
in the Baltic Sea in August
17–24, 1933 according to
Gustafsson and Kullenberg
(1936). The circular diagram
shows the current velocity on
August 21. From Leppäranta
and Myrberg (2009)
baroclinic mean circulation and the mean wind-driven circulation. Which one is
more important is difficult to determine in the case of such a strongly non-linear
system. The predominant component depends on the case studied and on the time
scale under investigation.
2.3.4 Inertial Oscillations
In physical oceanography inertial oscillations refer to the circular motions of water
in which the inertial and Coriolis acceleration balance each other. This situation occurs often, e.g., when wind ceases to drive a surface current. The observation of inertial oscillations in the Gotland Sea (Fig. 2.5) by Gustafsson and Kullenberg (1936)
was the first time when this phenomenon was documented in physical oceanography. Their study was based on current measurements collected during the summer
period in 1931–1933. Later on it became clear that the inertial motion is one of the
dominating features in the dynamics of the Baltic Sea.
Energy spectra of Baltic Sea current velocities show that the motions with the
inertial period of 13.2–14.5 hours are, on average, the strongest motions with a
more or less fixed period. At periods between 10 and 12 hours there are indications
of interaction between tidal waves and inertial oscillations. Also relatively intense
motions with shorter periods (8, 7.5, 6.5 and 4 hours) have been found.
When the external forcing ceases, the rotational effects take over and the inertial
oscillation becomes visible. Mathematical analysis of this situation is simple to perform using complex variables. Denote the velocity as q = u + iv, where i =
√ −1.