50
K. Myrberg and A. Lehmann
Fig. 2.7 The wind-driven
motion of ice and the currents
at different depths in the
Ekman layer in the Bay of
Bothnia, April 1975
(Leppäranta 1990)
On the other hand, contemporary 3D models of shallow shelf seas with very
high vertical resolution (the thickness of the uppermost model layers about 10 %
of the Ekman layer depth) apparently are capable of replicating, at least qualitatively, the Ekman spiral. Such models have been extensively used, for example, for
the modelling of the intensity and statistics of upwelling phenomena in the Baltic
Sea (Myrberg and Andrejev 2003; Lehmann and Myrberg 2008). It has been observed in various modelling examples during the last decade (from Andrejev et al.
2004 onwards) that a very thin upper layer (of only a few metres) can even move
in the opposite direction to the layers below it, and that the overall surface circulation pattern may, counter-intuitively, contain anticyclonic gyres in the northern
hemisphere (Beletsky et al. 2006; Soomere et al. 2011). This means in practice that
the actual Ekman-type layered flow may contain surprisingly thin layers and/or its
structure may substantially deviate from the theoretical predictions. These matters
of current dynamics are discussed to some extent in Chap. 9. This is a field where
intense research is also ongoing to understand the reasons behind different kinds of
behaviour and to quantify the actual dynamics more exactly (Heinloo and Toompuu
2011, 2012).
2.3.6 Geostrophic Flow
The geostrophic flow—the current driven by the pressure gradient on a rotating
planet—is a steady, frictionless current in which the Coriolis acceleration is balanced by the pressure gradient. In the vertical direction hydrostatic balance is assumed. The momentum equation is written for the geostrophic balance as
if q = −
1
ρ
∇p,
∂p
∂z
= −ρg.
(2.14)
K. Myrberg and A. Lehmann
Fig. 2.7 The wind-driven
motion of ice and the currents
at different depths in the
Ekman layer in the Bay of
Bothnia, April 1975
(Leppäranta 1990)
On the other hand, contemporary 3D models of shallow shelf seas with very
high vertical resolution (the thickness of the uppermost model layers about 10 %
of the Ekman layer depth) apparently are capable of replicating, at least qualitatively, the Ekman spiral. Such models have been extensively used, for example, for
the modelling of the intensity and statistics of upwelling phenomena in the Baltic
Sea (Myrberg and Andrejev 2003; Lehmann and Myrberg 2008). It has been observed in various modelling examples during the last decade (from Andrejev et al.
2004 onwards) that a very thin upper layer (of only a few metres) can even move
in the opposite direction to the layers below it, and that the overall surface circulation pattern may, counter-intuitively, contain anticyclonic gyres in the northern
hemisphere (Beletsky et al. 2006; Soomere et al. 2011). This means in practice that
the actual Ekman-type layered flow may contain surprisingly thin layers and/or its
structure may substantially deviate from the theoretical predictions. These matters
of current dynamics are discussed to some extent in Chap. 9. This is a field where
intense research is also ongoing to understand the reasons behind different kinds of
behaviour and to quantify the actual dynamics more exactly (Heinloo and Toompuu
2011, 2012).
2.3.6 Geostrophic Flow
The geostrophic flow—the current driven by the pressure gradient on a rotating
planet—is a steady, frictionless current in which the Coriolis acceleration is balanced by the pressure gradient. In the vertical direction hydrostatic balance is assumed. The momentum equation is written for the geostrophic balance as
if q = −
1
ρ
∇p,
∂p
∂z
= −ρg.
(2.14)