2 Topography, Hydrography, Circulation and Modelling of the Baltic Sea
49
Fig. 2.6 The Ekman profile
of current velocity in shallow
seas. Here d is the sea depth
and D is the Ekman depth.
From Leppäranta and
Myrberg (2009)
Thus the Ekman transport for a finite sea depth and zero bottom friction depends on
the eddy viscosity via the parameter λ.
The Ekman solution holds qualitatively in the Baltic Sea: The current speed reduces and the direction turns to the right with increasing depth. However, the approximation that the vertical eddy viscosity is constant is not always adequate here
and that is the most probable reason why the profiles observed in nature differ from
the theoretical one. For large sea depths the observed surface currents are about 2–
3 % of the wind speed, but the turning angle is 20–30 ◦ . This simple surface drift
model is still used in many practical applications like in forecasting the drifting of
floating objects. Figure 2.7 presents observations of freely moving sea ice and surface layer water velocity. In a qualitative sense the Ekman spiral is observed beneath
the ice 7 (Leppäranta 2010). The ice drifts by about 15 ◦ to the right from the wind
speed, with a drift speed about 2 % of the wind speed.
Probably the most widely known application of the Ekman theory explains the
phenomenon of wind-induced upwelling. If the wind blows parallel to the coast so
that the coast remains on the left on the northern hemisphere, the Ekman transport
is directed offshore. The drift of water to offshore is compensated by a vertical
movement of water from deeper layers towards the sea surface.
It is debatable whether, or how exactly, the structure of the Ekman layer is represented in the ocean modelling efforts. On the one hand, this phenomenon is obviously not replicated in two-dimensional (2D) models of circulation and in models
of the deep ocean with the thickness of the upper layer comparable with the Ekman layer depth. In cases where specifically the drift of substances or objects in the
uppermost layer is of importance (e.g., studies of pollution or oil propagation) it
has been customary to either insert artificially a similar drift of the uppermost layer
(e.g., Periáñez 2004) or to enhance the direct wind impact on the objects at the sea
surface (Verjovkina et al. 2010).
7 The Ekman spiral was first observed under the ice by Fridtjof Nansen in the 1890s. The spiral was
documented much later for the open ocean.
49
Fig. 2.6 The Ekman profile
of current velocity in shallow
seas. Here d is the sea depth
and D is the Ekman depth.
From Leppäranta and
Myrberg (2009)
Thus the Ekman transport for a finite sea depth and zero bottom friction depends on
the eddy viscosity via the parameter λ.
The Ekman solution holds qualitatively in the Baltic Sea: The current speed reduces and the direction turns to the right with increasing depth. However, the approximation that the vertical eddy viscosity is constant is not always adequate here
and that is the most probable reason why the profiles observed in nature differ from
the theoretical one. For large sea depths the observed surface currents are about 2–
3 % of the wind speed, but the turning angle is 20–30 ◦ . This simple surface drift
model is still used in many practical applications like in forecasting the drifting of
floating objects. Figure 2.7 presents observations of freely moving sea ice and surface layer water velocity. In a qualitative sense the Ekman spiral is observed beneath
the ice 7 (Leppäranta 2010). The ice drifts by about 15 ◦ to the right from the wind
speed, with a drift speed about 2 % of the wind speed.
Probably the most widely known application of the Ekman theory explains the
phenomenon of wind-induced upwelling. If the wind blows parallel to the coast so
that the coast remains on the left on the northern hemisphere, the Ekman transport
is directed offshore. The drift of water to offshore is compensated by a vertical
movement of water from deeper layers towards the sea surface.
It is debatable whether, or how exactly, the structure of the Ekman layer is represented in the ocean modelling efforts. On the one hand, this phenomenon is obviously not replicated in two-dimensional (2D) models of circulation and in models
of the deep ocean with the thickness of the upper layer comparable with the Ekman layer depth. In cases where specifically the drift of substances or objects in the
uppermost layer is of importance (e.g., studies of pollution or oil propagation) it
has been customary to either insert artificially a similar drift of the uppermost layer
(e.g., Periáñez 2004) or to enhance the direct wind impact on the objects at the sea
surface (Verjovkina et al. 2010).
7 The Ekman spiral was first observed under the ice by Fridtjof Nansen in the 1890s. The spiral was
documented much later for the open ocean.