107
Ekman spiral
walls of an aquarium, the hull of a ship. Edge effects make experiments with
pelagic organisms very difficult.
edge wave a wave that is trapped near a coast with a sloping beach. The rays of
this wave are gradually refracted towards the coast. For a given coastal angle
of incidence there will be a line at some distance along the coast where the
reflected rays run parallel to the coastline. Inshore of this line there is a pattern of standing (trapped) waves and offshore of it the amplitude will decay
exponentially. ~trapped wave.
eel grass ~sea grass.
EEZ ~Exclusive Economic Zone.
EFZ ~Exclusive Fishery Zone.
Ekman layer the surface layer over which in the case of the Ekman spiral, the current
becomes reduced to lIe (where e is the ground number of the naturallogarithm, 2.71828 ... ) of the surface value. The most important part of the drift current takes place in this layer. An Ekman layer is also present as a boundary layer
that forms the transition between a geostrophic current (~geostrophic balance)
and the near-bottom layer where the bottom friction is the main factor.
Ekman number a nondimensional number, E, that gives the ratio between the
frictional force (~friction) per unit mass to the Coriolis force. It is given by:
E=KlfJ), with K the coefficient of eddy viscosity,fthe Coriolis parameter and La
typical length scale. When the Ekman number is small, frictional forces can
be neglected.
Ekman spiral a solution of the equations of motion ( ~ocean dynamics) for the current
driven by a homogeneous wind stress was in 1905 given by the Swedish oceanographer v.w. Ekman (1874-1954), in order to explain the angle between wind
and ice drift as observed by Nansen on his drift through the North Polar Sea
with the Pram. A condition is that there are no horizontal pressure gradients
(sea surface slopes) and in the most elementary case it is further assumed that
the sea is very deep (no influence of bottom friction) and thus that the wind
stress and the Coriolis force are the only external forces. In large, deep oceans these
conditions are approximated. The result is a current that makes at the surface
an angle with the wind direction; to the right in the northern hemisphere, to
the left in the southern, and that with increasing depth decreases in strength
and further turns away from the wind direction. The point of the current vector thus describes a spatial spiral, the Ekman spiral. For other cases (shallow
seas) comparable solutions can be obtained. The theory of Ekman is important
in understanding processes like Ekman transport and upwelling. (Figure see p. 108)
Ekman spiral
walls of an aquarium, the hull of a ship. Edge effects make experiments with
pelagic organisms very difficult.
edge wave a wave that is trapped near a coast with a sloping beach. The rays of
this wave are gradually refracted towards the coast. For a given coastal angle
of incidence there will be a line at some distance along the coast where the
reflected rays run parallel to the coastline. Inshore of this line there is a pattern of standing (trapped) waves and offshore of it the amplitude will decay
exponentially. ~trapped wave.
eel grass ~sea grass.
EEZ ~Exclusive Economic Zone.
EFZ ~Exclusive Fishery Zone.
Ekman layer the surface layer over which in the case of the Ekman spiral, the current
becomes reduced to lIe (where e is the ground number of the naturallogarithm, 2.71828 ... ) of the surface value. The most important part of the drift current takes place in this layer. An Ekman layer is also present as a boundary layer
that forms the transition between a geostrophic current (~geostrophic balance)
and the near-bottom layer where the bottom friction is the main factor.
Ekman number a nondimensional number, E, that gives the ratio between the
frictional force (~friction) per unit mass to the Coriolis force. It is given by:
E=KlfJ), with K the coefficient of eddy viscosity,fthe Coriolis parameter and La
typical length scale. When the Ekman number is small, frictional forces can
be neglected.
Ekman spiral a solution of the equations of motion ( ~ocean dynamics) for the current
driven by a homogeneous wind stress was in 1905 given by the Swedish oceanographer v.w. Ekman (1874-1954), in order to explain the angle between wind
and ice drift as observed by Nansen on his drift through the North Polar Sea
with the Pram. A condition is that there are no horizontal pressure gradients
(sea surface slopes) and in the most elementary case it is further assumed that
the sea is very deep (no influence of bottom friction) and thus that the wind
stress and the Coriolis force are the only external forces. In large, deep oceans these
conditions are approximated. The result is a current that makes at the surface
an angle with the wind direction; to the right in the northern hemisphere, to
the left in the southern, and that with increasing depth decreases in strength
and further turns away from the wind direction. The point of the current vector thus describes a spatial spiral, the Ekman spiral. For other cases (shallow
seas) comparable solutions can be obtained. The theory of Ekman is important
in understanding processes like Ekman transport and upwelling. (Figure see p. 108)