196
P. Cipollini et al.
climate system so complex, intriguing and difficult to understand. Ocean mesoscale
features are one of the most important modes of energy transfer in the oceans as
well as one of the most important reflections of atmospheric forcing of the oceans.
Energy transfers can occur at a range of scales, from the smallest (such as the
transfer of wind energy to the ripples on the sea surface) to the very large (such
as the general overturning circulation that ensures that heat is transferred from the
equator to the poles). Features of the mesoscale have spatial scales of the order of a
few tens to 200 km, periods of the order of 10–100 days and speeds of the order of
a few cm/s.
Features at larger scales than this are sensitive to the gradient of Coriolis parameter with latitude. They are characterized by meridional (north-south) oscillatory
flow with a westward propagation of their phase. Their behaviour can be easily
explained when considering the Earth’s shape and rotation: when a parcel of water
previously at rest, i.e. with no relative vorticity, 1 is displaced northwards (southward) then its planetary vorticity will increase (decrease). In order to conserve the
absolute vorticity it must acquire a negative (positive) relative vorticity, which translates into a counter-clockwise (clockwise) rotation. If a line of particles is subjected
to these motions then the changes in the relative vorticity will induce a net westward
propagation of the disturbance (Killworth and John, 2001). That these features can
propagate at all within the ocean is due to the fact that the ocean behaves as a waveguide: the ocean floor and surface effectively confine energy within these boundaries
thus allowing for energy to propagate horizontally within them (Gill, 1982),
although there is no westward translation of the water mass associated with the
wave propagation. These are called planetary waves (also known as Rossby waves).
Features that propagate westwards can be linear (their propagation speed is
largely independent of their amplitude) or non-linear (speed depends on their amplitude). Planetary waves are nearly linear and predominant at the larger scales,
(300 km or longer). In contrast larger mesoscale eddies in the form of closed rings
(normally of diameters around 100–200 km, see Chelton et al., 2007) propagate with
non-linear characteristics, and transfer mass as they propagate. As we will illustrate in detail later, altimetry can be used very successfully to observe both classes
of phenomena. Their energy dominates the ocean’s energy spectrum at long timescales (Killworth and John, 2001); for example, the kinetic energy associated to
mesoscale eddies alone is more than an order of magnitude greater than the ocean’s
mean (Chelton et al., 2007).
The importance of the westward propagating features within the climate system
cannot be underestimated: they have been linked to major climate oscillations such
as El Niño Southern Oscillation (ENSO) (Jacobson and Spiesberger, 1998; Fu and
Qiu, 2002) and the North Atlantic Oscillation (NAO), they are known to interact
with the Meridional Overturning Circulation (Hirschi et al., 2007), they interact
1 Relative vorticity is the vertical component of the vorticity relative to the earth’s rotating frame
of reference; planetary vorticity is the vorticity due to the earth’s rotation; absolute vorticity is the
sum of the relative and planetary vorticity.
P. Cipollini et al.
climate system so complex, intriguing and difficult to understand. Ocean mesoscale
features are one of the most important modes of energy transfer in the oceans as
well as one of the most important reflections of atmospheric forcing of the oceans.
Energy transfers can occur at a range of scales, from the smallest (such as the
transfer of wind energy to the ripples on the sea surface) to the very large (such
as the general overturning circulation that ensures that heat is transferred from the
equator to the poles). Features of the mesoscale have spatial scales of the order of a
few tens to 200 km, periods of the order of 10–100 days and speeds of the order of
a few cm/s.
Features at larger scales than this are sensitive to the gradient of Coriolis parameter with latitude. They are characterized by meridional (north-south) oscillatory
flow with a westward propagation of their phase. Their behaviour can be easily
explained when considering the Earth’s shape and rotation: when a parcel of water
previously at rest, i.e. with no relative vorticity, 1 is displaced northwards (southward) then its planetary vorticity will increase (decrease). In order to conserve the
absolute vorticity it must acquire a negative (positive) relative vorticity, which translates into a counter-clockwise (clockwise) rotation. If a line of particles is subjected
to these motions then the changes in the relative vorticity will induce a net westward
propagation of the disturbance (Killworth and John, 2001). That these features can
propagate at all within the ocean is due to the fact that the ocean behaves as a waveguide: the ocean floor and surface effectively confine energy within these boundaries
thus allowing for energy to propagate horizontally within them (Gill, 1982),
although there is no westward translation of the water mass associated with the
wave propagation. These are called planetary waves (also known as Rossby waves).
Features that propagate westwards can be linear (their propagation speed is
largely independent of their amplitude) or non-linear (speed depends on their amplitude). Planetary waves are nearly linear and predominant at the larger scales,
(300 km or longer). In contrast larger mesoscale eddies in the form of closed rings
(normally of diameters around 100–200 km, see Chelton et al., 2007) propagate with
non-linear characteristics, and transfer mass as they propagate. As we will illustrate in detail later, altimetry can be used very successfully to observe both classes
of phenomena. Their energy dominates the ocean’s energy spectrum at long timescales (Killworth and John, 2001); for example, the kinetic energy associated to
mesoscale eddies alone is more than an order of magnitude greater than the ocean’s
mean (Chelton et al., 2007).
The importance of the westward propagating features within the climate system
cannot be underestimated: they have been linked to major climate oscillations such
as El Niño Southern Oscillation (ENSO) (Jacobson and Spiesberger, 1998; Fu and
Qiu, 2002) and the North Atlantic Oscillation (NAO), they are known to interact
with the Meridional Overturning Circulation (Hirschi et al., 2007), they interact
1 Relative vorticity is the vertical component of the vorticity relative to the earth’s rotating frame
of reference; planetary vorticity is the vorticity due to the earth’s rotation; absolute vorticity is the
sum of the relative and planetary vorticity.