12 Oceanic Planetary Waves and Eddies
197
and maintain western boundary currents and they can also affect the phytoplankton
distribution (hence the biology) of the oceans (Killworth et al., 2004), as we will
discuss later.
12.3 Observational Evidence of Planetary Waves and Eddies
Of the two classes of ocean phenomena that we review here, eddies are the one
that is more easily observed. Most eddies have a clear thermal signature, which
makes them observable in SST images, as satellite-borne infrared radiometers have
demonstrated since the late 1970s. Moreover, several other water properties in an
eddy have significantly different values from the surrounding ocean (because an
eddy tends to retain water in its core as it propagates), which along with their limited
spatial scale (a few hundred km at maximum) makes them visible in hydrographic
sections from ships and has allowed them to be studied extensively since the 1950s.
Detecting planetary waves is more complicated. These waves are essentially internal
waves, characterized by displacements of the isopycnals (levels of constant potential
density) of a few tens of metres, and their signature in the surface elevation if of the
order of just a few cm over length scales of hundreds of km. Until the early 1990s
there had been only scarce observational evidence of these waves, despite a sound
theoretical consensus on their existence for dynamical reasons (Anderson and Gill,
1975; Pedlosky, 1987; Fu and Chelton, 2001).
The advent of high-accuracy satellite altimetry in the early 1990s, with the
TOPEX/Poseidon and ERS-1 missions, dramatically changed our viewing capabilities of planetary waves and also opened the way for great improvements in
the observation and characterization of eddy dynamics. For the first time we were
able to produce a synoptic, global view of the sea surface elevation, showing the
ubiquity of westward propagating features at multiple scales, with the clearest
signals being initially interpreted as planetary waves in a seminal paper by Chelton
and Schlax (1996).
Due to the westward propagation of planetary waves and eddies, it is sufficient to
plot east–west (zonal) sections of altimetric Sea Surface Height (SSH) at a chosen
latitude (i.e. a section of the data cube in longitude and time, yielding a characteristic
diagram called longitude/time plot or Hovmöller plot), to observe some noticeable
signals that propagate to the west over time, with the range of speeds expected for
eddies and planetary waves. More usually, the quantity plotted is the SSH anomaly
with respect to the local mean, which removes the signature of residual geoid errors
and of the mean ocean currents. Figure 12.1 shows one such longitude-time plot in
the South Pacific at 14 ◦ S. The diagonal features apparent in the diagram (diagonal
alignments of positive and negative anomalies moving to the west over time) are the
surface signatures of planetary waves and eddies, with amplitudes in the range of
∼10 cm and propagation speeds of about 10–15 cm/s. The changing slope of the
alignments in the longitude/time domain indicates some variability in the propagation speed. A closer look at the figure also reveals a multitude of horizontal scales for
the westward propagating features – the most readily visible propagating features
197
and maintain western boundary currents and they can also affect the phytoplankton
distribution (hence the biology) of the oceans (Killworth et al., 2004), as we will
discuss later.
12.3 Observational Evidence of Planetary Waves and Eddies
Of the two classes of ocean phenomena that we review here, eddies are the one
that is more easily observed. Most eddies have a clear thermal signature, which
makes them observable in SST images, as satellite-borne infrared radiometers have
demonstrated since the late 1970s. Moreover, several other water properties in an
eddy have significantly different values from the surrounding ocean (because an
eddy tends to retain water in its core as it propagates), which along with their limited
spatial scale (a few hundred km at maximum) makes them visible in hydrographic
sections from ships and has allowed them to be studied extensively since the 1950s.
Detecting planetary waves is more complicated. These waves are essentially internal
waves, characterized by displacements of the isopycnals (levels of constant potential
density) of a few tens of metres, and their signature in the surface elevation if of the
order of just a few cm over length scales of hundreds of km. Until the early 1990s
there had been only scarce observational evidence of these waves, despite a sound
theoretical consensus on their existence for dynamical reasons (Anderson and Gill,
1975; Pedlosky, 1987; Fu and Chelton, 2001).
The advent of high-accuracy satellite altimetry in the early 1990s, with the
TOPEX/Poseidon and ERS-1 missions, dramatically changed our viewing capabilities of planetary waves and also opened the way for great improvements in
the observation and characterization of eddy dynamics. For the first time we were
able to produce a synoptic, global view of the sea surface elevation, showing the
ubiquity of westward propagating features at multiple scales, with the clearest
signals being initially interpreted as planetary waves in a seminal paper by Chelton
and Schlax (1996).
Due to the westward propagation of planetary waves and eddies, it is sufficient to
plot east–west (zonal) sections of altimetric Sea Surface Height (SSH) at a chosen
latitude (i.e. a section of the data cube in longitude and time, yielding a characteristic
diagram called longitude/time plot or Hovmöller plot), to observe some noticeable
signals that propagate to the west over time, with the range of speeds expected for
eddies and planetary waves. More usually, the quantity plotted is the SSH anomaly
with respect to the local mean, which removes the signature of residual geoid errors
and of the mean ocean currents. Figure 12.1 shows one such longitude-time plot in
the South Pacific at 14 ◦ S. The diagonal features apparent in the diagram (diagonal
alignments of positive and negative anomalies moving to the west over time) are the
surface signatures of planetary waves and eddies, with amplitudes in the range of
∼10 cm and propagation speeds of about 10–15 cm/s. The changing slope of the
alignments in the longitude/time domain indicates some variability in the propagation speed. A closer look at the figure also reveals a multitude of horizontal scales for
the westward propagating features – the most readily visible propagating features