12 Oceanic Planetary Waves and Eddies
205
2004), whose concentration has been rising in recent years and is approaching the
desired level of one float for every 3 ◦ × 3 ◦ box.
12.4.2 Westward Propagation in Temperature and Ocean Colour
The existence of a signature of westward propagating features in satellite-derived
Sea Surface Temperature (SST) data has been known for some time. Hill et al.
(2000) were the first to show, using SST data from the ATSR infrared radiometer on
board ERS-1, that this signature was almost ubiquitous and at speeds close to those
expected for planetary waves, therefore strongly supporting the hypothesis that, in
addition to eddies, planetary waves are also visible in these data. This SST signature
is important as influential for the processes of ocean-atmosphere interaction.
An even bigger surprise came to the scientific community when, at the beginning of this century, a couple of studies (Cipollini et al., 2001; Uz et al., 2001)
showed global, unambiguous evidence of wave-like, westward propagating signals
in longitude-time plots of chlorophyll concentrations from ocean colour satellites.
Figure 12.5 shows an example of this at around 32 ◦ N in the North Atlantic. This
prompted several questions on which mechanisms provoke this signature, and more
importantly whether its presence indicates a net effect on primary production and
ultimately on the carbon budget. A number of studies have investigated these issues
in recent years, but this new field of research on the biological effect of planetary
waves has not been completely explored and more surprises could be around the
corner.
The possible mechanisms involved in the generation of the ocean colour signature of planetary waves can be horizontal – like horizontal advection of meridional
(i.e. north–south) gradients of phytoplankton – and/or vertical – like vertical advection of phytoplankton or even upwelling of nutrient due to the passage of the wave,
that in turn stimulates growth: this latter mechanism has been dubbed rototiller effect
(Siegel, 2001). It has also been suggested that the features could be due to convergence/divergence of particles at the surface, with the waves acting as a “hay rake”
(Dandonneau et al., 2003) but there does not seem to be widespread consensus on
this (Killworth, 2004). While little or no impact on production is to be expected
from horizontal and surface mechanisms, the vertical ones are more interesting for
their potential effects on the carbon cycle, so it is crucial to ascertain whether and
where they occur.
To date, the most comprehensive attempt to model the signals due to all the different processes by which planetary waves would impact on the ocean colour field is
the one by Killworth et al. (2004). In parallel to their process modelling, Killworth
et al. (2004) performed a global cross-spectral analysis of satellite-derived SSH and
chlorophyll which allowed them to estimate which processes were taking place in
the real ocean, through the comparison of the observed cross-spectral amplitudes
and phases with those predicted for the various processes. Their conclusion is that
horizontal advection seems to be the dominant mechanism, but vertical mechanisms
cannot be completely ruled out, due to both phase ambiguities between different
205
2004), whose concentration has been rising in recent years and is approaching the
desired level of one float for every 3 ◦ × 3 ◦ box.
12.4.2 Westward Propagation in Temperature and Ocean Colour
The existence of a signature of westward propagating features in satellite-derived
Sea Surface Temperature (SST) data has been known for some time. Hill et al.
(2000) were the first to show, using SST data from the ATSR infrared radiometer on
board ERS-1, that this signature was almost ubiquitous and at speeds close to those
expected for planetary waves, therefore strongly supporting the hypothesis that, in
addition to eddies, planetary waves are also visible in these data. This SST signature
is important as influential for the processes of ocean-atmosphere interaction.
An even bigger surprise came to the scientific community when, at the beginning of this century, a couple of studies (Cipollini et al., 2001; Uz et al., 2001)
showed global, unambiguous evidence of wave-like, westward propagating signals
in longitude-time plots of chlorophyll concentrations from ocean colour satellites.
Figure 12.5 shows an example of this at around 32 ◦ N in the North Atlantic. This
prompted several questions on which mechanisms provoke this signature, and more
importantly whether its presence indicates a net effect on primary production and
ultimately on the carbon budget. A number of studies have investigated these issues
in recent years, but this new field of research on the biological effect of planetary
waves has not been completely explored and more surprises could be around the
corner.
The possible mechanisms involved in the generation of the ocean colour signature of planetary waves can be horizontal – like horizontal advection of meridional
(i.e. north–south) gradients of phytoplankton – and/or vertical – like vertical advection of phytoplankton or even upwelling of nutrient due to the passage of the wave,
that in turn stimulates growth: this latter mechanism has been dubbed rototiller effect
(Siegel, 2001). It has also been suggested that the features could be due to convergence/divergence of particles at the surface, with the waves acting as a “hay rake”
(Dandonneau et al., 2003) but there does not seem to be widespread consensus on
this (Killworth, 2004). While little or no impact on production is to be expected
from horizontal and surface mechanisms, the vertical ones are more interesting for
their potential effects on the carbon cycle, so it is crucial to ascertain whether and
where they occur.
To date, the most comprehensive attempt to model the signals due to all the different processes by which planetary waves would impact on the ocean colour field is
the one by Killworth et al. (2004). In parallel to their process modelling, Killworth
et al. (2004) performed a global cross-spectral analysis of satellite-derived SSH and
chlorophyll which allowed them to estimate which processes were taking place in
the real ocean, through the comparison of the observed cross-spectral amplitudes
and phases with those predicted for the various processes. Their conclusion is that
horizontal advection seems to be the dominant mechanism, but vertical mechanisms
cannot be completely ruled out, due to both phase ambiguities between different