5 Mesoscale Dynamics in the Canary Islands Area . . .
101
5.2.2 Properties of Retrieved Geophysical Parameters
The objective of this subsection is to provide a brief overview of how the geophysical parameters retrieved from RS measurements are related to the observation of
mesoscale features in the Canary Islands area. The interested reader is referred
to Robinson (2010) for a more detailed review of the relationship between the
knowledge of mesoscale ocean processes and RS measurements.
SST and Chl-a are frequently used to observe ocean processes. The contrast
between cold and productive waters, normally associated to coastal upwelling areas and cyclonic eddies, and warm and poorer waters, related to oceanic areas
and anticyclonic eddies, frequently makes both these parameters useful to visualize ocean mesoscale features. As for SLA, it is directly related to ocean dynamics.
This connection is set through geostrophic balance, which is expressed as:
u g = −
g
f
Δη
Δy
, v g =
g
f
Δη
Δx
(5.1)
where η is the SLA, f is the Coriolis parameter (that is positive in the Northern
Hemisphere), and u g and v g are the geostrophic velocity anomalies (Ducet et al.
2000). Then, in Northern Hemisphere, anticyclonic eddies, in which water rotates
in a clockwise sense around the eddy center, appear as concave downward in SLA
anomaly maps, while cyclonic eddies, in which water rotates in anticlockwise sense,
appear as concave upward.
The basic correlation between variability of these satellite-derived geophysical
parameters and mesoscale dynamics is not always evident. In the case of infrared
radiometers, the electromagnetic radiation recorded by the sensor comes from a layer
adjacent to the surface that is only 10-20 μm thick. Under strong insolation and
light wind conditions, a highly stable diurnal thermocline is formed, and satellitederived SST does not correspond to mixed layer temperature, which is related to
ocean dynamics. Figure 5.1b shows a diurnal MODIS SST image, with warm wakes
downwind of some of the islands and with a high-temperature feature in the top left
part of the image. These patterns are not observed in the coincident MODIS Chl-a
image shown in Fig. 5.1c, because the signal recorded in the visible spectral interval
is integrated over a deeper layer of water.
Mesoscale features like eddies can be detected in SST (or BT) and Chl-a scenes,
because they can produce strong vertical isopycnal displacements that change the
surface value of these parameters. In a stable eddy in geostrophic balance, this
displacement requires isopycnals (and isotherms) to dip down in the center of an
anticyclonic eddy and upwards in a cyclonic eddy. In order to reach this dynamical
balance, there is initially a downwelling at the center of anticyclonic eddies which
draws down the isotherms, so warmer and nutrient depleted water penetrates deeper
than normal. Subsequently warm water is dynamically maintained in this position.
The converse situation occurs in cyclonic eddies. So anticyclonic (cyclonic) eddies
can show in the center high (low) values of SST and low (high) values of Chl-a as
observed in satellite images.
101
5.2.2 Properties of Retrieved Geophysical Parameters
The objective of this subsection is to provide a brief overview of how the geophysical parameters retrieved from RS measurements are related to the observation of
mesoscale features in the Canary Islands area. The interested reader is referred
to Robinson (2010) for a more detailed review of the relationship between the
knowledge of mesoscale ocean processes and RS measurements.
SST and Chl-a are frequently used to observe ocean processes. The contrast
between cold and productive waters, normally associated to coastal upwelling areas and cyclonic eddies, and warm and poorer waters, related to oceanic areas
and anticyclonic eddies, frequently makes both these parameters useful to visualize ocean mesoscale features. As for SLA, it is directly related to ocean dynamics.
This connection is set through geostrophic balance, which is expressed as:
u g = −
g
f
Δη
Δy
, v g =
g
f
Δη
Δx
(5.1)
where η is the SLA, f is the Coriolis parameter (that is positive in the Northern
Hemisphere), and u g and v g are the geostrophic velocity anomalies (Ducet et al.
2000). Then, in Northern Hemisphere, anticyclonic eddies, in which water rotates
in a clockwise sense around the eddy center, appear as concave downward in SLA
anomaly maps, while cyclonic eddies, in which water rotates in anticlockwise sense,
appear as concave upward.
The basic correlation between variability of these satellite-derived geophysical
parameters and mesoscale dynamics is not always evident. In the case of infrared
radiometers, the electromagnetic radiation recorded by the sensor comes from a layer
adjacent to the surface that is only 10-20 μm thick. Under strong insolation and
light wind conditions, a highly stable diurnal thermocline is formed, and satellitederived SST does not correspond to mixed layer temperature, which is related to
ocean dynamics. Figure 5.1b shows a diurnal MODIS SST image, with warm wakes
downwind of some of the islands and with a high-temperature feature in the top left
part of the image. These patterns are not observed in the coincident MODIS Chl-a
image shown in Fig. 5.1c, because the signal recorded in the visible spectral interval
is integrated over a deeper layer of water.
Mesoscale features like eddies can be detected in SST (or BT) and Chl-a scenes,
because they can produce strong vertical isopycnal displacements that change the
surface value of these parameters. In a stable eddy in geostrophic balance, this
displacement requires isopycnals (and isotherms) to dip down in the center of an
anticyclonic eddy and upwards in a cyclonic eddy. In order to reach this dynamical
balance, there is initially a downwelling at the center of anticyclonic eddies which
draws down the isotherms, so warmer and nutrient depleted water penetrates deeper
than normal. Subsequently warm water is dynamically maintained in this position.
The converse situation occurs in cyclonic eddies. So anticyclonic (cyclonic) eddies
can show in the center high (low) values of SST and low (high) values of Chl-a as
observed in satellite images.