6
Chapter 1: Toward an Ecological Geography of the Sea
infrared portions of the electromagnetic spectrum. The infrared sensor enables solar
radiation at the sea surface to be measured to ±10 Wm
−2 . Apart from High Resolution
Picture transmission as full-resolution data in real time to dedicated ground stations,
AVHRR data are rendered in two formats. Local Area Coverage images are obtained at
1.1 km resolution, but are available only for limited periods and places, whereas Global
Area Coverage data have a resolution of 4 km. Although the primary mission of AVHRR
is to study the solid earth and terrestrial vegetation, SST data are accumulated as weekly
and monthly global integrations that may be consulted or downloaded from several Web
sites. Ten-day global integrations are available for the decade of the 1990s on the WOCE
CD-ROMs.
One of the immediate and (relatively) unsophisticated benefits of the data from the
new sensors has been an unprecedented ability to locate and map thermal fronts at sea
down to the kilometer scale. In this way, there has already been a rapid increase in our
knowledge of the locations of individual fronts, their evolution, and the physical processes
that maintain them. Such information has been particularly valuable in understanding
the nature of fronts at the shelf edge and also those associated with mesoscale eddies in
the open ocean. By inference, and by comparison with chlorophyll images, the ecological
significance of these features is now much better understood. However, it is as well to
bear in mind that SST is not a conservative property and that global, monthly integrations
can provide little more detail than is shown in “old-fashioned” seasonal maps of SST.
Integrated at 15-day intervals, however, the LAC images are striking and contain rich
information on the regional distribution of surface water masses.
The use of active altimetry from space to observe the topography of the sea surface
(SSH) was initiated in 1973 from Skylab and was progressively developed during the
1980s by a series of U.S. Navy missions, including SeaSat and GEOSAT during which
precision and sustainability in space were progressively improved to fully operational
status in the early 1990s. TOPEX-POSEIDON is a U.S.–France observing mission that
is currently in orbit at a height of 1336 km and uses active radar altimetry to measure
the elevation of the surface of the ocean in relation to the geoid. More than 90% of the
ice-free ocean is observed in each 10-day period, during which 127 orbits are completed.
Sea truth is obtained at 10-day intervals at two sites, one off Corsica and the other off
California, to calibrate the altimeter so that it maintains the desired accuracy of ±3 cm,
compared with 50 cm for SeaSat in 1982. Data have flowed from TOPEX-POSEIDON
since the instrument was launched in 1992 and will be continued by the planned follow-up
mission, Jason 1.
From the details of the shape of the returned radar pulses, useful information is
extracted concerning wind speed (to ±2 m sec
−1 ) and wave height (to ±03 m). From
the slope of the regional sea surface, geostrophic velocity vectors can be quantified. From
the topography represented by anomalies in regional sea surface height in relation to the
geoid, the mesoscale eddy field can be mapped. Maps of each of these products may be
obtained from browse files representing shorter or longer periods at several Web sites.
Personally, I find the University of Colorado global near-real-time SSH data viewer to
be extremely effective; here, the user easily specifies an area and period for viewing as a
color-coded and/or contoured image, invaluable for the interpretation of features in the
sea-surface chlorophyll field.
Perhaps even more importantly for ecological analysis, sea-level anomalies (SLAs)
serve as an inverse proxy for anomalies in the topography of the thermocline: small
changes in sea surface elevation require much larger, inverse changes in the mixed layer
which is relatively deep below an anticyclonic elevation in the sea surface compared
with surrounding areas, and vice versa. This is important because all changes in the
pycnocline depth, even when wind mixing is not the cause, may have consequences for
nutrient availability within the euphotic zone. A recent study in the equatorial Pacific used
Chapter 1: Toward an Ecological Geography of the Sea
infrared portions of the electromagnetic spectrum. The infrared sensor enables solar
radiation at the sea surface to be measured to ±10 Wm
−2 . Apart from High Resolution
Picture transmission as full-resolution data in real time to dedicated ground stations,
AVHRR data are rendered in two formats. Local Area Coverage images are obtained at
1.1 km resolution, but are available only for limited periods and places, whereas Global
Area Coverage data have a resolution of 4 km. Although the primary mission of AVHRR
is to study the solid earth and terrestrial vegetation, SST data are accumulated as weekly
and monthly global integrations that may be consulted or downloaded from several Web
sites. Ten-day global integrations are available for the decade of the 1990s on the WOCE
CD-ROMs.
One of the immediate and (relatively) unsophisticated benefits of the data from the
new sensors has been an unprecedented ability to locate and map thermal fronts at sea
down to the kilometer scale. In this way, there has already been a rapid increase in our
knowledge of the locations of individual fronts, their evolution, and the physical processes
that maintain them. Such information has been particularly valuable in understanding
the nature of fronts at the shelf edge and also those associated with mesoscale eddies in
the open ocean. By inference, and by comparison with chlorophyll images, the ecological
significance of these features is now much better understood. However, it is as well to
bear in mind that SST is not a conservative property and that global, monthly integrations
can provide little more detail than is shown in “old-fashioned” seasonal maps of SST.
Integrated at 15-day intervals, however, the LAC images are striking and contain rich
information on the regional distribution of surface water masses.
The use of active altimetry from space to observe the topography of the sea surface
(SSH) was initiated in 1973 from Skylab and was progressively developed during the
1980s by a series of U.S. Navy missions, including SeaSat and GEOSAT during which
precision and sustainability in space were progressively improved to fully operational
status in the early 1990s. TOPEX-POSEIDON is a U.S.–France observing mission that
is currently in orbit at a height of 1336 km and uses active radar altimetry to measure
the elevation of the surface of the ocean in relation to the geoid. More than 90% of the
ice-free ocean is observed in each 10-day period, during which 127 orbits are completed.
Sea truth is obtained at 10-day intervals at two sites, one off Corsica and the other off
California, to calibrate the altimeter so that it maintains the desired accuracy of ±3 cm,
compared with 50 cm for SeaSat in 1982. Data have flowed from TOPEX-POSEIDON
since the instrument was launched in 1992 and will be continued by the planned follow-up
mission, Jason 1.
From the details of the shape of the returned radar pulses, useful information is
extracted concerning wind speed (to ±2 m sec
−1 ) and wave height (to ±03 m). From
the slope of the regional sea surface, geostrophic velocity vectors can be quantified. From
the topography represented by anomalies in regional sea surface height in relation to the
geoid, the mesoscale eddy field can be mapped. Maps of each of these products may be
obtained from browse files representing shorter or longer periods at several Web sites.
Personally, I find the University of Colorado global near-real-time SSH data viewer to
be extremely effective; here, the user easily specifies an area and period for viewing as a
color-coded and/or contoured image, invaluable for the interpretation of features in the
sea-surface chlorophyll field.
Perhaps even more importantly for ecological analysis, sea-level anomalies (SLAs)
serve as an inverse proxy for anomalies in the topography of the thermocline: small
changes in sea surface elevation require much larger, inverse changes in the mixed layer
which is relatively deep below an anticyclonic elevation in the sea surface compared
with surrounding areas, and vice versa. This is important because all changes in the
pycnocline depth, even when wind mixing is not the cause, may have consequences for
nutrient availability within the euphotic zone. A recent study in the equatorial Pacific used