318
9 Experimental Methods in Fluid Mechanics
is affected by the ionosphere, the components of the atmosphere and by the
orbit of the satellite. Many of these effects can be eliminated by corrections
introduced for each of the effects.
Geostrophic ocean currents flow with a velocity proportional to the local
transverse slope. For example, the Gulf Stream which transports about 100
million m 3 of water per second has a transverse slope of a meter over several hundred kilometres. Maps of the global one-year mean topography also
show the seasonal variability due to monsoon influence in the Indian Ocean
(Tapley et al., 1994). During the northern winter, due to cooler and denser
air over Southeast Asia than air over the Indian Ocean, a northeasterly wind
(the North-East Monsoon) blows from the continent towards the ocean. After
some months, when the air over the land warms up, the pressure gradient reverses and warm and moist air (the South-West Monsoon) starts to blow from
the ocean to the continent. This shift in wind is responsible for generation
of the current system in the Indian Ocean, which is seasonally variable and
clearly evident in the lO-day dynamic ocean topography anomaly maps from
TOPEX/POSEIDON.
The sequence of topography maps (Tapley et al., 1994) illustrates the development and propagation of equatorial Kelvin waves (with speed of 3 m/s) in
the Pacific, beginning in December 1992. As we described in Sect. 7.6.4, the
travel of Kelvin waves eastward is a key element for the 'delayed action oscillator' scenario of the EI Nino event. These TOPEX/POSEIDON observations are
also consistent with observed changes in the equatorial trade winds and other
in situ observations of strengthening of the 1992 EI Nino (Tapley et al., 1994).
Similar analysis carried out for the East Auckland Current by using the
TOPEX/POSEIDON and ERS-l radar altimeter showed patterns of variability in dynamic sea-surface height consistent with in situ measurements (Laing,
1996). In particular, a strong east-southeastward flowing current between 33.5°
and 34°S, with a return flow which meandered between 32° and 33°S has been
detected. Also, an anticyclonic eddy to the north-east of New Zealand has been
identified moving south-eastward during the spring of 1993.
9.3.5 Sea Surface Temperature Measured by Satellites
Sea surface temperature is an important indicator of the heat content of the
upper ocean, and a tracer of surface velocities. It also controls air-sea exchange
of gases and influences biological activity. Monitoring of Sea Surface Temperature (SST) by satellite observations is derived from a split window algorithm
applied to the brightness temperatures issued from the infrared channels.
Another satellite sensor, known as Advanced Very High Resolution Radiometer (AVHRR) provides an opportunity to identify and describe an upwelling
and frontal activity, which are the cause of high biological productivity (see
Chap. 15). The AVHRR is a four- or five-channel radiometer with channels in
the visible (0.6~0.7 /Lm), near infrared (0.7~1.1 /Lm) and thermal infrared (3.5~
3.9 /Lm, 1O.5~11.5 /Lm, and 11.5~12.5 /Lm). The channel centered at 3.7 /Lm,
9 Experimental Methods in Fluid Mechanics
is affected by the ionosphere, the components of the atmosphere and by the
orbit of the satellite. Many of these effects can be eliminated by corrections
introduced for each of the effects.
Geostrophic ocean currents flow with a velocity proportional to the local
transverse slope. For example, the Gulf Stream which transports about 100
million m 3 of water per second has a transverse slope of a meter over several hundred kilometres. Maps of the global one-year mean topography also
show the seasonal variability due to monsoon influence in the Indian Ocean
(Tapley et al., 1994). During the northern winter, due to cooler and denser
air over Southeast Asia than air over the Indian Ocean, a northeasterly wind
(the North-East Monsoon) blows from the continent towards the ocean. After
some months, when the air over the land warms up, the pressure gradient reverses and warm and moist air (the South-West Monsoon) starts to blow from
the ocean to the continent. This shift in wind is responsible for generation
of the current system in the Indian Ocean, which is seasonally variable and
clearly evident in the lO-day dynamic ocean topography anomaly maps from
TOPEX/POSEIDON.
The sequence of topography maps (Tapley et al., 1994) illustrates the development and propagation of equatorial Kelvin waves (with speed of 3 m/s) in
the Pacific, beginning in December 1992. As we described in Sect. 7.6.4, the
travel of Kelvin waves eastward is a key element for the 'delayed action oscillator' scenario of the EI Nino event. These TOPEX/POSEIDON observations are
also consistent with observed changes in the equatorial trade winds and other
in situ observations of strengthening of the 1992 EI Nino (Tapley et al., 1994).
Similar analysis carried out for the East Auckland Current by using the
TOPEX/POSEIDON and ERS-l radar altimeter showed patterns of variability in dynamic sea-surface height consistent with in situ measurements (Laing,
1996). In particular, a strong east-southeastward flowing current between 33.5°
and 34°S, with a return flow which meandered between 32° and 33°S has been
detected. Also, an anticyclonic eddy to the north-east of New Zealand has been
identified moving south-eastward during the spring of 1993.
9.3.5 Sea Surface Temperature Measured by Satellites
Sea surface temperature is an important indicator of the heat content of the
upper ocean, and a tracer of surface velocities. It also controls air-sea exchange
of gases and influences biological activity. Monitoring of Sea Surface Temperature (SST) by satellite observations is derived from a split window algorithm
applied to the brightness temperatures issued from the infrared channels.
Another satellite sensor, known as Advanced Very High Resolution Radiometer (AVHRR) provides an opportunity to identify and describe an upwelling
and frontal activity, which are the cause of high biological productivity (see
Chap. 15). The AVHRR is a four- or five-channel radiometer with channels in
the visible (0.6~0.7 /Lm), near infrared (0.7~1.1 /Lm) and thermal infrared (3.5~
3.9 /Lm, 1O.5~11.5 /Lm, and 11.5~12.5 /Lm). The channel centered at 3.7 /Lm,