9 Determining Ocean Circulation and Sea Level from Satellite Altimetry
159
nadir-looking Jason-class altimeter. The combined two swaths plus the nadir observation will provide a swath of width of 130 km for studying the mesoscale and
submesoscale processes over the ocean and the storage and discharge of freshwater
on land.
Because of the finite swath coverage, it takes at least 22 days to cover the Earth
between the inclination latitudes without gaps. The repeat period of SWOT is set
to 22 days. To avoid sun-synchronous orbits for sampling tides, the inclination of
the orbit is set to 78 ◦ . At the oceanic mesoscales and submesoscales, shallow water
tides and internal tides become key concerns for studying ocean circulation. These
tides have not been well resolved by conventional altimetry and pose new challenges for SWOT. As noted in Section 9.7, tides in their own right are important
to the understanding of ocean circulation. The choice of Ka-band for SWOT is primarily for meeting the interferometry measurement requirement because the height
errors are proportional to the ratio of radar wavelength to the length of the mast.
Furthermore, at Ka-band, the range delay caused by the ionospheric free electrons
becomes negligible. To correct for the errors cause by the tropospheric water vapor,
a multi-frequency microwave radiometer will be included in the payload.
9.10 Conclusions
Satellite altimetry has revolutionized oceanography since the 1990s. Precision missions like T/P and its follow-ons have provided the first view of large-scale ocean
circulation, its variability, and the global mean sea level. The new observations have
motivated the advancement in ocean modeling and data assimilation leading to the
development of ocean state estimation for a variety of applications. The discovery
of the high-frequency large-scale variability led to a new view and appreciation of
the barotropic processes in the ocean. The decade-long data record provides the first
global view of the decadal change in ocean circulation and its geographic variability. The capability of detecting the rate of global mean sea level change at a level of
uncertainty less than 1 mm/year represents the state-of-the-art of precision altimetry.
One must realize that T/P and its follow-ons were not designed for reaching this
level of performance. The achievement was made by a dedicated effort of a large
team of scientists and engineers to push the limit of the measurement system. While
altimetry system is being transitioned from research to operation, we must recognize the critical importance of maintaining such a team effort to ensure the precision
and stability of the measurement into the future.
Combined data from multiple altimeters have enabled a wide range of advances
in ocean dynamics. Further, the combination with surface drifter data has led to
the most detailed knowledge of the global ocean general circulation, revealing the
unexpected ubiquitous presence of small-scale striations in ocean currents. It is now
possible to conduct detailed analysis of the balance of vorticity, a high-order computation, of large-scale ocean currents. For the first time, one can track the movement
of ocean eddies around the world’s oceans and determine their pathways and interaction with mean circulation and ocean topography. The analysis of the balance of
159
nadir-looking Jason-class altimeter. The combined two swaths plus the nadir observation will provide a swath of width of 130 km for studying the mesoscale and
submesoscale processes over the ocean and the storage and discharge of freshwater
on land.
Because of the finite swath coverage, it takes at least 22 days to cover the Earth
between the inclination latitudes without gaps. The repeat period of SWOT is set
to 22 days. To avoid sun-synchronous orbits for sampling tides, the inclination of
the orbit is set to 78 ◦ . At the oceanic mesoscales and submesoscales, shallow water
tides and internal tides become key concerns for studying ocean circulation. These
tides have not been well resolved by conventional altimetry and pose new challenges for SWOT. As noted in Section 9.7, tides in their own right are important
to the understanding of ocean circulation. The choice of Ka-band for SWOT is primarily for meeting the interferometry measurement requirement because the height
errors are proportional to the ratio of radar wavelength to the length of the mast.
Furthermore, at Ka-band, the range delay caused by the ionospheric free electrons
becomes negligible. To correct for the errors cause by the tropospheric water vapor,
a multi-frequency microwave radiometer will be included in the payload.
9.10 Conclusions
Satellite altimetry has revolutionized oceanography since the 1990s. Precision missions like T/P and its follow-ons have provided the first view of large-scale ocean
circulation, its variability, and the global mean sea level. The new observations have
motivated the advancement in ocean modeling and data assimilation leading to the
development of ocean state estimation for a variety of applications. The discovery
of the high-frequency large-scale variability led to a new view and appreciation of
the barotropic processes in the ocean. The decade-long data record provides the first
global view of the decadal change in ocean circulation and its geographic variability. The capability of detecting the rate of global mean sea level change at a level of
uncertainty less than 1 mm/year represents the state-of-the-art of precision altimetry.
One must realize that T/P and its follow-ons were not designed for reaching this
level of performance. The achievement was made by a dedicated effort of a large
team of scientists and engineers to push the limit of the measurement system. While
altimetry system is being transitioned from research to operation, we must recognize the critical importance of maintaining such a team effort to ensure the precision
and stability of the measurement into the future.
Combined data from multiple altimeters have enabled a wide range of advances
in ocean dynamics. Further, the combination with surface drifter data has led to
the most detailed knowledge of the global ocean general circulation, revealing the
unexpected ubiquitous presence of small-scale striations in ocean currents. It is now
possible to conduct detailed analysis of the balance of vorticity, a high-order computation, of large-scale ocean currents. For the first time, one can track the movement
of ocean eddies around the world’s oceans and determine their pathways and interaction with mean circulation and ocean topography. The analysis of the balance of