Background - The Rationale for Jason-1
Measuring Ocean Topography for Understanding and Predicting
Most of the heat stored in the earth's hydrosphere resides in the
ocean. The upper 3 meters of the ocean contains the same amount
of heat as that stored in the entire atmosphere. This enormous reservoir
is moved around the world by ocean currents, affecting the earth's
climate and its variability in a profoundly complicated manner.
The ocean circulation is fundamentally turbulent, always changing,
and its observation, especially on a global scale for addressing
its climatic impact, has posed a tantalizing challenge to oceanographers
for a long time.
Surface geostrophic currents, strongly linked to the circulation
of the deep ocean, can be determined from the ocean topography,
defined as the height of ocean surface relative to one of the earth's
equi-geopotential surfaces, called the geoid. In addition, ocean
topography directly reflects the heat content of the ocean and its
changes. Knowledge of the ocean topography is thus very useful to
the determination of ocean circulation and its heat transport.
Altimeters for Ocean Topography
The concept of using a spaceborne radar altimeter to measure ocean
topography was formulated in the late 1960s. The concept was first
demonstrated by Seasat (1978), followed by Geosat (1985-89) and
reached its current state-of-the-art via the Joint US/ France TOPEX/POSEIDON
Mission (1992-present). The measurement principle is straightforward
but the challenge is to reach the exceptionally demanding accuracy
and precision at the level of one centimeter for adequately determining
the oceanic transport of mass, heat, freshwater, and chemicals,
to which the earth's climate system is extremely sensitive.
TOPEX/POSEIDON has demonstrated that the time variation of ocean
topography can be determined with an accuracy and precision of a
few centimeters, an order of magnitude improvement over its predecessors.
The five and half years' worth of data from T/P have revolutionized
the way the global ocean is studied. For the first time, the seasonal
cycle and other temporal variabilities of the ocean have been determined
globally with high accuracy, yielding fundamentally important information
for testing ocean circulation models (Stammer et al., 1996; Fu and
Smith, 1996). The characteristics of planetary scale waves of critical
importance to climate have been redefined (Chelton and Schlax, 1996).
The evolution of the state of the global ocean on interannual time
scales is being monitored, providing the first near real-time, global
view of the 1997-98 El Niņo event. New models of global ocean tides
have been developed (Shum et al., 1997) and shed new light on the
mixing mechanism of the deep ocean, which is important for understanding
large-scale ocean circulation and its climatic effects (Munk and
Wunsch, 1998). Furthermore, this new capability has motivated the
booming discipline of data assimilation by ocean models, paving
the way to optimal estimation of the state of the ocean for wide
ranges of both research and applications.
The need for Jason-1
Continuation of the measurement made by TOPEX/POSEIDON is imperative
to the understanding of ocean circulation and its effects on climate.
The ocean retains a memory of past climate states (parts of the
ocean are imprinted with atmospheric conditions up to 1000 years
ago) and it continues to respond to those forces. In addition, much
of the modern climate state has important stochastic elements, meaning
that no particular year or even decade can be regarded as necessarily
reflective of typical conditions. For instance, TOPEX/POSEIDON has
captured three El Niņo events - two mild ones (1992-93 and 1994-95)
and a major one (1997-98), as part of the somewhat different climatic
state of the Pacific Ocean that began in the late 1970s as compared
to the short record of what had come before. To observe and understand
how this climatic state will evolve in the next decade is essential
to the understanding of long-term climate change. TOPEX/POSEIDON
has also established a new capability for monitoring the global
sea level change through the aid of a small network of well surveyed
tide gauges. Such a combination will make the detection of a trend
in sea level rise much more efficient and lead to a timely determination
on whether the sea level rise is accelerating or not. In summary,
the information content of ocean topography measurement makes it
an indispensable component of a global ocean observing system.
Recognizing the importance of continuing ocean topography measurement,
NASA and the French space agency CNES have approved Jason-1 as a
joint US/France follow-on to T/P. It will be launched in 2000.
Material courtesy of:
Lee-Lueng Fu, Jet Propulsion Laboratory,
Carl Wunsch , Massachusetts Institute of Technology
Robert Cheney , NOAA
Chester Koblinsky, NASA Goddard Space Flight Center