Background - The Rationale for Jason-1

Measuring Ocean Topography for Understanding and Predicting Climate Change

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 Niño 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 Niño 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.