Earth's oceans are the greatest influence on global climate. Only from space can we observe our vast oceans on a global scale and monitor critical changes in ocean currents and heat storage. Continuous data from satellites like TOPEX/Poseidon and Jason help us understand and foresee the effects of the changing oceans on our climate and on catastrophic climate events such as El Niño and La Niña. View this interactive Flash module about Ocean Surface Topography to understand the important role the oceans play in our daily lives. This wonderful new tool was developed by NASA Oceanography.
Our home, the Earth, experiences some of the most varied and interesting weather in the Solar System. This is because the Earth is actually a water-world, with some 70% of its surface covered by a vast, deep ocean. The world-ocean is a huge sink for solar energy. Through currents the ocean transports this energy as heat, and releases it back into the atmosphere as water vapor, which is then returned back to the oceans and land as rain or snow. This completes the hydrologic cycle.
How can we measure and track changes in the oceanic heat-budget? We need to know both ocean currents and heat storage of the ocean. Like winds blow around the highs and lows of atmospheric pressure, ocean currents flow around highs and lows of oceanic pressure that can be determined from the height of sea surface, called ocean surface topography. Ocean current velocity can thus be computed from the slope of the ocean surface. Furthermore, as water heats up, it expands, and as it cools, it contracts, affecting the height of sea surface as well. Measuring the ocean surface topography thus gives the required information for studying global ocean circulation and the oceans heat budget. Consistent monitoring of the ocean surface to maintain a database of ocean surface topography can help predict short-term changes in weather and longer-term patterns of climate.
Since 1992 NASA, NOAA and our European partners have been tracking global ocean surface topography with joint ocean altimeter satellite missions from an orbit 1336 km above the ocean surface. The spacecrafts' radar altimeters measure the precise distance between the satellite and sea surface. The round-trip travel time of microwave pulses bounced from the spacecraft to the sea surface and back to the spacecraft provides data indicating sea surface height and the topography of the ocean surface. The precise altitude of the satellite is determined by a sophisticated estimation procedure based on instrument systems onboard the satellite and a network of ground recievers across the globe. The details of the shape of the returned radar pulses also give information on wind speed and the wave height.
Ocean altimeter missions monitor large-scale features like Rossby and Kelvin waves, track El Niño's like the large event of 1997-1998 and the subsequent La Niña events, and explores long-term changes such as the Pacific Decadal Oscillation.
The high accuracy of these measurements has made satellite altimetry an efficient method for monitoring the variation of global mean sea level in relation to global climate change. TOPEX/Poseidon was joined in 2001, and later replaced after the conclusion of a Tandem Mission, by Jason-1, which continues to build the database. The two GRACE (the Gravity Recovery and Climate Experiment) spacecrafts, which launched March 2002, are refining global measurements, increasing the utility of all previous altimetry data. The Ocean Surface Topography Mission or Jason-2 (OSTM/Jason-2), launched in June 2008, is taking ocean surface topography measurements into an operational mode for continued climate forecasting research as well as scientific and industrial applications.
A PDF of the brochure Changing Climates: the Ocean Connection (PDF, 2.56 MB) includes information on the early ocean altimeter missions, as well as other NASA Earth observing missions.