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. Regular scanning 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.

Ocean heating up

Overview – Ocean Surface Topography

Topography is the configuration of a surface, including its relief. The topography of the sea surface is influenced by both gravity and ocean circulation.

Artist’s illustration of ocean surface topography

The actual shape of the Earth is not a perfect sphere — it’s not even a perfect ellipsoid (which would account for the effect of its rotation). If you measure the strength of the gravity field across the Earth’s surface, you will find local variations caused by irregularities in the crust, which produce an irregular figure called the geoid — the actual shape of the Earth. To find the surface caused by ocean circulation, we mathematically remove the sea height caused by gravity (the geoid) and create maps of ocean topography. We then calculate the speed and direction of ocean currents similar to how meteorologists use atmospheric pressure maps to calculate winds.

Altimetry is the measurement of the height of the sea surface above (or below) some reference level. Every 10 days, Jason-1 measures heights of more than 90% of the world’s ice-free oceans with the radar altimeter and completes 127 revolutions, or orbits, around the Earth.

To measure sea surface height we need the satellite’s position in its orbit and the height between the satellite and the ocean’s surface. We may also have to correct for variable amounts of water vapor in the lower atmosphere and free electrons in the upper atmosphere. Both of these can delay microwave pulses.

To learn more visit these sites:
http://topex-www.jpl.nasa.gov/overview/overview.html
http://topex-www.jpl.nasa.gov/overview/topography.html

SOURCE:http://www.nasa.gov