Ocean currents can raise sea surface height by up to a metre higher over the surrounding area. Currents can therefore be mapped by measuring height variations. Satellite altimetry, supplying continuous worldwide observations, has been increasing our knowledge of the ocean circulation since the 1978 Seasat mission.
A view of the global ocean circulation shows currents swirling around the hills and valleys at the sea surface. In the Northern Hemisphere, currents flow around hills in a clockwise direction and in an anticlockwise direction (the opposite occurs in the Southern Hemisphere) around valleys. These currents form gyres on either side of the equator. Planetary waves are other large-scale phenomena.
The mesoscale circulation is defined like a class of energetic phenomena of spatial scales ranging from about ten to several hundred kilometers and time scales ranging from a few days to several months. Its forcing mechanisms are mainly instabilities from the large-scale circulation and interactions between currents and bathymetry and the direct forcing by the wind.
Ocean tides represent more than 80% of the variability of the surface in the open ocean. Satellite altimetry provides regular measurements of sea level in the middle of the ocean, which means that forecasting of tides has been improved.
As global temperatures rise, mean sea level is rising with them. Altimetry satellites are a crucial aid for observing variations in mean sea level and can confirm trends and attempt to mitigate their effects.
The ocean is responding to the seasons, as the atmosphere. Just now, in May, it is beginning to warm in the Northern Hemisphere, and to cool in the Southern. The amplitude of the variations in sea level due to the seasons is one of Topex/Poseidon's discoveries. This shows the quantity of heat kept in stock in the ocean, and thus its impact on the climate.
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