OCEAN'S SURFACE is far from flat. The Moon's tug creates a great bulge of water that comes ashore as the tide. And thermal changes, such as those caused by ENSO also raise and lower the average sea level. The first indication that El Nio was being supplanted by La Nia were images captured by the TOPEX/Poseidon satellite showing a depressed sea level in the western tropical Pacific, indicating the presence of more dense and cold seawater.
The composite images clearly show the differences in sea level created by ENSO. They are comprised of sea surface topography from TOPEX, sea surface temperature's from NOAA's AVHRR satellite sensor and sea temperature below the surface as measured by the network of TAO moored buoy's. Red is 30 degrees C and blue is 8 degrees C.; the thermocline (the boundary between warm and cold sea water) is the border between the dark blue at the bottom and the cyan. The thermocline exists at 20 degrees C.
The first image, based on data from January 1997 when El Nio was still strengthening shows a sea level rise along the Equator in the eastern Pacific Ocean of up to 34 centimeters with the red colors indicating an associated change in sea surface temperature of up to 5.4 degrees C. The lower image, made at the outset of the current La Nia episode in March 1998 reveals cool water (yellow, blue) at the western margin of the Pacific and a depressed sea level.
TOPEX/Poseidon, launched in August 1992, bounces radar signals off the ocean's surface to get precise measurements of the distance between the satellite and the sea surface. Every ten days, the data is complied to produce a complete map of global ocean topography, the barely perceptible hills and valleys as small as 0.16 inches.
A team headed by R. Steven Nerem of the Center for Space Research at the University of Texas at Austin recently concluded that the ENSO induced changes in sea level are not confined to the Pacific but effect sea level on a global basis. Data indicate that during the 1997-98 El Nio the average sea level rose about eight-tenths of an inch before it returned to normal levels.
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