Sea-Level vs. Sea Surface Temperature
Another important type of instrument used to measure El Niņo is the Advanced Very-High Resolution Radiometer (AVHRR). These instruments are also based in space, and convert the observed infrared radiation into Sea Surface Temperature (SST). The SST measured by these satellites is really the temperature on the very thin layer at the top of the water column. Since sea-level change during El Niņo is caused mainly by heating variations, one would expect that SST and sea-level would be very similar.
However, there are significant differences. Ignoring for a moment the sea-level effects of currents, winds, and other forces, recall the previous discussion that much of the early El Niņo signal occurs below the ocean surface. It takes some time for heating at 100 m to reach the surface. However, the changes will be seen in sea-level measured by altimetry or the moored buoys which monitor temperatures to a deep level as they are happening. This means that altimetry can actually see El Niņo signals slightly before the SST measurements can. The difference in time is only slight, about 2 weeks to a month. However, by combining the two measurements, one can get a more complete picture of how El Niņo evolves.
The next four slides will demonstrate this by showing complementary sea-level and SST anomalies for each month during 1997. The fifth slide following will summarize important events during the evolution of the 1997 El Niņo.