El Niño has received a lot of attention this year, mostly due to the fact that several different instruments, both in the ocean and in space, detected signals of an impending El Niño nearly a year before the warming peaked. One of these instruments was a radar altimeter on a spacecraft flying nearly 1300 km (780 mi.) over the ocean called TOPEX/Poseidon (T/P). T/P was launched in September 1992 and is a joint mission between the United States and France. It is managed by NASA's Jet Propulsion Laboratory and the Centre National d'Etudes Spatiales (CNES).

In this presentation, I will describe in general the altimeter measurement and discuss how it can be used to monitor El Niño. In particular, I will discuss how the altimetry signal is related to sea surface temperature, and how it is different, and how altimetry can detect El Niño signals months before peak warming occurs in the eastern Pacific.

To begin with, I would like to show two recent images of the sea-level measured by TOPEX/Poseidon and point out the El Niño signatures. Then, I will discuss the measurements that went into the image and discuss how we know they are really measuring what we think they are.

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