Read all about El Niño at the El Niño Pages of NOAA/PMEL or KNMI.
Figure 1a shows an image of sea
level anomalies on 30 Nov 1997,
clearly illustrating the effect of El Niño: high
sea levels near the coast of Ecuador and Columbia, but also as north as
Mexico; low sea levels in the Western Pacific.
Figure 1b gives the situation on 30 Aug 1998 when the opposite phenomenon is happening. Now sea levels much lower than normal are seen along the Equator. This is as deep as the La Niña became following the 1997/1998 El Niño.
Figure 1c is the most recent snapshot of sea level anomalies (10 Mar 2013). The rising warm pool in the western Pacific suggests that a new El Niño is in the make.
Figure 1. The sea level of the Equatorial Pacific Ocean on 30 Nov 1997 (a, top), on 30 Aug 1998 (b, middle) and on 10 Mar 2013 (c, bottom), based on ERS satellite radar altimeter data spanning a 16-day period centred on this epoch. The CLS01 long-term mean sea level model is used as reference for these sea level anomalies. (Click on any of the maps to get a time-longitude diagram for the latitude indicated by the vertical position of your cursor.)
Another way to quantify the state of the Southern Oscillation is through sea surface temperature (SST). Although sea surface temperature is only measured at few places scattered throughout the tropical Pacific, these measurements are fed into ocean models such that it is possible to estimate the SST anywhere in the tropical Pacific. Using this information as input, the Nino indices were developed. Each of them are determined by the average SST anomalies over a specific area:
Similar to the Nino indices we have created Nino-A indices, based on sea surface height (SSH) anomalies, rather than temperature anomalies. In order to give these values the same range as the Nino indices, the SSH anomalies are normalised by a fixed value of 7 cm. Figure 2 shows in 4 graphs the negative of the SOI index (-SOI) and the respective Nino and Nino-A indices. It appears that the Nino3A index gives a good characterisation of the El Niño/La Niña condition and resembles close the Nino3 and -SOI indices. Note however that the Nino3A index is a straightforward representation of the measured sea surface height and does not require ocean modelling to derive.
Negative of the Southern Oscillation Index (-SOI), and the Nino and Nino-A
indices (see description in text). The monthly SOI and conventional Nino
indices are from NOAA/CPC.
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