There are several indices used to monitor the tropical Pacific,
all of which are based on Sea Surface temperature (SST) anomalies averaged across a given region.
In general, the anomalies are computed relative to a base (climatological mean) period of 30 years.
The Niño1+2 SST index, which is obtained from the SST anomalies (SSTAs) averaged in Niño1+2 region (90°W - 80°W, 10°S – 0°),
is an indicator of far eastern tropical Pacific SST condition.
The Niño3 SST index, which is obtained form the SSTAs averaged in Niño3 region (150°W - 90°W, 5°S –5°N),
is an indicator of eastern tropical Pacific SST condition.
The Niño3.4 SST index, which is obtained form the SSTAs averaged in Niño3.4 region (170°W - 120°W, 5°S –5°N),
is an indicator of central-to-eastern tropical Pacific SST condition.
The Niño4 SST index, which is obtained form the SSTAs averaged in Niño4 region (160°E - 150°W, 5°S –5°N),
is an indicator of central tropical Pacific SST condition.
Trenberth, Kevin E., 1997: The Definition of El Niño. Bull. Amer. Meteor. Soc., 78, 2771–2777
Trenberth, K. E., and David P. Stepaniak , 2001: Indices of El Niño evolution. Journal of Climate., 14, 1697-1701
The Indian Ocean Dipole Mode Index (DMI) is defined as the SSTA difference
between the western (50°E-70°E, 10°S-10°N) and southeastern (90°E-110°E, 10°S to equator) tropical Indian Ocean.
Saji, N. H., B. N. Goswami, P. N. Vinayachandran and T. Yamagata,
1999. A Dipole Mode in the tropical Indian Ocean. Nature, 401 (23): 360363.
The Tropical North Atlantic Index is defined as the SSTAs averaged in 5.5°N-23.5°N and 15°W-57.5°W.
The Tropical South Atlantic Index is defined as the SSTAs averaged in Eq-20°S and 10°E-30°W.
Enfield, D.B., A.M. Mestas, D.A. Mayer, and L. Cid-Serrano (1999),
How ubiquitous is the dipole relationship in tropical Atlantic sea surface temperatures, JGR-O, 104, 7841-7848