A question germane to the study of climate is: When does an El Niño/Southern Oscillation (ENSO) event occur?
The Japanese Meteorological Agency(JMA) considers an El Niño (the warm extreme of the ENSO cycle) to be underway when sea surface temperature (SST) in the tropical Pacific Ocean is a minimum of 0.5°C warmer than normal for at least six consecutive months. This definition does not account for what causes the El Niño: a large, down-welling Kelvin wave propagating eastward across the equatorial Pacific Ocean [Busalacchi and O'Brien, 1981]. A new definition of El Niño based on sea level (SL) takes this into account and modifies our understanding of the global climate cycle. (Sea level at tropical locations such as Galapagos Islands can be used as a surrogate variable for thermocline depth.)