The variation of El Nino Southern Oscillation (ENSO) events from the mid-nineteenth century until the beginning of the twenty-first century is explored using an ocean reanalysis. Decadal variability and trends in the strength, frequency, duration, and propagation direction of ENSO events is investigated. The hypothesis that there are different types of ENSO and that the location of ENSO is shifting to the west Pacific is also studied. The study uses the latest version of an ocean reanalysis, called SODA 2.2.4 (Simple Ocean Data Assimilation), which covers the period from 1871 through 2008. The reanalysis uses an eddy-permitting resolution model of the ocean forced with boundary conditions available from an atmospheric reanalysis that covers the same period. SODA 2.2.4 assimilates all available hydrographic and surface marine observations of temperature and salinity to produce a "best estimate" of the ocean state in terms of temperature, current, salinity, and sea surface height. A new index based on the first moment of anomalous sea surface temperature over the equatorial Pacific is used to describe the location and strength of warm and cold events. The results show strong decadal variability in the strength of El Nino events but little trend during the period of 1871-2008. The strength of La Nina events has neither prominent decadal variation nor a trend. The index also documents changes in frequency, duration, and location of ENSO events. The study shows that the frequency of El Nino varies considerably over the record. Given the large variance in the period of ENSO it is difficult to reliably determine if there has been a change in the period of El Nino events. The location of warming during El Nino can be described by a normal distribution centered at about 140 degrees W. The strength and frequency of ENSO events have very little trend indicating negligible impact from global warming.
