El Nino variability in simple ocean data assimilation (SODA), 1871-2008

A new ocean reanalysis that covers the period from 1871 to 2008 is used to explore the time-evolving characteristics of El Nino. The new reanalysis assimilates all available hydrographic and sea surface temperature data into a model of the global ocean forced with surface boundary conditions from an atmospheric reanalysis that also covers the period from 1871 through 2008. Using traditional measures of El Nino, our reanalysis shows that the timing of El Nino events is in agreement with sea surface temperature reconstructions, but El Nino in the reanalysis is stronger, particularly from 1871 to 1920. A new index based on the first moment of the temperature anomaly is introduced. The new index is used to characterize the strength and location of El Nino events and has the advantage that it is independent of the location of El Nino. Using the new index, El Nino in the reanalysis shows prominent decadal variability of strength but relatively little long-term trend. El Nino events were strong in the last part of the 19th century and first part of the 20th century and again in the latter part of the 20th century, with weak El Nino events in the middle of the 20th century. The location of El Nino also varies considerably, ranging from the western to the eastern Pacific near the coast of South America. However, the null hypothesis that the location of El Nino can be represented as a random distribution about a central longitude of about 140 degrees W cannot be rejected.