ENSO Characteristics in SODA and CMIP5

Changes in ENSO from the mid-1800s to the present is explored using an ocean reanalysis (Simple Ocean Data Assimilation: SODA 2.2.6) and historical runs of Coupled Model Intercomparison Project Phase 5 (CMIP5). The ocean reanalysis is constructed from an ensemble of individual ocean reanalyses that span the period from 1865 to 2010. The reanalysis assimilates Sea Surface Temperature (SST) observations and forcing for the reanalysis comes from the 20th Century Reanalysis project (20CRv2). Most CMIP5 historical runs span the period from 1850 to 2005. ENSO in the reanalysis shows a broad range of variability of strength, location, duration, and frequency that extends on time scales from event-to-event to multiple decades. The largest variation of ENSO characteristics is in its strength, with strong ENSO in the latter part of the 19th Century and early 20th Century and again in the latter part of the 20th Century, with weak ENSO in the middle of the 20th Century. There also appears to be two kinds of ENSO in the first half of the record; a long ENSO with duration of about 24 months and a short ENSO with a duration of about 12 months. In the last half of the record the duration of ENSO is more uniform, with duration of about 12-15 months. Despite the considerable variability of ENSO characteristics, there is little long-term trend in any of the characteristics. ENSO in the CMIP5 runs has considerable variability from model to model. Some of the models have remarkably realistic structure, including the location of warming and multi-decadal variation of ENSO strength. Other models show markedly different behavior when compared with the reanalysis. Several of the models place ENSO temperature anomalies too far to the west. All of the models tend to show fairly symmetric behavior between warm and cold events, whereas the reanalysis shows considerable asymmetry between warm and cold events.