Multidecadal Variability of ENSO in a Recharge Oscillator Framework

We use a conceptual recharge oscillator model for the tropical Pacific to identify multidecadal changes in El Niño-Southern Oscillation (ENSO) statistics and dynamics during the observational record. The model, defined by only two variables, sea surface temperature (SST) and warm water volume (WWV), is fitted to the observations for the period 1901-2010. The variability of ENSO has increased during the 20th century. The model simulates similar changes in the variance of SST and WWV. The coupling between SST and WWV is weaker and SST and WWV are more strongly damped during the first half of 20th century. From the 1970s onwards, both observations and model output show that the SST drives WWV anomalies with a 10 month leadtime and that the WWV feedbacks onto the SST with a leadtime of about 8 months. The latter is reminiscent of a recharge-discharge mechanism of the upper ocean heat content. Before the 1970s only the impact of SST on WWV (through implied wind changes) is observed and is reproduced by the model. The periodicity of ENSO has also changed; ENSO has become more frequent changing from a 5-yr periodicity in the beginning of 20th century to a 7-yr periodicity in the recent decades. We find that the full recharge-discharge mechanism of the equatorial upper ocean heat content that characterizes the dynamics of the model is only active from the 1970s onwards and is likely a consequence of a stronger observed coupling between WWV and SST and of the leading role of the thermocline feedback. We also find that the multidecadal changes in ENSO can be partly explained by a global warming signal and a remote impact signal related to the multidecadal variability of tropical Atlantic SSTs.