ENSO coupling to the equatorialAtlantic: Analysis with an improved recharge oscillator model

Introduction: Observational and modeling studies have examined theinteractions between El Niño-Southern Oscillation (ENSO) and the equatorialAtlantic variability as incorporated into the classical charge-recharge oscillatormodel of ENSO. These studies included the role of the Atlantic in thepredictability of ENSO but assumed stationarity in the relationships, i.e., thatmodels’ coefficients do not change over time. A recent work by the authors haschallenged the stationarity assumption in the ENSO framework but withoutconsidering the equatorial Atlantic influence on ENSO.Methods: The present paper addresses the changing relationship betweenENSO and the Atlantic El Niño using an extended version of the rechargeoscillator model. The classical two-variable model of ENSO is extended byadding a linear coupling on the SST anomalies in the equatorial Atlantic. Themodel’s coefficients are computed for different periods. This calculation isdone using two methods to fit the model to the data: (1) the traditional method(ReOsc), and (2) a novel method (ReOsc+) based on fitting the Fisher’s Ztransform of the auto and cross-correlation functions.Results: We show that, during the 20th century, the characteristic damping rateof the SST and thermocline depth anomalies in the Pacific have decreased intime by a factor of 2 and 3, respectively. Moreover, the damping time of theENSO fluctuations has doubled from 10 to 20 months, and the oscillationperiod of ENSO has decreased from 60-70 months before the 1960s to 50months afterward. These two changes have contributed to enhancing ENSOamplitude. The results also show that correlations between ENSO and theAtlantic SST strengthened after the 70s and the way in which the impact of theequatorial Atlantic is added to the internal ENSO variability.Conclusions: The remote effects of the equatorial Atlantic on ENSO must beconsidered in studies of ENSO dynamics and predictability during specifictime-periods. Our results provide further insight into the evolution of the ENSOdynamics and its coupling to the equatorial Atlantic, as well as an improved toolto study the coupling of climatic and ecological variables.