Multidecadal ENSO Amplitude Variability in a 1000-yr Simulation of a Coupled Global Climate Model: Implications for Observed ENSO Variability

The amplitude ofthe El Ni~lation(ENSO) can varynaturally over multidecadal time scales and can be influenced by climate change. However, determining the mechanism for this variation is difficult becauseofthepaucityofobservationsoversuchlongtimescales.Usinga1000-yrintegrationofacoupledglobal climate model and a linear stability analysis, it is demonstrated that multidecadal modulation of ENSO amplitudecanbedrivenbyvariations inthegoverningdynamics.Inthismodel, themodulationiscontrolled bythe underlying thermocline feedback mechanism, which in turn is governed by the response of the oceanic thermocline slope across the equatorial Pacific to changes in the overlying basinwide zonal winds. Furthermore, the episodic strengthening and weakening of this coupled interaction is shown to be linked to the slowly varying background climate. In comparison with the model statistics, the recent change of ENSO amplitude in observations appears to be still within the range of natural variability. This is despite the apparent warming trend in themeanclimate.Hence,thisstudysuggeststhatitmaybedifficulttoinferaclimatechangesignalfromchanges in ENSO amplitude alone, particularly given the presently limited observational data.