Simulated ENSO in a Global Coupled Ocean–Atmosphere Model: Multidecadal Amplitude Modulation and CO2Sensitivity

An analysis is presented of simulated ENSO phenomena occurring in three 1000-yr experiments with a lowresolution (R15) global coupled ocean-atmosphere GCM. Although the model ENSO is much weaker than the observed one, the model ENSO's life cycle is qualitatively similar to the ''delayed oscillator'' ENSO life cycle simulated using much higher resolution ocean models. Thus, the R15 coupled model appears to capture the essential physical mechanism of ENSO despite its coarse ocean model resolution. Several observational studies have shown that the amplitude of ENSO has varied substantially between different multidecadal periods during the past century. A wavelet analysis of a multicentury record of eastern tropical Pacific SST inferred from 18 O measurements suggests that a similar multidecadal amplitude modulation of ENSO has occurred for at least the past three centuries. A similar multidecadal amplitude modulation occurs for the model ENSO (2-7-yr band), which suggests that much of the past amplitude modulation of the observed ENSO could be attributable to internal variability of the coupled ocean-atmosphere system. In two 1000-yr CO 2 sensitivity experiments, the amplitude of the model ENSO decreases slightly relative to the control run in response to either a doubling or quadrupling of CO 2 . This decreased variability is due in part to CO 2 -induced changes in the model's time-mean basic state, including a reduced time-mean zonal SST gradient. In contrast to the weaker overall amplitude, the multidecadal amplitude modulations become more pronounced with increased CO 2 . The frequency of ENSO in the model does not appear to be strongly influenced by increased CO 2 . Since the multidecadal fluctuations in the model ENSO's amplitude are comparable in magnitude to the reduction in variability due to a quadrupling of CO 2 , the results suggest that the impact of increased CO 2 on ENSO is unlikely to be clearly distinguishable from the climate system ''noise'' in the near future-unless ENSO is substantially more sensitive to increased CO 2 than indicated in the present study.