Characterization of ENSO in a stratosphere-resolving version of the EC-EARTH model: comparison with observations and with the low-top version

Abstract

A stratosphere-resolving configuration (HIGH-TOP) of the EC-EARTH global climate model (GCM) is used to characterize the El Ni~no-Southern Oscillation (ENSO) and compare it to observations. Despite the state of the art stratosphere, HIGH-TOP still presents some biases in the seasonal cycle and in the ENSO spatial pattern common to other GCMs. The interannual peak in the Ni~no3.4 index power spectrum is broadly captured, but the model is biased towards lower frequencies. To assess the effects of a well-resolved stratosphere in the tropical Pacific and, in particular, on ENSO, HIGH-TOP is compared with a low-top version of the model (LOW-TOP). HIGH-TOP displays systematic and statistically significant higher interannual variability in the central-eastern Pacific, along with an improvement in the ENSO pattern of sea surface temperature (SST) anomalies. The comparison of the ENSO-induced tropospheric circulation anomalies reveals a strengthening of the eastern Pacifc intertropical convergence zone (ITCZ) in HIGH-TOP with respect to LOW-TOP. Additionally, in the equatorial eastern Pacific, HIGH-TOP correctly simulates anomalous upward motion, while LOW-TOP simulates anomalous downward motion that enhances the climatological Walker circulation. The stronger ITCZ and the anomalous ascent in the eastern equatorial Pacific in HIGH-TOP are consistent with the higher total interannual variability in SST and precipitation in HIGH-TOP. Hence, incorporating a well-resolved stratosphere in the EC-EARTH model yields a more realistic representation of the variability in the central-eastern Pacific and tropical ENSO pattern.