Reduced ENSO variability driven by mean state changes in the Pliocene: insights from PlioMIP

Abstract:

Since the modern geographic distribution of the continents has been established at ~3.5 Ma BP, the Pliocene was the period with the highest temperatures compared to today (+2-3K). This way, the Pliocene is considered useful analog to the end-of-century climate. Here we investigate the ENSO response within the Pliocene using the Pliocene Model Intercomparison Project (PlioMIP). We find that 21 out of 23 models simulated up to 65% reduction in ENSO. The analysis of the Bjerknes stability index indeed indicates increased stability in the Pliocene simulations, however the mechanisms differ among models. Although the BJ index framework successfully describes the feedbacks that act on a developing SST anomaly in the eastern Pacific, it lacks the effect of meridional SST gradients and possible ENSO triggers on its amplitude. We find a robust intermodel relationship between ENSO amplitude change and ITCZ shifts, where the further north the mean ITCZ is displaced, the more ENSO weakens in the PlioMIP models. Additionally, a northward ITCZ shift is also associated with intensified winds in the western Pacific. In this scenario, a northward ITCZ shift reduces the probability of occurrence of deep convection in the central-eastern Pacific and intensified trades winds can possibly nullify the trigger effect of westerly winds anomalies in the Western Pacific. These results constrain possible changes in ENSO amplitude across climates to the ITCZ position.