Projected future changes in the spatiotemporal evolutions of El Niño

Abstract:

Previous studies based on the climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) have suggested an increase in the frequency and amplitude of El Niño - Southern Oscillation (ENSO) and associated land precipitation anomalies in the 21^st century (21C) in response to increasing greenhouse gases. However, future changes in the spatiotemporal evolution of El Niño, especially during the onset and decay phases, have received little attention. This work investigates the spatiotemporal evolution of El Niño events in the late 21C and the dominant drivers that modulate the projected changes using a large ensemble simulation of a couple general circulation model under two greenhouse gases emission scenarios. Our analysis indicates that El Niño is projected (1) to develop earlier in boreal spring, (2) to grow at a faster rate in summer, and (3) to persist longer in the following spring over the eastern and far eastern Pacific. Further analysis suggests that the fast growing and slow dissipating El Niño in the late 21C can be largely explained by the projected changes in the equatorial Pacific atmosphere-ocean mean states and the associated atmosphere-ocean feedback processes (i.e., thermocline, zonal advective and Ekman feedbacks). Additionally, we find a significant projected increase in the stochastic forcing in the form of westerly wind bursts (WWBs), and a significant shift of the WWBs forcing to occur earlier in the year, further supporting the fast growing El Niño in the late 21C. An important implication is that the climate impacts of increasing greenhouse gases may become more significant and persistent, owing to the increased persistence of El Niño events. Another important implication is the lead-time for skillful seasonal El Niño forecasts may be decreased in the late 21C due the faster onset of El Niño events.