Dynamics and impacts of Coastal El Niño events in global climate models

Coastal El Niño events, characterized by anomalous warming confined to the coastal regions of Peru and Ecuador, have a significant impact on regional hydroclimate, often mirroring or surpassing the effects of basin-scale El Niño events. The recent extreme events in boreal spring of 2017 and 2023 have renewed attention to the dynamics of coastal El Niño. However, key uncertainties persist regarding the relative roles of remote versus local wind forcing —through equatorial Kelvin waves and coastal upwelling—or local surface heat flux anomalies in the onset and development of these events. Here, we develop a suite of metrics to identify coastal El Niño events and diagnose their underlying mechanisms in CMIP6 model simulations. More than half of the models either strongly underestimate or overestimate the number of coastal events per century, while the simulated SST and precipitation response patterns also exhibit biases. These biases are associated with basin-scale model discrepancies in simulating tropical climate - such as the zonal temperature gradient and the Intertropical Convergence Zone (ITCZ) - as well as the strength of local feedback processes in the far eastern Pacific. These in turn manifest into considerable model spread in projections of the frequency of coastal events under high-emission scenarios and the resulting precipitation extremes in the affected regions.