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Abstract

In early 2017 an extreme coastal warming occurred off the coast of Peru, causing heavy rainfalls and flooding over land. Remarkably, during this coastal event, the central Pacific did not show any significant sea surface temperature anomalies. Several studies have investigated Pacific Coastal Niños with a focus on their onset mechanisms. Here we analyze their evolution and decay and in particular their connection to the central equatorial Pacific, using the coupled climate model FOCI (Flexible Ocean Climate Infrastructure). We performed sets of 2-year long sensitivity experiments under pre-industrial conditions in which a coastal warming is generated by a local wind stress anomaly utilizing a partial coupling approach. Once the warming is initiated by reduced upwelling, the wind forcing is switched off and the model can evolve freely, which enables us to investigate the evolution and decay of the warming. By varying the background conditions in terms of equatorial heat content and the timing of the warming, the influences of both the background state of the equatorial Pacific during the Coastal Niño and the seasonality of the coastal warming are investigated. We find that both the local shortwave radiation feedback and the equatorial Ekman response are strongest in boreal spring. Thus, the evolution of Coastal Niños is closely tied to the seasonal cycle. Interannual variations of the oceanic background state can further regulate the strength of different feedbacks.