Changes and mechanisms of long-lived warm blobs in the Northeast Pacific in low-warming climates

Abstract

In the last decade, three persistent warm blob events (2013–2014, 2015, and 2019–2020) in the Northeast Pacific (NEP) have been hotly debated given their substantial effects on climate, ecosystem, and socioeconomy. This study investigates the changes of such long-lived NEP warm blobs in terms of their intensity, duration, structure, and occurrence frequency under Shared Socioeconomic Pathway (SSP) 119 and 126 low-warming scenarios of the Coupled Model Intercomparison Project Phase 6. Results show that the peak timing of the warm blobs shifts from cold season to boreal summer. For the summer-peak warm blobs, their maximum intensity increases by 6.7% (10.0%) under SSP119 (SSP126) scenario, but their duration reduces by 31.0% (20.4%) under SSP119 (SSP126) scenario. In terms of their vertical structure, the most pronounced temperature signal is located at the surface, and their vertical penetration is mostly confined to the mixed layer, which becomes shallower in warming climates. Based on a mixed-layer heat budget analysis, we reveal that shoaling mixed layer depth plays a dominant role in driving stronger intensity of the warm blobs under low-warming scenarios, while stronger magnitude of ocean heat loss after their peaks explains the faster decay and thus shorter duration. Regarding occurrence frequency, the total number of the warm blobs does not change robustly in the low-warming climates. Following the summer peak of the warm blobs, extreme El Niño events may occur more frequently under the low-warming scenarios, possibly through stronger air-sea coupling induced by tropical Pacific southwesterly anomalies.