Contrasting the Eastern Pacific El Niño and the Central Pacific El Niño: Process-based Feedback Attribution

Abstract:

This paper examines the roles of the radiative and non-radiative air-sea coupled thermodynamic processes in modifying the SST anomalies driven by air-sea coupled oceanic dynamic processes. This helps to isolate their contributions to the key differences between the Eastern Pacific (EP) El Niño and the Central Pacific (CP) El Niño. The attribution is achieved by decomposing sea surface temperature (SST) anomalies into partial SST anomalies due to individual processes using a coupled atmosphere-surface climate feedback-responses analysis method.

Oceanic processes induce warming from the central to the eastern equatorial Pacific and cooling over the western basin with the maximum warming center in the central Pacific for both types of El Niño. The processes that act to oppose the oceanic process-induced SST anomalies are surface latent flux, sensible heat flux, cloud, and atmospheric dynamic feedbacks. The cooling due to each of the four negative-feedback processes is strongest in the region where the initial warming due to oceanic processes is maximum. Water vapor feedback is the sole process that acts to enhance the initial warming induced by oceanic processes. The increase of atmospheric water vapor over the eastern Pacific is much stronger for EP El Niño than CP El Niño. Therefore, it is the strong water vapor feedback over the eastern Pacific together with the strong negative feedbacks over the central equatorial Pacific that helps to relocate the maximum warming center from the central Pacific to the eastern basin for EP El Niño.This paper examines the roles of the radiative and non-radiative air-sea coupled thermodynamic processes in modifying the SST anomalies driven by air-sea coupled oceanic dynamic processes. This helps to isolate their contributions to the key differences between the Eastern Pacific (EP) El Niño and the Central Pacific (CP) El Niño. The attribution is achieved by decomposing sea surface temperature (SST) anomalies into partial SST anomalies due to individual processes using a coupled atmosphere-surface climate feedback-responses analysis method.

Oceanic processes induce warming from the central to the eastern equatorial Pacific and cooling over the western basin with the maximum warming center in the central Pacific for both types of El Niño. The processes that act to oppose the oceanic process-induced SST anomalies are surface latent flux, sensible heat flux, cloud, and atmospheric dynamic feedbacks. The cooling due to each of the four negative-feedback processes is strongest in the region where the initial warming due to oceanic processes is maximum. Water vapor feedback is the sole process that acts to enhance the initial warming induced by oceanic processes. The increase of atmospheric water vapor over the eastern Pacific is much stronger for EP El Niño than CP El Niño. Therefore, it is the strong water vapor feedback over the eastern Pacific together with the strong negative feedbacks over the central equatorial Pacific that helps to relocate the maximum warming center from the central Pacific to the eastern basin for EP El Niño.