ENSO‐like interdecadal variability in the Pacific Ocean as simulated in a coupled general circulation model

Abstract

Spatial and temporal structures of interdecadal variability in the Pacific Ocean are investigated using results from an atmosphere‐ocean coupled general circulation model (AOGCM). The model shows a basin‐wide spatial pattern of the principal sea surface temperature (SST) variability similar to the observed one. Both interdecadal and interannual temporal structures of the SST variability agree well between the observation and the AOGCM. On the other hand, a slab ocean model coupled to the same atmospheric model as the AOGCM fails to simulate the observed temporal structure. Therefore the timescale of the coupled variability is associated with dynamical processes in the ocean. A distinct interdecadal mode of the coupled atmosphere‐upper ocean temperature variability is found in the AOGCM, with a spatiotemporal structure coherent with the SST variability. The mode accompanies an El Niño‐Southern Oscillation (ENSO)‐like spatial pattern of SST and the surface wind and behaves like a delayed oscillator in ENSO. A wedge‐shaped anomaly pattern of the upper thermocline temperature is formed in the eastern Pacific, and its northern subtropical signal propagates westward, enhanced by a subtropical wind forcing at the central basin. Arrival of the subtropical signal at the western Pacific around 20°N switches the anomaly of subsurface temperature in the equatorial region through anomalous oceanic heat transport along the western boundary. The travel time of the trans‐Pacific signal in the subtropics appears to be responsible for the timescale of this mode. The AOGCM successfully simulated the second mode of SST with a major variation in the midlatitude North Pacific as in the observed SST. In the upper ocean heat content we found another distinct mode, which is characterized by a midlatitude‐subtropics dipole pattern migrating around the North Pacific subtropical gyre. However, the associated SST variation of this mode shows a poor correspondence in the dominant interdecadal modes for the observed SST.