Interdecadal variability over the North Pacific in a multi-century climate simulation*

Interdecadal variability in the North Pacific region is investigated in a 500-y control integration of the Hamburg ECHAMcLSG coupled ocean-atmosphere general circulation model. The spectrum is predominantly red, but a significant peak with a period of about 18 y is detected in the spectrum of sea surface temperature (SST). This peak is shown to be associated with an irregular oscillation that involves both the model ocean and atmosphere. The SST, sea-level pressure, and geopotential height at 500 hPa all undergo a primarily standing oscillation with an extensive monopole structure centered near the date line. The surface anticyclone is situated to the northeast of the warm SST anomaly, and there is a small westward tilt with height; temporal changes are approximately in phase. The anomalous surface heat flux accompanying the warm phase of SST is primarily out of the ocean, but is compensated by anomalous warm advection by surface currents, allowing the SST anomaly to persist. Oceanic thermocline anomalies propagate northward in the western Pacific, and lag the atmosphere indicating a disequilibrium with the atmosphere; sub-surface thermal advection appears to play an important role. A comparison is made between the model’s 18-y oscillation and oscillatory components identified in an analysis of the GISST observational SST dataset, which have periods of approximately 6 and roughly 30 y.