Decadal North Pacific Bred Vectors in a Coupled GCM

Abstract

A number of recent studies done with simple numerical models suggest that the decadal variability in the extratropical North Pacific Ocean is a result of the excitation of low-frequency ocean basin modes. To test this assumption, low-frequency North Pacific variability was examined using a state-of-the-art coupled general circulation model (CGCM). Earlier studies had shown that slowly varying dynamical modes in a CGCM can be effectively isolated using the breeding technique. In this study, the breeding method was applied to the Center for Ocean-Land-Atmosphere Studies (COLA) anomaly coupled GCM (ACGCM), and it was found that several types of slow modes can be isolated depending on the parameters of the breeding cycle. Tropical bred vector SST and upper-ocean heat content are dominated by the ENSO, which is consistent with the results obtained earlier using other CGCMs. Extratropical bred vector upper-ocean heat content is dominated by oceanic instability localized east of Japan, varying on seasonal-to-interannual time scales, and decadal modes with the large-scale pattern over the central and eastern extratropical North Pacific. Similar to ocean basin modes, the decadal modes have a signature of westward-propagating long baroclinic Rossby waves, but do not exhibit the global imprint typical for global basin modes. The relationship between the decadal bred vectors and the background anomalies is consistent with linear damped dynamics. Presumably, the growth of the decadal bred vectors is due to the atmospheric stochastic forcing, but the existence of extratropical instability on decadal time scales still needs to be verified.