| Abstract: | When considering physical mechanisms for decadal-timescale climate variability in the North Pacific, it is useful to describe in detail the expected response of the ocean to the chaotic atmospheric forcing. The expected response to this white-noise forcing includes strongly enhanced power in the decadal frequency band relative to higher frequencies, pronounced changes in basin-wide climate that resemble regime shifts, preferred patterns of spatial variability, and a depth-dependent profile that includes variability with a standard deviation of 0.2–0.4°C over the top 50–100 m. Weak spectral peaks are also possible, given ocean dynamics. Detecting coupled ocean–atmosphere modes of variability in the real climate system is difficult against the spectral and spatial structure of this ‘null-hypothesis’ of how the ocean and atmosphere interact, especially given the impossibility of experimentally decoupling the ocean from the atmosphere. Turning to coupled ocean–atmosphere models to address this question, a method for identifying coupled modes by using models of increasing physical complexity is illustrated. It is found that a coupled ocean–atmosphere mode accounts for enhanced variability with a time scale of 20 years/cycle in the Kuroshio extension region of the model's North Pacific. The observed Pacific Decadal Oscillation (PDO) has many similarities to the expected noise-forced response and few similarities to the model's coupled ocean–atmosphere variability. However, model deficiencies and some analyses of observations by other workers indicate that the possibility that part of the PDO arises from a coupled ocean–atmosphere mode cannot be ruled out. |
