Distinguishing coupled ocean-atmosphere interactions from background noisein the North Pacific

When considering physical mechanisms for decadal-timescale climate variabil ity in the North Pacific, it is useful to describe in detail the expected r esponse of the ocean to the chaotic atmospheric forcing. The expected respo nse to this white-noise forcing includes strongly enhanced power in the dec adal frequency band relative to higher frequencies, pronounced changes in b asin-wide climate that resemble regime shifts, preferred patterns of spatia l variability, and a depth-dependent profile that includes variability with a standard deviation of 0.2-0.4 degreesC over the top 50-100 m. Weak spect ral peaks are also possible, given ocean dynamics. Detecting coupled ocean- atmosphere modes of variability in the real climate system is difficult aga inst the spectral and spatial structure of this 'null- hypothesis' of how t he ocean and atmosphere interact, especially given the impossibility of exp erimentally decoupling the ocean from the atmosphere. Turning to coupled oc ean-atmosphere models to address this question, a method for identifying co upled modes by using models of increasing physical complexity is illustrate d. It is found that a coupled ocean-atmosphere mode accounts for enhanced v ariability with a time scale of 20 years/cycle in the Kuroshio extension re gion 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 indic ate that the possibility that part of the PDO arises from a coupled ocean-a tmosphere mode cannot be ruled out. (C) 2001 Elsevier Science Ltd. All righ ts reserved.