Chaos in the North Pacific: spatial modes and temporal irregularity

The small amount of North Pacific SST variance explained by the first two organized modes, as well as the irregular nature of the decadal-interdecadal variability of SSTs and spatial atmospheric forcing, suggest that a low order, nonlinear, chaotic system of atmospheric/oceanic variability may be acting in the North Pacific. Systems, which visit many states yet tend to return to the vicinity of previously observed patterns, have properties similar to mathematical chaos. Wavelet analyses of the wintertime Pacific Decadal Oscillation, Aleutian Low intensity from 1900, and Sitka air temperatures from 1832 suggest broad-banded time series with irregular oscillatory behavior. The different frequency bands do not appear to be independent: their alignment has resulted in major shifts around 1847, 1880?, 1925, 1945, 1977, and minor shifts in 1958 and 1989. Although the observational time series are too short to prove that the coupled atmosphere and ocean in the North Pacific is chaotic, the notion that it is stable for decadal periods and then exhibits rapid transitions is the basis for our conceptual model. This model suggests that the North Pacific system is less sensitive to external forcing when it is near one of its stable states. At other times the system is more responsive to short-term extreme events such as ENSO, Siberian snow cover, hemispheric atmospheric modes, or local SST anomalies. Increased interannual variability is expected around the times of interdecadal shifts. Recent persistence of La Nina as well as enhanced interannual variability in several other time series, are consistent with a possible shift of the PDO in 1999. (C) 2000 Published by Elsevier Science Ltd.