North Pacific regime shifts: definitions, issues and recent transitions

The many recent publications on regimes and shifts highlight the importance of decadal variability in understanding climate and ecosystems and their connectivity. This paper explores several issues in the application of regime concepts. Even the definition of regimes is unclear, as usage by different authors highlight: (1) displacement or shifts in timeseries, (2) underlying mechanisms, and (3) the distinction between external forcing and internal reorganization of ecosystems. Such differences arise, and cannot be easily resolved, because of the relatively short duration of available physical and biological timeseries, and the complexity of multivariate process in marine systems with unknown variables and relationships. Climate indices often show a rather Gaussian distribution of values with a single mean, rather than clearly separated discrete multiple states. These physical indices can be represented by a red noise long memory process, where the index can, in fact, deviate substantially from the long term mean for multiple years. If we consider changes in timeseries themselves, then climate variables for the North Pacific display shifts near 1977, 1989 and 1998. Recent variability suggests considerable uncertainty in the current state of the North Pacific. Biological variables often show a broader distribution of shifts over time, which is consistent with different types of responses to climate for different ecosystem elements and the importance of time lags in response to changes in physical forcing. Our current understanding of regime shifts is not a deterministic one, and while one can discuss amplitudes and mean duration of regimes, we cannot predict their precise timing other than to say that they will be a main feature of future climate and ecosystem states. While the authors believe that a single definition for regimes is currently not possible, the concept continues to be useful in moving the discussion of ecosystems away from the assumptions of single species and stationary processes.