Fig. 5: Conceptual model explaining changing PDO–biology relationships through time under long-term secular warming.

This model assumes that a theoretical biological response (for example, salmon abundance) is described by a normal distribution as a function of SST, and that the range of SST anomalies variability associated with the PDO is maintained but shifted in absolute SST with PBP warming. In scenario A, the response positively correlates with the PDO index, as increasing PDO index values correspond to stronger biological responses, consistent with observed salmon catches in the GOA before 1989. In scenario B, with basin-wide warming, PDO variability shifts along the biological response curve towards the thermal optimum, resulting in decorrelation between the PDO index and biological response, as observed for GOA salmon catches from 1989–2013. In scenario C, as warming continues, PDO variability is shifted beyond the thermal optimum, resulting in negative correlations with the PDO index, as is seen for GOA salmon catches from 2014–2019. m is the slope of the linear correlation between PDO index values and the biological response. Salmon image from PhyloPic under a Creative Commons license PDM 1.0.