The changing physical and ecological meanings of North Pacific Ocean climate indices

Abstract

Climate change is likely to change the relationships between commonly used climate indices and underlying patterns of climate variability, but this complexity is rarely considered in studies using climate indices. Here, we show that the physical and ecological conditions mapping onto the Pacific Decadal Oscillation (PDO) index and North Pacific Gyre Oscillation (NPGO) index have changed over multidecadal timescales. These changes apparently began around a 1988/1989 North Pacific climate shift that was marked by abrupt northeast Pacific warming, declining temporal variance in the Aleutian Low (a leading atmospheric driver of the PDO), and increasing correlation between the PDO and NPGO patterns. Sea level pressure and surface temperature patterns associated with each climate index changed after 1988/1989, indicating that identical index values reflect different states of basin-scale climate over time. The PDO and NPGO also show time-dependent skill as indices of regional northeast Pacific ecosystem variability. Since the late 1980s, both indices have become less relevant to physical-ecological variability in regional ecosystems from the Bering Sea to the southern California Current. Users of these climate indices should be aware of nonstationary relationships with underlying climate variability within the historical record, and the potential for further nonstationarity with ongoing climate change.

Keywords: North Pacific Gyre Oscillation; Pacific Decadal Oscillation; climate change; climate index; nonstationary relationship.

Fig. 1.

Changes in North Pacific atmosphere and ocean climate after 1988/1989. (A) Increasing winter SST anomalies. (B) Decreasing temporal variance (SD over 11-y rolling windows) in winter Aleutian Low SLPa values. (C) Increasing correlation between PDO and NPGO indices (correlation over 11-y or 132-mo rolling windows). The vertical dashed lines indicate 1988/1989; trend lines are best stepwise linear (A) or nonparametric (B and C) regressions.

Fig. 2.

Differences in atmospheric forcing of the PDO and NPGO indices before and after 1988/1989. Coefficients (Pa) for regression of SLP (November to January) onto PDO (February to April) index (A) 1950 to 1988, (B) 1989 to 2012, and (C) Difference in era coefficients (1989 to 2012) − (1950 to 1988). (D–F) The same regressions for the NPGO index.

Fig. 3.

Dominant spatial patterns in winter (November to March) North Pacific SST anomalies, 1951 to 2018. Six leading patterns (nodes) from self-organizing map (SOM) analysis. (A and B) Nodes 1 and 2 are PDO-like patterns occurring primarily before 1988/1989, (C–E) nodes 3–5 occur only after 1988/1989, and (F) node 6 is a NPGO-like pattern occurring almost exclusively before 1988/1989.

Fig. 4.

Changing incidence of spatial patterns associated with the PDO and NPGO indices. Years of occurrence for the six nodes from SOM analysis (Fig. 3) plotted against corresponding winter (November to March) PDO–NPGO values.

Fig. 5.

Changes in physical and ecological conditions mapping onto the PDO and NPGO indices. (A) Location of physical (environmental) and biological time series used in analysis. Numbers in parentheses indicate the number of individual time series in each group, spread over season, space, or species. (B–D) Bayesian linear regression results for changes in relationships between regional variables and the PDO and NPGO indices after 1988/1989: posterior distributions of the ratios of slope in era 2 (after 1988/1989) to slope in era 1 (before 1988/1989). Values between −1 and 1 indicate a slope that has become weaker or smaller in magnitude after 1989, values <0 indicate a switch in the sign of the relationship between eras, and values less than −1 or >1 indicate a stronger association (greater slope) in era 2. Separate models were fit to environmental variables (B), productivity for three salmon species (C), and other biology time series (D). The dots indicate posterior medians, light (heavy) horizontal lines indicate 90% (50%) range of distributions, and vertical line indicates ratio of 1, or no change in slope between eras.