Fig. 1: The positive relationship between the inter-decadal Pacific Oscillation and the Southern Ocean inter-decadal variability.

a Time series of observed SO (50°S-70°S) annual-mean SST anomalies (black), the normalized Southern Ocean inter-decadal variability (SOIDV) index (red), and normalized inter-decadal Pacific Oscillation (IPO) index (blue) over 1900–2013. The observed SO SST anomalies are referenced to the climatology of 1900–2013. b The first principal pattern, representing the SOIDV spatial pattern, is obtained by applying an EOF to the quadratically detrended, 10-year low-pass-filtered annual-mean SST anomalies over the SO domain, explaining 28.2% of the total variance (ERSST v4). Beyond the SO domain, the associated pattern is obtained by regressing SST anomalies onto the first principal component (representing the SOIDV in a). c the synchronous correlation coefficients with the IPO index. The global SST has been quadratically detrended and 10-year low-pass filtered. All correlation coefficients are multiplied by −1 to facilitate the comparison with recent observed SST trends. d Observed SST trends during 1980–2013. e The 1980–2013 observed SST trends linearly congruent with the IPO-index trend (IPO-congruent). The IPO-congruent trends are computed by multiplying the regression coefficients for the IPO at each grid point in the linear regression of annual-mean SST anomalies against the IPO during 1900–2013 by the observed IPO-index trend over 1980–2013 (see Methods). Stippling indicates regions above the 95% significant level based on a Student’s two-sided t-test (b, c) and the Mann–Kendall test (d, e). The purple lines demarcate the SO domain (50°S-70°S). The observed SST dataset is based on ERSST V4.