The North Pacific Oscillation (NPO) is a dipole oscillation of the sea level pressure (or the atmospheric mass) between the subpolar low and subtropical high over the North Pacific Ocean. A study published in Nature Climate Change showed that the center of the NPO shifted eastward during the recent decades (1995-2014), and thus increased the frequency of abnormal winter weather extremes over North America. The study suggested that the recent zonal displacement of the NPO is linked to the La Nina-like mean state condition in the tropical Pacific. More specifically, the enhanced (suppressed) deep tropical convection over the Maritime continents (central equatorial Pacific), through atmospheric Rossby wave responses, increased the atmospheric mean baroclinicity over the extratropical North Pacific, which further increased the rate of available potential energy conversion that feeds the NPO. Thus, the NPO under the La Nina-like mean state condition grows stronger and expands more to the east across the North America.
Figure 2 from Sung et al. (2019): Observed and simulated horizontal structure of the NPO by decade and corresponding correlation with SAT. a–c, SLP anomalies regressed onto the NPO index (contours) and the correlation coefficients for the NPO and winter SAT (shading) for (a) 1948–1969 (P1), (b) 1970–1994 (P2) and (c) 1995–2014 (P3) periods. d, Running correlation between the NPO and temperature in central NA (120–60° W, 45–60° N, green rectangular region in a) using 25-yr windows. Red indicates values that exceed the 90% confidence level. e,f, As for a–c but describing features for the NPOeast and NPOwest periods of the model, respectively. The contour interval is 1 hPa for the observations and 0.7 hPa for the model, and the zero lines are omitted. The stippled area indicates significant correlation values at the 95% confidence level.
Sung, M.-K., H.-Y. Jang, B.-M. Kim, S.-W. Yeh, Y.-S. Choi, and C. Yoo, 2019: Tropical influence on the North Pacific Oscillation drives winter extremes in North America. Nature Climate Change, 9, 413–418, https://www.nature.com/articles/s41558-019-0461-5
Baxter, S., 2019: Oscillating American winter temperatures. Nature Climate Change, 9, 354–355, https://www.nature.com/articles/s41558-019-0469-x