The Arctic Oscillation

The Arctic Oscillation (AO) can be defined as a zonally symmetric seesaw in sea level pressure (SLP) between the polar and temperate latitudes. The number of centers in the temperate latitudes, as well the strength of correlation between SLP in those centers and SLP at the North Pole vary depending on the season, but usually the Atlantic center is the strongest. In April, for example, there are four centers in the temperate latitudes, with the correlation coefficient for the Atlantic center greater than 0.5 (Fig. 1).

The month of April was chosen here for two reasons. First, it was noticed that the correlation coefficients for the Atlantic and Pacific centers of the AO have substantially weakened and even changed signs in recent years (Fig. 2). Second, recent studies suggest that boreal spring AO can exert a significant influence on the outbreak of ENSO events in the following winter and, therefore, could be an effective predictor for those events (Chen, 2014).

To determine if the weakening of the Atlantic and Pacific centers was a gradual process or represented a regime shift, the STARS was applied to both centers using the target p = 0.1 and cut-off length l = 25. All SLP time series in this example are normalized (by their respective standard deviations) anomalies relatively to the 1981-2000 base period. Since no stepwise trends and changes in the variance were detected in those series, the first two steps in the 3-step procedure were skipped.

As shown in Fig. 3, the correlation coefficient between SLP variations at the North Pole and those in the Atlantic center abruptly changed in 1997, from -0.57 during 1948-1996 to 0.20 during 1997-2014. The difference between the correlation coefficients for these two periods is statistically significant at p = 0.006.

The shift in the correlation coefficient for the Pacific center also occurred in 1997 (Fig. 4). Although this center is weaker than the Atlantic one, the shift was almost equally impressive, from -0.41 during 1948-1996 to 0.35 during 1997-2014, which is statistically significant at p = 0.009.

These structural changes in the AO suggest weakening of the zonal SLP gradient and more active air mass exchange between the Arctic and temperate latitudes. It appears that the zonal type of atmospheric circulation that dominated prior to the 1997 shift, was replaced by a meridional one characterized by amplified planetary waves.

The shift in the spring AO was followed by an El Niño event of 1997/98, one of the strongest in recent history. After that event, a regime of cooler background sea-surface temperatures was established in the equatorial Pacific, with no major El Niños but several significant La Niñas. Simultaneously, the Pacific Decadal Oscillation entered a negative (cool) phase that persisted through at least 2013.