An interdecadal change in the influence of the spring Arctic Oscillation on the subsequent ENSO around the early 1970s

Previous studies suggested that the springtime Arctic Oscillation (AO) influences the El Nio-Southern Oscillation (ENSO) outbreak in the following winter. Using the HadISST, HadSLP2r, ERSSTv3b and NCEP-NCAR reanalysis data for the period 1948-2012, this analysis further reveals that the AO-ENSO relationship experienced a pronounced interdecadal shift. The spring AO influence on the subsequent ENSO is weak before 1970; while the influence becomes strong and statistically significant in the 1970s and 1980s. We then compare the spring AO associated circulation, SST and precipitation anomalies between the PRE (1949-1968) and POST (1970-1989) epochs to explore this interdecadal change of the AO-ENSO relationship. The spring AO-related anomalies of atmospheric circulation over the North Pacific mid-latitudes, cyclonic circulation over the subtropical western-central Pacific, and westerly winds in the tropical western-central Pacific are found to be stronger in the POST epoch than in the PRE epoch. The intensity of spring Pacific synoptic-scale eddy activity is seen to experience a significant interdecadal change around the early-1970s from a weak regime to a strong regime. Thus the strength of synoptic-scale eddy feedback to the low frequency flow becomes stronger after 1970. In the POST epoch, the strong synoptic-scale eddy feedback provides a favorable condition for the formulation of the spring AO-related cyclonic circulation and westerly wind anomalies over the western North Pacific. The tropical SST, precipitation and atmospheric circulation anomalies sustain and develop from spring to winter through the positive Bjerknes feedback, leading to an El Nio-like warming in the tropical central-eastern Pacific in the following winter.