Understanding the sub-seasonal variation in the wintertime AO spatial pattern from the viewpoint of El Niño-Southern Oscillation

The wintertime Arctic Oscillation (AO) manifests a high degree of uncertainty in its spatial structure, which was suggested to be associated with some other extratropical atmospheric variabilities (e.g., the Aleutian-Icelandic Low seesaw and the stratospheric polar vortex). Based on reanalysis and model simulation data in 1950–2019, the present work investigates the possible effects of El Niño-Southern Oscillation (ENSO) on the AO spatial structure. It is found that there exist obvious differences in AO structure between early (November–December) and late (January–February) winter. In early winter, the AO is manifested by its traditional pattern with three action centers, one of which is over the Arctic and the other two are with the opposite sign over the North Atlantic and North Pacific. However, the AO pattern in late winter changes remarkably, resembling a regional North Atlantic Oscillation (NAO) pattern with the absence of its significant Pacific center. The noteworthy sub-seasonal variation in the AO spatial pattern can be largely ascribed to the observed fact that ENSO atmospheric teleconnections to the Northern Hemisphere are significantly enhanced in late winter compared with early winter. The enhanced ENSO-associated teleconnection during late winter can significantly weaken the teleconnectivity of the Arctic center with the Pacific center, resulting in the absence of the AO’s significant Pacific center. The understanding of ENSO impacts on the AO structure has some significance for the AO-associated climate prediction around the North Pacific region.