Recent weakening of the linkage between the spring Arctic Oscillation and the following winter El Niño-Southern Oscillation

Abstract

Previous studies indicated that the spring Arctic Oscillation (AO) is an important extratropical forcing of the occurrence of El Niño-Southern Oscillation (ENSO) in the following winter. This study reveals an interdecadal weakening in the spring AO-winter ENSO connection around the early-1990s and investigates the reason of this change. Before the early-1990s, in association with the positive phase of the spring AO, an anomalous anticyclone appears over the mid-latitude North Pacific, accompanied by an anomalous cyclone over the subtropical North Pacific that is supported by the wave-mean flow interaction. Correspondingly, the sea surface temperature (SST) warming and its associated positive precipitation anomalies develop over the subtropical North Pacific, which play a crucial role in forming and maintaining the westerly wind anomalies over the tropical western Pacific (TWP) from spring to the following summer. The TWP westerly wind anomalies would induce the El Niño event in the following winter. After the early-1990s, by contrast, the Pacific component of the circulation anomalies related to the spring AO is weak and shifts northward. This is followed by weak anomalies of SST and precipitation over the subtropical North Pacific, as well as weak TWP westerly wind anomalies from spring to the following summer. Hence, the spring AO-winter ENSO connection is weak after the early-1990s. The change of the spring AO associated circulation anomalies over the North Pacific around the early-1990s tends to be related to the interdecadal change in the intensity of the Aleutian Low.