Spatial variation of the Arctic Oscillation and its long-term change

The Arctic Oscillation (AO) is spatially varied, but lack of study. The authors defined the spatial variation of AO as the temporal variation of the ensemble of grids with sea level pressure (SLP) varying consistent with the AO index in certain time span. The region of the ensemble is called as 'AO-dominant region', identified by the running correlation coefficient of gridded SLP with the AO index. The positive and negative AO-dominant regions show that the SLP oscillated sometimes between the polar and mid-latitude regions and sometimes between land and ocean. Along the Atlantic-Pacific section, the North Atlantic Oscillation exists as a stationary seesaw-like dipole, while on the Pacific side the oscillation is intermittent with lower intensity and swinging boundary. The long-term spatial variation of the AO with three stages is clearly identified by the relative area of the SLP anomaly regions. Positive SLP anomaly area dominated before 1970, showing the state before the global warming. Negative SLP anomaly area dominated during 1971-1995, indicating the effect of global warming before the Arctic warming being apparent. Since 1996 both positive and negative SLP anomaly areas are all small, being possibly caused by the sea-ice retreat during the Arctic warming.