Spatial Variation of the Arctic Oscillation and its Long-term Change

The Arctic Oscillation (AO) is commonly defined by a space-stationary and time-fluctuating result of Empirical Orthogonal Function (EOF) method for sea level pressure (SLP). Since the SLP is a spatiotemporal variable, the AO should be a spatially varying oscillation. The space-stationary EOF pattern is equivalent to a spatial distribution of full-time correlation between AOI and gridded SLP. When the part-time RCC is calculated, the variable spatial patterns of AO-dominant regions emerge. The sequential maps of RCC patterns identify the time-dependent AO-dominant regions to illustrate the spatial variation of the AO.

The spatial pattern of the AO-dominant region changes interannually, which explains the regional difference of SLP in the north hemisphere. Of the RCC patterns in the past 56 years, the oscillation occurred sometimes between the polar and mid-latitude regions, and sometimes between the landmass and ocean. In most cases, the RCC spatial patterns are quite complicated.

The spatiotemporal variation of the RCC is displayed along two longitudinal sections. Along Atlantic-Pacific Section, a stationary seesaw-like oscillation exists between the Arctic and North Atlantic, i.e., the NAO. On the Pacific side, an oscillation occurs between the Arctic and Pacific oceans intermittently with lower correlation and swinging boundary between the positive and negative RCC regions. In the section across the Eurasian and North American continents, the oscillation is unclear, with complicated multiyear variation.

Based on the result that the AO index is negatively correlated with averaged SLP of the Arctic Oscillation Core Region, long-term spatial variation of the AO can be identified by the relative area of positive and negative SLP anomaly regions. The difference of the relative areas (DRA) is chosen as index that shows the dominating SLP anomaly in different stages of the spatial variation. The spatial variation of the AO is divided into three stages naturally and is speculated to correlate with global climate change. Before 1970, the positive SLP anomaly was dominant in the NH, indicating the state before the global warming. During 1971-1995, negative SLP anomaly region was dominant; this is a period when the global warming started but the Arctic did not show obvious warming. From 1996 to present, the areas of the positive and negative SLP anomaly regions are similar and small, showing the sea-ice retreat during the Arctic warming.