The summertime annular mode in the Northern Hemisphere and its linkage to the winter mode

Abstract

The seasonal variations of the Northern Hemisphere annular mode (NAM) are investigated through empirical orthogonal function analysis of the zonally averaged geopotential height fields for each individual calendar month. Patterns of the winter and summer NAMs differ not only in the geopotential height fields but also in the mean meridional circulation and eddy structure. The summer NAM has a smaller meridional scale and is displaced poleward as compared to the winter NAM. The antinode on the lower‐latitude side in the summer NAM is at the nodal latitude of the winter NAM. The summer NAM is more strongly related to surface air temperatures over Eurasia than the original Arctic Oscillation. The summer NAM is a wave‐driven internal atmospheric mode that is maintained by both stationary and transient waves. The summer NAM is associated with the Arctic front, polar jet, and storm track around the Arctic Ocean. The winter‐to‐summer linkage described by M. Ogi et al. can be interpreted as a preferred transition from one polarity of the winter annular mode to the same polarity of the summer annular mode. The spring cryosphere, i.e., snow in Eurasia and sea ice in the Barents Sea, plays a supporting role in this transition.