Influence of the Aleutian-Icelandic low seesaw on the Arctic Oscillation

The Arctic Oscillation (AO), defined as the leading EOF of monthly extratropical SLP anomalies within the entire cold season, is identified as one of the most dominant variability in the wintertime Northern Hemisphere (NH) circulation. The climatic significance of the AO lies in its strong projection upon other climate variables such as the polar vortex intensity, total column ozone, surface air temperature over Eurasia, precipitation over Europe, and Arctic sea ice. The tropospheric AO (mainly, a polar-midlatitude seesaw over the Euro-Atlantic sector) is strongly coupled with zonally-symmetric seesaw-like oscillation between the polar vortex and its surroundings in the lower stratosphere (namely, the gannular modeh) through a vertical coupling that is manifested in its deep equivalent barotropic structure. The AO (or the NH annular mode) is well represented in the zonally-averaged zonal wind anomalies on the meridional plane, although its spatial pattern in the troposphere has a weak zonally-asymmetric signature.

Recently, we found that the seesaw between the Aleutian and Icelandic lows (AL and IL, respectively) becomes more apparent during late winter (February to mid-March) than in early winter and it exhibits an equivalent barotropic structure extending from the surface to the upper troposphere. Our analysis indicates that the AL-IL seesaw is related to the weak zonally-asymmetric component of the surface AO. After the formation of the AL-IL seesaw in February, the annular mode-like pattern is no longer identified as the leading variability within the troposphere. The annular pattern is modified accordingly in late winter by the superposition of the distinct signature of the AL-IL seesaw upon the annular mode. Nevertheless, because of a particular geographical alignment between the anomalous AL and IL, zonal wind anomalies associated with their late-winter seesaw yield a strong projection on the meridional plane, whose latitudinal profile is almost indistinguishable from the counterpart of the annular mode.

We argue that the AO in the troposphere may be interpreted as a mixed signature of the surface manifestation of the annular mode and the late winter AL-IL seesaw. An implication of our result is that the AO does not necessarily represent the ggenuineh annular mode but it is more or less subject to contamination by the AL-IL seesaw signal. The late-winter formation of an interannual seesaw between the surface AL and IL is shown to significantly modify the characteristics of the leading interannual variability in the tropospheric circulation over the extratropical NH.