Abstract
Through analysis of observational data for the period of 1973–94, the late-winter formation of an interannual seesaw between the surface Aleutian and Icelandic lows (AL and IL, respectively) is shown to significantly impact the covariance structure of the leading mode of the interannual variability in the geopotential height field over the extratropical Northern Hemisphere. The tropospheric leading mode for early winter (November–January) is characterized by a polar–midlatitude dipole over the Euro–Atlantic sector with a high degree of the annularity, coupled with the anomalous lower-stratospheric polar vortex. Over the North Pacific, no significant anomalies are associated with this mode. After the formation of the AL–IL seesaw, however, the dipole no longer dominates in the upper-tropospheric variability. The dipole signature is masked in late winter (February–April) by the predominant combined signature of the so-called Pacific–North American pattern and a meridional dipole over the northwestern Atlantic as an upper-level manifestation of the seesaw. Though somewhat less pronounced, the leading mode of the near-surface variability is modified accordingly in late winter by the superposition of the distinct signature of the AL–IL seesaw. The annularity of the leading mode of the tropospheric variability is thus reduced in late winter, particularly at the upper levels. Nevertheless, because of the particular geographical alignment between the anomalous AL and IL, their seesaw changes the zonal wind coherently between the two ocean basins, yielding a strong projection on the meridional plane whose latitudinal profile is almost indistinguishable from the counterpart of the Arctic–midlatitude dipole.
It is argued that what is called the Arctic oscillation in some recent literature, defined as the leading mode of the sea level pressure variability for the entire cold season, may be interpreted as a superposition of the AL–IL seesaw upon a dominant signal of the Arctic–midlatitude dipole. The corresponding leading mode for the upper troposphere primarily represents the variability associated with the seesaw. It is also argued that the late-winter tropospheric variability over the North Atlantic may not necessarily be associated with the Arctic–midlatitude dipole. The remote influence of the North Pacific variability accounts for as much as 30%–50% of the variance in the vicinity of the IL for the data period considered.
Corresponding author address: Dr. Hisashi Nakamura, Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, 113-0033 Japan. Email: hisashi@eps.s.u-tokyo.ac.jp
