Interannual Seesaw between the Aleutian and Icelandic Lows. Part II: Its Significance in the Interannual Variability over the Wintertime Northern Hemisphere

Abstract

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 northweste...