North Atlantic Storm Track Variability and Its Association to the North Atlantic Oscillation and Climate Variability of Northern Europe

Abstract

The primary mode of North Atlantic storm track variability is identified using rotated principal component analysis (RPCA) on monthly fields of root-mean-squares of daily high-pass filtered (2?8-day periods) sea level pressures (SLP) for winters (December?February) 1900?92. It is examined in terms of its association with 1) monthly mean SLP fields, 2) regional low-frequency teleconnections, and 3) the seesaw in winter temperatures between Greenland and northern Europe. The principal storm track component is characterized by high synoptic variability preferring one of two areas at any given time. The northeastern Atlantic center (identified by positive RPCA scores) is characterized by deep cyclones in the area extending from Iceland northeastward to the Norwegian and Barents Seas, whereas the Bay of Biscay center (negative scores) is linked to cyclone activity around that area and into the Mediterranean basin. Combined principal component analysis is used to link the high-frequency storm track pressure variability with that of lower frequencies (monthly mean pressures). In this, the primary storm track pattern is linked to large monthly mean SLP variations around the Bay of Biscay and near northern Scandinavia and the Barents Sea. This pattern does not suggest a strong storm track link to the North Atlantic Oscillation (NAO). Instead, the results presented indicate that the dominant mode of variability in the storm track is associated with low-frequency SLP anomalies in the extreme northeastern Atlantic. When the component scores reach their highest positive values, both the mean Atlantic subpolar low and subtropical high are unusually strong and displaced farther northeast than normal. The pressure field intensifies to the northeast and produces strong zonal flow extending into Europe, bringing abnormally high surface air temperatures as far east as Siberia and below normal temperatures over Greenland and northern Africa (the ?Greenland below? seesaw mode, GB). Besides this eastward extension of the mean pressure field, anomalously high European winter temperatures can also be somewhat less frequently caused by mild return flow around the Siberian high, which is displaced farther west than normal. In this situation the Icelandic low is in its normal Denmark Strait location and cyclones move along the more southerly storm track toward the Mediterranean basin, contributing to the synoptic forcing that helps develop the westward extended high. The NAO appears to be only indirectly linked to the European component of the GB mode of the winter surface air temperature seesaw.