Reversals in the usual monthly mean sea level pressure (SLP) gradient over the North Atlantic during winter—an extreme mode of the North Atlantic Oscillation—are documented back to 1867. Such events are typically associated with well-below-normal temperatures in north-west Europe and above-normal precipitation amounts in the western Mediterranean area. Also noted during these events is the tendency for above-normal temperatures in western Greenland and below-normal temperatures in the eastern United States.
Temporal changes in the frequency of SLP reversals relate well to trends in the strength of the zonal wind and the frequency of blocking in the north-east Atlantic area. Part of the fluctuations in temperature in western Europe during the last century can be explained by these changes in circulation. Nevertheless, major trends in winter temperature over this period, particularly the early 20th century rise in temperature, remain when adjustments are made to account for the effects of circulation changes during reversal winters. This suggests that mechanisms other than changes in the frequency of circulation regimes also play an important role in determining hemispheric temperature trends on these time scales.