SOLAR-CYCLE INFLUENCES IN TROPOSPHERIC CIRCULATION

The mean latitude at which surface depressions cross a given longitude in the N.E. Atlantic/N.W. Europe sector during the winter months of the year has been found to show a solar cycle dependence over the last five cycles. The average track crossing the region at latitudes north of 50°N is some 2 1 2° of latitude further south at sunspot maximum than at sunspot minimum. The effect arises from an increase at sunspot maximum in the proportion of depressions that are diverted from a main track across the Norwegian Sea into a subsidiary track further south crossing the North Sea. For the much smaller number of depressions which follow a more southerly route (south of 50°N) towards the Mediterranean there is a suggestion of a latitude change with the solar cycle in the opposite sense to that found for the northerly depressions. This phenomenon has also been examined in relation to the possible steering of cyclone tracks by the mean tropospheric flow at the 500 mb level. The results are discussed in relation to other published work, and some suggestions are made linking the phenomenon with other aspects of solar activity influences on the general circulation of the atmosphere. © 1979.