A comparative climatology of blocking action in the two hemispheres

Blocking action in the northern hemisphere Is a well-documented phenomenon and several climatological studies hate been carried PH which show It to be most commonly initiated in the northeast Atlantic and northeast Pacific oceans. Other features such as its tendency to occur most frequently in spring and the movement of its central location both seasonally and during a single event are well described. In contrast. the phenomenon in the southern hemisphere has teethed little attention. most likely because its manifestations are on average less intense and less long lasting than its counterpart north of the equator. The existing statistics, or blocking anion in both hemispheres are reviewed and compared, although the multiplicity of definitions and a lack of common techniques blur some of the frequency and duration comparisons. Since its occurrence in both hemispheres appears to be determined by selective longitudinal forcing of the large-scale circulation, the temporal and spatial relationships between blocking and the mean phases and amplitudes of the long waves are described. Finally, statistics ate presented which sagged that thermal forcing may play a dominant role in blocking action In both hemispheres.