Variations occur in response to seasonal changes in hemispheric temperature differences.
In winter, with a generally strong north-south temperature gradient, the resultant thermally driven winds are much stronger than in summer, when temperature differences are somewhat less pronounced, due to the summer hemisphere receiving more insolation. These complex 'feedback loops' are a common feature in meteorology. The position of these features, plus their semi-permanence, suggest that the massive mountain chains involved are the primary reason for their existence. The definition of atmospheric pressure is the force per unit area due to the weight of the atmosphere above any defined point. The opposite occurs for cooling of a column.