Changes in the Atmospheric Circulation as Indicator of Climate Change

Abstract

The strength, direction, and steadiness of the prevailing winds are crucial for climate. Winds associated with the atmospheric circulation lead to transports of heat and moisture from remote areas and thereby modify the local characteristics of climate in important ways. Specific names, such as extratropical Westerlies, tropical Trades, and equatorial Doldrums remind us of the significance of winds for the climate of a region and for the human societies living in it. This chapter discusses changes in the structure of the atmospheric circulation and its associated winds that have taken place during recent decades. These changes are best described as poleward displacements of major wind and pressure systems throughout the global three-dimensional atmosphere. The associated trends are important indicators of climate change and are likely to have profound influences on ecosystems and societies. It is focused on two important examples: first, tropical circulation change related to a poleward expansion of the Hadley cell (HC) and second, extratropical circulation change, as manifested by a poleward shift of the zone of high westerly winds in the midlatitudes, also known as an enhanced positive phase of the annular modes (AMs). Although both changes are associated with similar poleward displacements, it still remains to be seen whether the two phenomena are directly connected. The most intriguing challenges regarding the atmospheric circulation and climate change are to understand what the nature of this change is, what the consequences for surface climate are, and what the underlying causes and mechanisms are.