Overview

Some aspects of the atmospheric circulation were introduced in Chapters 2 and 3. The present chapter begins by providing a historical perspective. We then make extensive use of data from the NCEP/NCAR reanalysis to provide a “top down” perspective, starting with the stratosphere and moving to the tropopause, troposphere and the surface. The focus is largely on the mean state, aspects of seasonality and key interactions – temporal variability and trends will be examined largely in Chapter 11. Topics introduced here help set the stage for understanding the surface energy and hydrologic budgets in Chapters 5 and 6.

The stratospheric circulation during winter is dominated by a strong polar vortex; along its margin is the polar night jet. The polar night jet decays through spring, to be replaced in summer by a weak circum-Arctic easterly flow. The winter vortex sometimes rapidly breaks down due to sudden stratospheric warmings, the effects of which can impact the troposphere and surface. At 500 hPa, a circumpolar vortex is present year-round. It is strongly asymmetric during winter, with major troughs over eastern North America and eastern Asia and a weaker trough over western Asia. A prominent ridge is located over western North America. The lowest winter pressure heights at 500 hPa are found over the Canadian Arctic Archipelago. This asymmetry is related to orography, large-scale land–ocean distributions and radiative forcing. The mid-tropospheric vortex weakens during spring and summer and becomes more symmetric than its winter counterpart.