General – Circulation

The atmosphere of the Earth has a diverse range of motions. The general circulation refers to the largerscale motions that have horizontal length scales greater than 1000 km and persist for a season or longer. In addition, this term includes all processes necessary to explain sufficiently, or maintain directly, the large-scale circulation. The general circulation encompasses more than simply the movement of the air. Understanding the circulation requires the examination of many other atmospheric quantities. Large-scale circulations are created by imbalances in the radiation fields that lead to temperature gradients that the atmosphere tries to eliminate. The circulations that develop are limited by various physical constraints such as energy balance, mass balance, and angular momentum balance. So, while the primary scope of this article is to descibe the mean properties of the circulation, it is also necessary to consider related variables that are directly observed. The related variables reveal the constraints on the circulation and the underlying laws of dynamics and thermodynamics. Other articles on the general circulation discuss how the circulation is maintained and how it can be simulated. The general circulation of the atmosphere has complex structure in all dimensions as it evolves over the seasons. To make a discussion of this subject manageable, it is traditional to examine first the properties of the general circulation when longitudinal averages are taken. Longitude averages are commonly labeled ‘‘zonal means.’’ Zonal means provide a useful starting point since many atmospheric variables have much symmetry in the longitudinal direction. For example, contours of temperature in the upper troposphere are oriented east–west to a large degree. Zonal means reduce the information that one needs to view in order to visualize the circulation. However, zonal averages miss important phenomena that can be seen in time averages. Time means reveal longitudinal variations that must be taken into account in order to understand both the properties and the maintenance of the zonal mean state. The general circulation undergoes seasonal change. In many fields the seasonal change is much less in the Southern Hemisphere. The difference arises because the middle latitudes in the Southern Hemisphere have a much higher fraction of ocean coverage than those in the Northern Hemisphere. Land and ocean have different thermodynamic properties: heating and cooling are more readily mixed through a greater amount of mass in the ocean than on land; and the albedo of land can change drastically with season, unlike that of the ice-free ocean. These factors magnify seasonal change over land. The difference in middle latitude land coverage has another implication. There are more major mountain ranges in the Northern Hemisphere. The mountains, together with differences that arise between land and sea areas, lead to more prominent planetary waves in the Northern Hemisphere. In contrast, many fields in the Southern Hemisphere tend to be more zonally uniform.