New Model for Global Atmospheric Circulation

23 August 2008

Schematic representation of the proposed model of global atmospheric circulation. Red and blue arrows show the moist branch of the circulation corresponding to the additional mass transport on moist isentropes, with warm moist air moving toward the mid-latitudes at low levels, rising into the upper troposphere within the storm tracks, and subsiding over the pole and dry cold air moving toward the equator near the surface. Click to enlarge. Credit: AAAS

The process of global atmospheric circulation creates weather patterns and influences the climate of the planet. Understanding this process is important in order to predict weather events, and to improve and test climate models. New research published in the 22 August edition of the journal Science concludes that the current models of air circulation are incomplete, and that air circulates above the Earth in four distinct cells, rather than two.

Previous theories have claimed that there are just two large circular systems of air in the atmosphere, one on either side of the equator. These theories suggested that air rises at the equator and then travels towards either the north or south polar regions, where it falls. The new research suggests instead that there are two cells in both the northern and southern hemispheres. In the first cell, air rises at the equator and then falls in the subtropics. In the second cell, air rises in the mid-latitudes—approximately 30 to 60 degrees north and south of the equator—and then falls in the polar regions.

The researchers say that this second cell of rising air is a mechanism responsible for setting the distribution of temperature and winds in the mid-latitudes which has not been fully appreciated before. The mid-latitudes include the UK, Europe and most of the United States.

Our model suggests that there is a second cell of air in each hemisphere which is characterized by air rising, clouds forming, storms developing and other processes associated with moisture in the air occurring in the mid-latitudes.

—Dr. Arnaud Czaja from Imperial College London’s Department of Physics and the Grantham Institute for Climate Change

The new theory stresses the importance of storms setting the temperature and winds in the mid-latitudes. Current theories to describe weather patterns in the mid-latitudes do not take these moisture-based processes into consideration. Dr. Czaja argues that these theories are therefore incomplete, and that water vapor plays as much of an important role in the weather systems of the mid-latitudes as it does in the tropics, where it is a well-documented driver of weather events.

This revised circulation emphasizes the importance of moist processes in mid-latitude dynamics. Although previous studies have noted some impacts of the moist processes on the isentropic circulation...we have shown that an analysis based on a dry framework systematically underestimates the atmospheric circulation by averaging out the moist branch of the circulation from the total mass transport. The moist branch is closely tied to the latent heat transport, which accounts for roughly half of the poleward energy transports.

Ascent of warm, moist air within the storm tracks occurs through a combination of deep and slantwise convection and results in enhanced precipitation. A key question is to what extent moist processes play an active role in setting the atmospheric lapse rate in mid-latitudes, as has been suggested recently. Without fully answering this question, our findings confirm that the circulation provides an ample supply of warm, moist air that should have a direct impact on the temperature structure in the mid-latitudes.

As Earth’s temperature rises, the amount of water vapor present in the atmosphere is extremely likely to increase as well. Understanding how changes in temperature and humidity affect the dynamics of the storm tracks and, in particular, the mass transport in the two branches of the circulation is a critical issue for better predicting mid-latitude climate in a warmer world.

—Pauluis et al. (2008)

Resources

• Olivier Pauluis, Arnaud Czaja, Robert Korty (2008) The Global Atmospheric Circulation on Moist Isentropes, Science, Vol. 321. no. 5892, pp. 1075 - 1078 doi: 10.1126/science.1159649