The Nature and Causes of Changes in the General Circulation of the Atmosphere (Invited Presentation)

A major theme of the studies led by Professor Donald Johnson involved physical understanding of processes maintaining the time averaged atmospheric circulation. The First Global Atmospheric Research Program (FGGE) considerably advanced the observational basis for such inquiries. Professor Johnson served on the FGGE Advisory Panel, and was also chairman of the First National Workshop on the Global Weather Experiment held in July 1984. There he presented diagnostic results on properties of the general circulation, providing a unique perspective (at least for climate studies) that stemmed from application of isentropic analysis. Its key attribute is that the spatial scale of thermodynamic forcing --- characterized by latent and sensible heating in the tropics and mid-latitudes and radiative cooling at high latitudes---drives a hemispheric scale mean meridional mass circulation. This was subsequently described as a “global monsoonal circulation”, with its thermally direct nature of planetary reach resembling early schematics of George Hadley. In the first part of the presentation, a brief summary of the isentropic perspective is provided that includes a historical context of how it related to existing theories of the general circulation.

Variability of the general circulation on multi-decadal to centennial time scales has become a topic of great interest in climate studies today. It is estimated that the combined radiative forcing related to increases in carbon dioxide, methane, and nitrous oxide is about 2-2.5 W/m2, and that its rate of increase during the industrial revolution has been unprecedented in over 10,000 years. Such disruption of the energy balance requires a change in atmospheric circulation. The presentation next examines new research describing how the general circulation of the atmosphere has changed over the last century focusing on the Hadley circulation, the trade winds, and the zonal mean westerlies. Observed estimates of change are presented using a reanalysis from a state-of-the art data assimilation system and surface pressure observations that have been used to create a four-dimensional global atmospheric dataset covering 1871-2010. This can be viewed in the lineage of the first FGGE reanalysis data that covered 1979 alone. Physical interpretation of changes is provided by presenting results of global climate model experiments that have been forced by specified changes in boundary and external radiative forcings. The relation of changes in the general circulation to conditions at the Earth's surface including statistics of extreme weather events is also briefly discussed.