Changes in the Width of the Tropical Belt due to Simple Radiative Forcing Changes in GeoMIP Simulations

Tuesday, July 28, 2015
Dian J Seidel, NOAA, Air Resources Laboratory, College Park, MD, United States, Thomas Birner, CO State Univ-Atmospheric Sci, Fort Collins, CO, United States, Nicholas Davis, Colorado State University, Fort Collins, CO, United States, Sean M Davis, Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States and Simone Tilmes, National Center for Atmospheric Research, Boulder, CO, United States
Abstract:
Various independent lines of observational evidence suggest that the width of the tropical belt increased during the period 1979-2010. Although the estimated rates of change vary considerably depending on the metric used to estimate the width, some estimates are much larger than trends obtained from simulations of this period by global climate models and chemistry-climate models. Those simulations are driven by a wide array of changes in radiative forcings, including greenhouse gases, stratospheric ozone, solar cycle, and aerosols, some having complex space-time structure, which complicates the problem of identifying possible cause(s) of tropical width changes.

Simulations from the Geoengineering Model Intercomparison Project (GeoMIP) offer an opportunity to examine the response of the tropical belt to much simpler, large changes in climate forcings. The GeoMIP experiments include a pre-industrial control run; a quadrupled carbon dioxide (4XCO2) run; and a solar constant reduction run (in which the solar constant is reduced to offset the forcing associated with 4XCO2). Previous studies of these GeoMIP simulations showed significant changes in temperature and precipitation patterns, some of which may be connected to changes in the latitudinal extent of the Hadley circulation and/or positions of the jet streams. We examine these GeoMIP simulations to determine the equilibrium response of the width of the tropical belt to these large changes in climate forcing.

Simulations from several different models indicate that the Hadley cell widens, by up to several degrees latitude in each hemisphere, in response to 4XCO2. Reducing the solar constant effectively negates the 4XCO2 Hadley cell widening. These Hadley cell responses are associated with changes in subtropical static stability and deep tropical tropospheric temperature. In contrast, the locations of the subtropical jets exhibit no coherent response to either 4XCO2 or solar constant reduction. These results suggest that the basic drivers of tropical belt width changes vary depending upon the parameter used to measure it, and that some of the hydrologic cycle changes seen in GeoMIP simulations may be linked to changes in the Hadley cell.

Previous Abstract | Next Abstract >>