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Report 33: Model Simulated Northern Winter Cyclone and Anticyclone Activity under a Greenhouse Warming Scenario

Zhang, Yi and Wei-Chyung Wang
March 1996, 26 pp.

Two one-hundred-year equilibrium simulations from the NCAR Community Climate Model (CCM1) are used to investigate the activity of northern winter extratropical cyclones and anticyclones under normal and greenhouse warming climate conditions. The first simulation uses the 1990 observed CO₂, CH₄, N₂O, CFC-11 and CFC-12 concentrations and the second adopts the year 2050 concentrations according to the IPCC business-as-usual scenario. Variables that describe the characteristic properties of the cyclone-scale eddies, such as surface cyclone and anticyclone frequency and band-passed root-mean-square (RMS) of 500 hPa geopotential height, along with the Eady growth rate maximum, form a framework for the analysis of the cyclone and anticyclone activity.

Objective criteria are developed for identifying cyclone and anticyclone occurrence based on the 1000 hPa geopotential height and vorticity fields and are tested using ECMWF analyses. The potential changes of the eddy activity under the greenhouse warming climate are then examined. Results indicate that the activity of cyclone-scale eddies decreases under the greenhouse warming scenario. This is not only reflected in the surface cyclone and anticyclone frequency and the bandpassed RMS of 500 hPa geopotential height, but is also discerned from the Eady growth rate maximum. Based on the analysis, three different physical mechanisms responsible for the decreased eddy activity are discussed: (1) decrease of the extratropical meridional temperature gradient from surface to mid-troposphere; (2) reduction in the land-sea thermal contrast in the east coastal regions of the Asian and North American continents; and (3) increase in the eddy meridional latent heat fluxes. Uncertainties of the results related to the limitations of the model and the model equilibrium simulations are discussed. (pdf file)

UCRL-MI-123395