report/ab33.html

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<h1>Report 33:&nbsp; Model Simulated Northern Winter Cyclone
and Anticyclone Activity under a Greenhouse Warming Scenario</h1>
<ul><b>Zhang, </b>Yi and Wei-Chyung Wang
<br>March 1996, 26 pp.&nbsp;
<hr>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 CO<sub>2</sub>,
CH<sub>4</sub>, N<sub>2</sub>O, 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.
<p>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. <a href="pdf/33.pdf">(pdf
file)</a>
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<p><font size=-1>UCRL-MI-123395</font></p>