HR: 11:55h
AN: A22B-09    [Abstracts]
TI: Response of the zonal mean atmospheric circulation to El Nino versus global warming
AU: * Lu, J
EM: jianlu@ucar.edu
AF: NCAR/CGD, 1850 Table Mesa Dr., Boulder, CO 80305, United States
AU: Chen, G
EM: gchenpu@mit.edu
AF: MIT, Building 54, Room 1726, Cambridge, MI 02139, United States
AU: Frierson, D
AF: University of Washington, 408 ATG Bldg. Box 351640, Seattle, WA 98195, United States
AB: The change in the zonal mean atmospheric circulation under global warming is studied in comparison with the response to El Nino forcing, by examining the model simulations conducted for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. In contrast to the strengthening and contraction of the Hadley cell and the equatorward shift of the tropospheric zonal jets in response to El Nino, the Hadley cell weakens and expands poleward, and the jets move poleward in a warmed climate, despite the "El Nino- like" enhanced warming over equatorial central and eastern Pacific. Two feasible mechanisms are proposed for the zonal mean circulation response to global warming: (1) The increase in static stability of the subtropical and mid-latitude troposphere, a result of the quasi-moist adiabatic adjustment to the surface warming, can stabilize the eddy growth on the equatorward side of the storm track and plausibly push the eddy activity and the associated eddy-driven wind and subsidence poleward, leading to the poleward expansion of the Hadley cell and the shift of jet; (2) the strengthening of the mid-latitude wind at the upper-troposphere and lower-stratosphere, arguably a consequence of the rise in the height of the tropopause and the associated increase in the meridional temperature gradient, can increase the phase speed of the eddies emanating from the mid-latitudes, and thus the critical latitudes (where the eddy phase speed matches the background zonal wind, and where the eddies break and extract angular momentum from the thermally driven wind) displace poleward together with the eddy-driven circulation. Both mechanisms are somewhat, if not completely, distinct from those in response to the El Nino condition. The hydrological impacts of global warming also exhibit distinct patterns over the subtropics and mid-latitudes in comparison to the El Nino. Since both mechanisms do not essentially depend on the details of the SST warming in the equatorial oceans and the variety of model physical parameterizations, these extratropical responses to global warming may be of increased credence due to the fact that they are dynamically determined.
DE: 1610 Atmosphere (0315, 0325)
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
DE: 3319 General circulation (1223)
DE: 3337 Global climate models (1626, 4928)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting