Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.a22b..11s&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #A22B-11
Physics
1610 Atmosphere (0315, 0325), 1620 Climate Dynamics (3309), 3210 Modeling, 8409 Atmospheric Effects (0370)
Scientific paper
All strong equatorial volcanic eruptions during the period of instrumental observations have forced a positive phase of the Arctic Oscillation (AO) for one or two years following each eruption. The conventional view is that the volcanic effect on the AO is caused by aerosol heating in the tropical lower stratosphere that produces a stronger polar vortex that prevents the propagation of planetary waves into the polar stratosphere. A shift from transparent to reflective (for planetary waves) stratosphere changes the "top boundary condition" for the tropospheric flow and affects the tropospheric circulation. Here we study the response of Arctic Oscillation to aerosols and observed ozone changes after the June 15, 1991 Mt. Pinatubo eruption in the SKYHI GCM to test the AO mechanism. An enhanced positive phase of the AO is reproduced in the model when forced with either aerosols or ozone. For the ozone case, stratospheric cooling, caused by ozone depletion in winter and early spring in the north polar region, increases the temperature gradient between the pole and midlatitudes in the lower stratosphere strengthening the polar vortex and the AO. Experiments without aerosol absorption (stratospheric heating) show as strong an AO response as with the total aerosol forcing. This suggests that aerosol stratospheric warming in the tropical lower stratosphere is not the dominant AO mechanism. Stratospheric aerosols can also affect the AO by cooling of the land surface and the lower troposphere. This cooling, which is strongest in low latitudes especially in winter, reduces the tropospheric meridional temperature gradient, which leads to a decrease of the mean zonal energy and amplitudes of planetary waves in the troposphere. The corresponding decrease of decelerating Eliassen-Palm flux into the lower stratosphere causes a strengthening of the polar vortex and triggers the "wave feedback," as previously discussed. We suggest that this mechanism can also be applicable to a long-term AO trend caused by greenhouse gases, because they, due to polar amplification, also weaken the tropospheric temperature gradient.
Hamilton Kathleen
Ramachandran Sujatha
Ramaswamy Venkatachalam
Robock Alan
Schwarzkopf Daniel M.
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