Effects of sea surface temperature and greenhouse gas changes on the transport between the stratosphere and troposphere

Abstract

The effects of sea surface temperature (SST) and greenhouse gas (GHG) changes on the mean age-of-air and water vapor are investigated using a state-of-the-art general circulation model (GCM), and general characteristics of tracer transport between the stratosphere and troposphere are analyzed. Downward tracer transport in the northern midlatitude stratosphere is found to be faster than at southern midlatitudes. The global mean downward transport to the troposphere from stratosphere mainly occurs during northern winter and the downward cross-tropopause transport is weakest from August to October. The maximum troposphere mean (TM) age-of-air, derived from an age tracer released near the stratopause (around 1 hPa), can reach 13 years and is much larger than the maximum stratosphere mean (SM) age-of-air derived from an analogous age tracer released in the troposphere, with the SM age-of-air in the Northern Hemisphere being younger than in the Southern Hemisphere. Increased SSTs tend to accelerate upward transport through the stratosphere and slow downward transport in midlatitudes and the tropical stratosphere. In the context of effects of GHG increases and the associated SST increases on the stratosphere mean age-of-air, the GHG effects dominate, i.e., changes in the stratospheric mean age-of-air caused by SST increases only are smaller than those caused by combined changes in SSTs and GHGs. An increase in SSTs enhances the upward Eliasen-Palm (EP) flux in the extratropics. A 7.7% enhancement of tropical upwelling can be caused by a uniform 1.5 K SST increase. When both SST and GHG values are increased to the 2100 conditions, the meridional heat flux decreases in both winter hemispheres (and statistically significantly in the Southern Hemisphere). Meanwhile, the EP flux in the Northern Hemisphere increases significantly and the tropical upwelling is enhanced by 15% compared to the present-day conditions. Copyright 2011 by the American Geophysical Union.