ON THE EFFECTS OF STRATOSPHERIC CIRCULATION CHANGES ON TRACE GAS TRENDS

Substantial decreases in the trends of three primary greenhouse gases, CH4, N2O, and CO2, occurred during 1992. Here we describe a physical mechanism that may help explain these observations. These changes in trends followed the eruption of Mount Pinatubo in June, 1991. We show that if heating from the volcanic aerosols were of sufficient magnitude to significantly increase stratospheric circulation, the response in the troposphere would be a decline in the increasing trends of these trace gases. The decline results from a higher degree of exchange between the stratosphere, which contains lower mixing ratios of all three gases, and the troposphere. Increasing stratospheric circulation is likely to have the largest effect on the mixing ratio trend of N2O, a moderate effect on the trend of CH4, and a minor effect on the trend of CO2. The magnitude of the tropospheric response of each gas to enhancements in both stratospheric circulation and interhemispheric exchange rates is related to the difference between the value of the trend and the value of the net flux between the troposphere and stratosphere and the net flux between hemispheres in the troposphere.