Articles | Volume 15, issue 7
Research article
07 Apr 2015
Research article |  | 07 Apr 2015

Methane as a diagnostic tracer of changes in the Brewer–Dobson circulation of the stratosphere

E. E. Remsberg

Abstract. This study makes use of time series of methane (CH4) data from the Halogen Occultation Experiment (HALOE) to detect whether there were any statistically significant changes of the Brewer–Dobson circulation (BDC) within the stratosphere during 1992–2005. The HALOE CH4 profiles are in terms of mixing ratio versus pressure altitude and are binned into latitude zones within the Southern Hemisphere and the Northern Hemisphere. Their separate time series are then analyzed using multiple linear regression (MLR) techniques. The CH4 trend terms for the Northern Hemisphere are significant and positive at 10° N from 50 to 7 hPa and larger than the tropospheric CH4 trends of about 3% decade−1 from 20 to 7 hPa. At 60° N the trends are clearly negative from 20 to 7 hPa. Their combined trends indicate an acceleration of the BDC in the middle stratosphere of the Northern Hemisphere during those years, most likely due to changes from the effects of wave activity. No similar significant BDC acceleration is found for the Southern Hemisphere. Trends from HALOE H2O are analyzed for consistency. Their mutual trends with CH4 are anti-correlated qualitatively in the middle and upper stratosphere, where CH4 is chemically oxidized to H2O. Conversely, their mutual trends in the lower stratosphere are dominated by their trends upon entry to the tropical stratosphere. Time series residuals for CH4 in the lower mesosphere also exhibit structures that are anti-correlated in some instances with those of the tracer-like species HCl. Their occasional aperiodic structures indicate the effects of transport following episodic, wintertime wave activity. It is concluded that observed multi-year, zonally averaged distributions of CH4 can be used to diagnose major instances of wave-induced transport in the middle atmosphere and to detect changes in the stratospheric BDC.

Short summary
Time series of the satellite-observed stratospheric tracer, CH4, are analyzed to see whether they indicate a significant trend for the hemispheric Brewer--Dobson circulation (BDC) for 1992-2005. Trends in CH4 for the lower stratosphere are generally positive and equivalent to those of the troposphere. However, the Northern Hemisphere BDC is clearly accelerated in the mid-stratosphere (20 to 7hPa). Corresponding trends for the Southern Hemisphere are smaller and less significant.
Final-revised paper