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

E E Remsberg

doi:10.5194/acp-15-3739-2015

Abstract

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.

Highlights

The dynamically forced, seasonal circulation in the meridional plane of the stratosphere is balanced by differential radiative heating between Equator and pole
The present analysis study makes use of time series of CH4 mixing ratio data as a function of pressure altitude for 1992–2005 from the Halogen Occultation Experiment (HALOE) aboard the Upper Atmosphere Research Satellite (UARS)
Time series data of the tracer-like molecule CH4 obtained with the UARS HALOE experiment are analyzed using multiple linear regression (MLR) techniques for their periodic and trend terms during 1992– 2005

Summary

Introduction

The dynamically forced, seasonal circulation in the meridional plane of the stratosphere is balanced by differential radiative heating between Equator and pole. That primary circulation is balanced at the middle latitudes by two-way exchanges or mixing processes (Birner and Bönisch, 2011; Garny et al, 2014; Ploeger et al, 2015a). Together, both mechanisms constitute the total or Brewer–Dobson circulation (BDC), generally having an upward component at low latitudes and a return, downward component at the extratropical latitudes of both the Northern Hemisphere and the Southern Hemisphere (e.g., Butchart, 2014; Plumb, 2007; Haynes et al, 1991). It is likely that there is some asymmetry for the BDC in the Northern Hemisphere versus the Southern Hemisphere stratosphere because winter wave forcing and mixing processes are more pronounced and frequent in the Northern Hemisphere (Shepherd, 2007)

Objectives

Results

Conclusion