Technical note: Multi-year changes in the Brewer-Dobson circulation from Halogen Occultation Experiment (HALOE) methane

This study makes use of Halogen Occultation Experiment (HALOE) methane (CH 4 ) in a search for multi-year changes in the Brewer-Dobson circulation (BDC). Changes in CH 4 are determined for three successive 5-year time spans from 1992 to 2005, and there are significant differences between them. There is a clear separation between the changes in the Northern Hemisphere near 30 hPa and at the transition of the shallow and deep branches of the BDC. The CH 4 changes are positive and large in the shallow branch following the eruption of Pinatubo, but they then decrease and agree with tropospheric trends in the late 1990s and early 2000s. CH 4 decreases in the upper part of the deep branch from 1992 to 1997 or following the eruption of Pinatubo. CH 4 continues to decrease in the deep branch in the late 1990s but then increases in the early 2000s, although those changes are small compared with the seasonal and interannual variations of CH 4 . Multi-year changes are due, in part, to wave forcings during the El Nino-Southern Oscillation (ENSO) of 1997-1998 and beyond and to episodic, sudden stratospheric warming (SSW) events during both time spans. It is concluded that time series of HALOE CH 4 provide effective tracer diagnostics for studies of the nature of the BDC from 1992 to 2005.