Technical Note: Multi-year Changes in the Brewer-Dobson Circulation from HALOE Methane
Abstract. This study makes use of Halogen Occultation Experiment (HALOE) methane (CH4) in a search for multi-year changes in the Brewer-Dobson Circulation (BDC). Changes in CH4 are determined for three, successive 5-yr time spans from 1992 to 2005, and there are significant differences in them. There is a clear separation for the changes in the northern hemisphere near 30 hPa or at the transition of the shallow and deep branches of the BDC. The CH4 changes were positive and large in the shallow branch following the eruption of Pinatubo, but they then decreased and agreed with tropospheric trends in the late 1990s and early 2000s. CH4 decreased in the upper part of the deep branch from 1992 to 1997 or following the eruption of Pinatubo. CH4 continued to decrease in the deep branch in the late 1990s but then increased in the early 2000s, although the changes were small compared with the seasonal and interannual variations of CH4. Those multi-year changes were due, in part, to wave forcings during 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 CH4 provide effective tracer diagnostics for studies of the nature of the BDC from 1992 to 2005.
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