QBO and QTO of atmospheric methane

Abstract

The 12-month running means of methane (CH 4) concentrations at the surface/lower troposphere averaged for latitude groups 1–8 (1, 82–63°N; 2, 56–53°N; 3, 47–36°N; 4, 32–26°N; 5, 20–13°N; 6, 5–14°S, 7, 41°S only; 8, 65–90°S) showed oscillations but the peak spacings were similar to that of 30 mb wind (∼30 months) only for groups 3, 4 and 7. A cross-correlation with 30 mb wind showed maximum good correlations (+0.7) with a phase lag of about 6 months for CH 4 in group 7 (in agreement with Camp et al. [2001. The sensitivity of tropospheric CH 4 to the interannual variability in stratospheric ozone. Chemosphere-Global Change Science 3, 147–156]) and about 12 months for CH 4 in groups 6 (+0.7). A spectral analysis indicated for 30 mb wind, a prominent Quasi-biennial oscillation (QBO) at 2.34 years and a minor peak at 2.06 years. For Southern Oscillation Index (SOI) (T−D), there was a prominent Quasi-triennial oscillation (QTO) peak at 3.6 years, but there was also a less prominent but significant QBO at 2.54 years. All CH 4 groups showed significant QBOs in the range 2.22–2.58 years but also showed more prominent QTOs in the range 3.2–4.1 years, and barely significant peaks in the range 1.6–1.9 years (19–22 months). To test the ozone mechanism proposed by Hamilton and Fan [2000. Effects of the stratospheric QBO on long-lived greenhouse gases in the troposphere. Journal of Geophysical Research 105, 20581–20587] (stratospheric wind QBO affecting stratospheric ozone, in turn affecting the filtered UV, the tropospheric OH radical, and finally tropospheric CH 4), TOMS-SBUV data were analyzed. Many CH 4 and ozone groups showed QBOs in the range 2.22–2.77 years, and lower periodicities in the range 1.60–1.91 years (19–22 months [Tung, K.K., Yang, H., 1994. Global QBO in circulation and ozone. Part I. Reexamination of observational evidence. Journal of Atmospheric Science 51, 2699–2707]), confirming that the ozone mechanism was operative. A disconcerting feature is the QTO of ∼3.5 years, which was present in CH 4 in all latitude groups, but in ozone, only in groups 3, 6, 7.