Spatialand temporal variability of thehydroxyl radical: Understandingthe role of large-scale climate featuresand their influence on OH through its dynamical and photochemical drivers

Abstract. The hydroxyl radical (OH) is the primary atmospheric oxidant, responsible for removing many important trace gases, including methane, from the atmosphere. Although robust relationships between OH drivers and modes of climate variability have been shown, the underlying mechanisms between OH and these climate modes, such as the El Nino Southern Oscillation (ENSO), have not been thoroughly investigated. Here, we use a chemical transport model to perform a 38-year simulation of atmospheric chemistry, in conjunction with satellite observations, to understand the relationship between tropospheric OH and ENSO, Northern Hemispheric modes of variability, the Indian Ocean Dipole, and monsoons. Empirical orthogonal function (EOF) and regression analyses show that ENSO is the dominant mode of global OH variability in the tropospheric column and upper troposphere, responsible for approximately 30 % of the total variance in boreal winter. Reductions in OH due to ENSO are centered over the tropical Pacific and Australia and can be as high as 10–15 % in the tropospheric column. The relationship between ENSO and OH is driven by changes in nitrogen oxides in the upper troposphere and changes in water vapor and O1D in the lower troposphere. While the spatial scale of the relationship between monsoons, other modes of variability, and OH are much smaller than ENSO, local changes in OH can be significantly larger than those caused by ENSO. These relationships also occur in multiple models that participated in the Chemistry Climate Model Initiative (CCMI), suggesting that the dependence of OH interannual variability on these well-known modes of climate variability is robust. Finally, modeled relationships between ENSO and OH drivers – such as carbon monoxide, water vapor, and lightning – closely agree with satellite observations. The ability of satellite products to capture the relationship between OH drivers and ENSO provides an avenue to an indirect OH observation strategy and new constraints on OH variability.