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Modulation of hydroxyl variability by ENSO in the absence of external forcing

A. Turner,I. Fung,V. Naik,L. Horowitz,R. Cohen

Published 2018 in Proceedings of the National Academy of Sciences of the United States of America

ABSTRACT

Significance The hydroxyl radical (OH) is central to tropospheric chemistry, but current measurements are insufficient to assess its effects on year-to-year changes in atmospheric methane. We use a 6,000-y control simulation in a global coupled chemistry-climate model to study the natural variability of OH. We find that natural OH variability can produce (unforced) methane trends as large as the observed changes in methane over the last few decades. Additionally, we find a link between OH and La Niña. While we cannot directly measure annual global mean OH, we can use what we know about La Niña to improve our understanding of OH. This may, in turn, improve our understanding of recent methane trends. The hydroxyl radical (OH) is the primary oxidant in the troposphere, and the impact of its fluctuations on the methane budget has been disputed in recent years, however measurements of OH are insufficient to characterize global interannual fluctuations relevant for methane. Here, we use a 6,000-y control simulation of preindustrial conditions with a chemistry-climate model to quantify the natural variability in OH and internal feedbacks governing that variability. We find that, even in the absence of external forcing, maximum OH changes are 3.8 ± 0.8% over a decade, which is large in the context of the recent methane growth from 2007–2017. We show that the OH variability is not a white-noise process. A wavelet analysis indicates that OH variability exhibits significant feedbacks with the same periodicity as the El Niño–Southern Oscillation (ENSO). We find intrinsically generated modulation of the OH variability, suggesting that OH may show periods of rapid or no change in future decades that are solely due to the internal climate dynamics (as opposed to external forcings). An empirical orthogonal function analysis further indicates that ENSO is the dominant mode of OH variability, with the modulation of OH occurring primarily through lightning NOx. La Niña is associated with an increase in convection in the Tropical Pacific, which increases the simulated occurrence of lightning and allows for more OH production. Understanding this link between OH and ENSO may improve the predictability of the oxidative capacity of the troposphere and assist in elucidating the causes of current and historical trends in methane.

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