Midlatitude atmospheric OH response to the most recent 11-y solar cycle

Abstract

The hydroxyl radical (OH) plays an important role in middle atmospheric photochemistry, particularly in ozone (O 3 ) chemistry. Because it is mainly produced through photolysis and has a short chemical lifetime, OH is expected to show rapid responses to solar forcing [e.g., the 11-y solar cycle (SC)], resulting in variabilities in related middle atmospheric O 3 chemistry. Here, we present an effort to investigate such OH variability using long-term observations (from space and the surface) and model simulations. Groundbased measurements and data from the Microwave Limb Sounder on the National Aeronautics and Space Administration's Aura satellite suggest an ∼7-10% decrease in OH column abundance from solar maximum to solar minimum that is highly correlated with changes in total solar irradiance, solar Mg-II index, and Lyman-α index during SC 23. However, model simulations using a commonly accepted solar UV variability parameterization give much smaller OH variability (∼3%). Although this discrepancy could result partially from the limitations in our current understanding of middle atmospheric chemistry, recently published solar spectral irradiance data from the Solar Radiation and Climate Experiment suggest a solar UV variability that is much larger than previously believed. With a solar forcing derived from the Solar Radiation and Climate Experiment data, modeled OH variability (∼6-7%) agrees much better with observations. Model simulations reveal the detailed chemical mechanisms, suggesting that such OH variability and the corresponding catalytic chemistry may dominate the O 3 SC signal in the upper stratosphere. Continuing measurements through SC 24 are required to understand this OH variability and its impacts on O 3 further.decadal variability | odd hydrogen Q uantifying effects of the solar cycle (SC) in Earth's atmosphere helps differentiate relative contributions of natural processes and anthropogenic activities to global climate change (1). From the 11-y SC maximum (max) to minimum (min), the total solar irradiance (TSI) varies only by ∼0.1%. However, changes in solar UV fluxes can be much larger (2). Thus, detectable SC impacts on Earth's climate are more likely to be linked to changes in middle (stratosphere and mesosphere, tropopause to ∼90 km) and upper (thermosphere and above) atmospheric composition through photochemistry in the UV region.A number of observational and modeling studies have characterized SC modulations in mesospheric and stratospheric chemistry, especially in ozone (O 3 ) (3-9). Changes in UV absorption by O 3 at low latitudes over the SC can lead to changes in thermal structures in the middle atmosphere, affecting tropospheric and polar climates, and may lead to changes in global circulations (1). Accurate simulations of the O 3 response to the SC are therefore required for better understanding the sun-climate relationship (10, 11). However, the SC signal in O 3 simulated by different models shows quantitative differences, which may be due to differences in model resolutions, model...

Stratospheric and mesospheric HO<sub>2</sub> observations from the Aura Microwave Limb Sounder

“…These SSI measurement-model differences have been proven to affect the HO x photochemistry (Haigh et al, 2010;Merkel et al, 2011;Ermolli et al, 2013); more UV irradiance leads to an enhancement of O 3 photolysis as well as H 2 O photodissociation, which leads to more HO x production through Reactions (R4)-(R8). Further, Wang et al (2013) showed that using a solar forcing derived from these SSI measurements the modelled OH variability agrees much better with observations. Lastly, (3) these discrepancies might be related to the WACCM representation of the mean meridional circulation which has been shown to have some deficiencies (Smith et al, 2011;Smith, 2012), suggesting that the gravity wave parametrization needs to be modified.…”

Effects of two large solar energetic particle events on middle atmosphere nighttime odd hydrogen and ozone content: Aura/MLS and TIMED/SABER measurements

“…The Microwave Limb Sounder (MLS) on board the Aura spacecraft observes thermal microwave emission from Earth's “limb” (the edge of the atmosphere) viewing forward along the spacecraft flight direction, scanning its view from near the surface to ~90 km altitude every ~25 s. It provides global measurements of HO x species in the stratosphere and mesosphere (~32 to 0.0032 hPa) on a daily basis during both day and night [ Waters et al , ; Pickett et al , , ; Livesey et al , ]. The unique HO x data, together with the simultaneous measurements of O 3 and other related parameters (including H 2 O and temperature), make it possible to investigate the detailed mechanism of the response of the HO x ‐O 3 system to solar forcing [ Wang et al , ]. O 3 profiles are retrieved at the same latitude every orbit, and 240 limb scans per orbit provide close to 3500 profiles per day.…”

Several notes on the OH* layer

“…Airglow intensity correlates with the Lyman-α 11-year solar cycle (Fukuyama, 1977;Pertsev and Perminov, 2008;Wang et al, 2013;Sonnemann et al, 2015). Indeed, years of solar maxima are characterized by stronger dissociation of molecular oxygen by the Lyman-α radiation at the mesopause/lower thermosphere and, consequently, enhanced production of atomic oxygen.…”