Will climate change impact polar NOx produced by energetic particle precipitation?

Ville Maliniemi,Hilde Nesse Tyssøy,Christine Smith-Johnsen,Daniel R Marsh

doi:10.5194/egusphere-egu2020-5120

Abstract

&lt;p&gt;Energetic electron precipitation (EEP) is an important source of polar nitrogen oxides (NOx) in the upper atmosphere. During winter, mesospheric NOx has a long chemical lifetime and is transported to the stratosphere by the mean meridional circulation. Climate change is expected to accelerate this circulation and therefore increase polar mesospheric descent rates. We investigate the southern hemispheric polar NOx distribution during the 21&lt;sup&gt;st&lt;/sup&gt; century under a variety of future scenarios using simulations of the Whole Atmosphere Community Climate Model (WACCM). Each future scenario has the same moderate variable solar activity scenario, where EEP activity is lower than during the 20&lt;sup&gt;th&lt;/sup&gt; century. We simulate stronger polar mesospheric descent in all future scenarios that increase the atmospheric radiative forcing. By the end of 21&lt;sup&gt;st&lt;/sup&gt; century polar NOx in the upper stratosphere is significantly enhanced in two future scenarios with the largest increase in radiative forcing. This indicates that the ozone depleting NOx cycle will become more important in the future, especially if stratospheric chlorine species decline. Thus, EEP-related atmospheric effects may become more prominent in the future.&lt;/p&gt;

Highlights

Energetic particle precipitation (EPP) is an important source of ionization in the upper atmosphere (Brown, 1966; Sinnhuber et al, 2012)
In this letter we have shown that winter polar nitrogen oxides (NOx) is strongly related to the geomagnetic activity and solar proton events
Polar NOx response to solar proton events (SPE) is relatively constant over different winter months peaking in the lower mesosphere

Summary

Introduction

Energetic particle precipitation (EPP) is an important source of ionization in the upper atmosphere (Brown, 1966; Sinnhuber et al, 2012). SPE are sporadic events of high-energy proton injection directly from the solar wind They can penetrate over the entire polar cap, down to the upper stratosphere (Jackman et al, 2008). GCR come from outside of the solar system and have enough energy to precipitate down to the troposphere (Usoskin & Kovaltsov, 2006) Their total number and average ionization rate are several magnitudes smaller than that of EEP or SPE in the mesosphere and upper stratosphere (Mironova et al, 2015). EEP-related polar NOx enhancements are observed in the stratosphere, but with a time lag relative to the mesospheric enhancements (Randall et al, 2006) This is because the dominating mean atmospheric flow is a poleward and downward circulation in the high latitudinal mesosphere during winter (Andrews et al, 1987), and this circulation slowly transports mesospheric NOx into the stratosphere.

Data and Methods

Historical Polar NOx Response to Different External Forcings

Future Polar NOx Ditributions in Different SSPs

Findings

Conclusions