Differences in the QBO response to artificial stratospheric aerosol injections depending on injection strategy and species
Authors: Franke Henning , Niemeier Ulrike 
Published: March 2021

&lt;p&gt;&lt;span&gt;A known adverse side effect of stratospheric aerosol modification (SA&lt;/span&gt;&lt;span&gt;M)&lt;/span&gt; &lt;span&gt;by artificial aerosol injections&lt;/span&gt;&lt;span&gt; is the alteration of the quasi-biennial oscillation (QBO), which is caused by the stratospheric heating associated with an artificial aerosol layer. Multiple studies found the QBO to slow down or even completely vanish for point-like injections of SO&lt;sub&gt;2&lt;/sub&gt;&lt;/span&gt; &lt;span&gt;at the equator. The &lt;/span&gt;&lt;span&gt;reason&lt;/span&gt;&lt;span&gt; for this was found to be a modification of the thermal wind balance and a&lt;/span&gt;&lt;span&gt;n acceleration of the residual circulation leading to a &lt;/span&gt;&lt;span&gt;stronger tropical upwelling. For other injection strategies, different responses of the QBO have been observed &lt;/span&gt;&lt;span&gt;in model simulations&lt;/span&gt;&lt;span&gt;. It has not yet been presented a theory which is able to explain those differences in a&lt;/span&gt; &lt;span&gt;comprehensive manner. This is further complicated by the fact that the simulated QBO response is highly sensitive to the used model even under identical boundary conditions.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Therefore, within &lt;/span&gt;&lt;span&gt;our&lt;/span&gt;&lt;span&gt; study we investigate&lt;/span&gt;&lt;span&gt;d&lt;/span&gt;&lt;span&gt; the response of the QBO to &lt;/span&gt;&lt;span&gt;continuous artificial aerosol injections&lt;/span&gt;&lt;span&gt; for three different injection strategies (point-like injection at the equator, point-like injection at 30&lt;/span&gt;&lt;span&gt;&amp;#176;&lt;/span&gt;&lt;span&gt;N and 30&lt;/span&gt;&lt;span&gt;&amp;#176;&lt;/span&gt;&lt;span&gt;S simultaneously, and areal injection into a 60&lt;/span&gt;&lt;span&gt;&amp;#176; &lt;/span&gt;&lt;span&gt;wide belt along the equator)&lt;/span&gt;&lt;span&gt;, and 3 different injection rates (5, 10, 25 Tg(S)&lt;/span&gt; &lt;span&gt;yr&lt;/span&gt; &lt;sup&gt;&lt;span&gt;-1&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;).&lt;/span&gt; &lt;span&gt;For &lt;/span&gt;&lt;span&gt;each injection scenario&lt;/span&gt; &lt;span&gt;we &lt;/span&gt;&lt;span&gt;ran&lt;/span&gt;&lt;span&gt; 10&lt;/span&gt;&lt;span&gt;-year&lt;/span&gt;&lt;span&gt; AMIP-style &lt;/span&gt;&lt;span&gt;simulations &lt;/span&gt;&lt;span&gt;with&lt;/span&gt;&lt;span&gt; the &lt;/span&gt;&lt;span&gt;general circulation model MAECHAM5, which was coupled interactively to the aerosol microphysical model HAM.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Our simulations &lt;/span&gt;&lt;span&gt;show&lt;/span&gt;&lt;span&gt; that the QBO response significantly depends on the injection location. Based on thermal wind balance, we demonstrate that this dependency is explained by differences&lt;/span&gt; &lt;span&gt;in the meridional structure of the aerosol-induced stratospheric warming, i.e. the location and meridional extension of the maximum warming, &lt;/span&gt;&lt;span&gt;rather than its absolute &lt;/span&gt;&lt;span&gt;magnitude&lt;/span&gt;&lt;span&gt;. Additionally, we tested two different injection species, SO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;and H&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;SO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;4&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;since t&lt;/span&gt;&lt;span&gt;he injection of H&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;SO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;4&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;has been recently proposed as an alternative to an injection of SO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; as first studies indicate that an injection of &lt;/span&gt;&lt;span&gt;H&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;SO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;4&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; may &lt;/span&gt;&lt;span&gt;be more&lt;/span&gt;&lt;span&gt; efficie&lt;/span&gt;&lt;span&gt;nt&lt;/span&gt;&lt;span&gt; than an injection of SO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;Our &lt;/span&gt;&lt;span&gt;simulation&lt;/span&gt;&lt;span&gt;s indicate that&lt;/span&gt;&lt;span&gt; t&lt;/span&gt;&lt;span&gt;he QBO response is qualitatively similar for both investigated injection species, &lt;/span&gt;&lt;span&gt;but quantitatively stronger for an injection of H&lt;/span&gt;&lt;sub&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;SO&lt;/span&gt;&lt;sub&gt;&lt;span&gt;4&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;