Differences in the QBO response to artificial stratospheric aerosol injections depending on injection strategy and species

A known adverse side effect of stratospheric aerosol modification (SAM) by artificial aerosol injections 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 SO₂ at the equator. The reason for this was found to be a modification of the thermal wind balance and an acceleration of the residual circulation leading to a stronger tropical upwelling. For other injection strategies, different responses of the QBO have been observed in model simulations. It has not yet been presented a theory which is able to explain those differences in a 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.

Therefore, within our study we investigated the response of the QBO to continuous artificial aerosol injections for three different injection strategies (point-like injection at the equator, point-like injection at 30°N and 30°S simultaneously, and areal injection into a 60° wide belt along the equator), and 3 different injection rates (5, 10, 25 Tg(S) yr ^-1). For each injection scenario we ran 10-year AMIP-style simulations with the general circulation model MAECHAM5, which was coupled interactively to the aerosol microphysical model HAM.

Our simulations show that the QBO response significantly depends on the injection location. Based on thermal wind balance, we demonstrate that this dependency is explained by differences in the meridional structure of the aerosol-induced stratospheric warming, i.e. the location and meridional extension of the maximum warming, rather than its absolute magnitude. Additionally, we tested two different injection species, SO₂and H₂SO₄, since the injection of H₂SO₄has been recently proposed as an alternative to an injection of SO₂ as first studies indicate that an injection of H₂SO₄ may be more efficient than an injection of SO₂. Our simulations indicate that the QBO response is qualitatively similar for both investigated injection species, but quantitatively stronger for an injection of H₂SO₄.