Quasi-biennial oscillation of the tropical stratospheric aerosol layer

R Hommel,M A Giorgetta,H F Graf,C Timmreck

doi:10.5194/acp-15-5557-2015

R Hommel, M A Giorgetta + Show 2 more

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https://doi.org/10.5194/acp-15-5557-2015

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Abstract

Abstract. This study describes how aerosol in an aerosol-coupled climate model of the middle atmosphere is influenced by the quasi-biennial oscillation (QBO) during times when the stratosphere is largely unperturbed by volcanic material. In accordance with satellite observations, the vertical extent of the stratospheric aerosol layer in the tropics is modulated by the QBO by up to 6 km, or ~ 35% of its mean vertical extent between 100–7 hPa (about 16–33 km). Its largest vertical extent lags behind the occurrence of strongest QBO westerlies. The largest reduction lags behind maximum QBO easterlies. Strongest QBO signals in the aerosol surface area (30 %) and number densities (up to 100% e.g. in the Aitken mode) are found in regions where aerosol evaporates, that is above the 10 hPa pressure level (~ 31 km). Positive modulations are found in the QBO easterly shear, negative modulations in the westerly shear. Below 10 hPa, in regions where the aerosol mixing ratio is largest (50–20 hPa, or ~ 20–26 km), in most of the analysed parameters only moderate statistically significant QBO signatures (< 10%) have been found. QBO signatures in the model prognostic aerosol mixing ratio are significant at the 95% confidence level throughout the tropical stratosphere where modelled mixing ratios exceed 0.1 ppbm. In some regions of the tropical lower stratosphere the QBO signatures in other analysed parameters are partly not statistically significant. Peak-to-peak amplitudes of the QBO signature in the prognostic mixing ratios are up to twice as large as seasonal variations in the region where aerosols evaporate and between 70–30 hPa. Between the tropical tropopause and 70 hPa the QBO signature is relatively weak and seasonal variations dominate the variability of the simulated Junge layer. QBO effects on the upper lid of the tropical aerosol layer turn the quasi-static balance between processes maintaining the layer's vertical extent into a cyclic balance when considering this dominant mode of atmospheric variability. Global aerosol-interactive models without a QBO are only able to simulate the quasi-static balance state. To assess the global impact of stratospheric aerosols on climate processes, those partly nonlinear relationships between the QBO and stratospheric aerosols have to be taken into account.

Highlights

The stratospheric aerosol layer, referred to as the Junge layer (Junge et al, 1961), is a key constituent in the Earth’s atmosphere
In this study we address certain aspects of the coupling between stratospheric dynamics and aerosol microphysical processes, as they are important to understand the contributions of quasi-biennial oscillation (QBO) and natural variability to recent observed changes of stratospheric aerosol, and as they are key to evaluate stratospheric geoengineering options
In the HOM11 study, the Junge layer behaves almost statically, in the tropics only being influenced by temperature variations in the tropical tropopause layer (TTL) and wind alterations related to the semi-annual oscillation (SAO) in the mesosphere and upper stratosphere (Fig. 3a; see Giorgetta et al, 2006)

Summary

Introduction

The stratospheric aerosol layer, referred to as the Junge layer (Junge et al, 1961), is a key constituent in the Earth’s atmosphere. During times of low volcanic activity, the stratospheric aerosol load inevitably degrades towards a so-called background state representing the lowest possible self-maintaining aerosol level in the stratosphere. This natural balance may be influenced by sulfur-releasing anthropogenic activities (Hofmann et al, 2009; Neely et al, 2013). Together with the sporadically occurring volcanic perturbations, human activities alter the Earth’s radiative balance, in turn affecting the longterm trend of the global aerosol load (Solomon et al, 2011)

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