Changes in Northern Hemisphere stratospheric variability under increased CO₂ concentrations

Abstract

† The contribution of J. Kettleborough was written in the course of his employment at the Met Office, UK, and is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.The robustness of stratospheric circulation changes under increased concentrations of carbon dioxide are investigated using the Met Office HadSM3-L64 model. Equilibrium climate change simulations employing forcing of two and four times pre-industrial CO 2 are presented, with particular focus on the temperature response of the Arctic lower stratosphere during Northern Hemisphere winter. High CO 2 loading provides the ability to attain the statistical significance of any response, typically a problem given the large component of interannual variability common to the region. In response to CO 2 , the expected global stratospheric cooling is modified by an anomalous dynamical warming of the Arctic winter lower stratosphere. This warming is shown to be associated with an increase in frequency of stratospheric sudden warming (SSW) events. At four times pre-industrial CO 2 , the frequency of SSW events per year is doubled with respect to the control simulation. Further, by comparing winters with and without SSW events, it is shown that the warming of the lower stratosphere cannot be achieved without the presence of a frequency modulation of SSW events.

The influence of the stratosphere on the tropospheric zonal wind response to CO<sub>2</sub> doubling

“…The studies of Sigmond et al (2004) and Gillett et al (2003) describe the effect of CO 2 doubling in terms of temperature and wind changes. Bell et al (2010) further show that the doubled CO 2 response in the UM model is robust, as a similar structure of the response, but with larger amplitude, is found under quadrupled CO 2 concentrations. Here we focus on changes in the stratospheric PV that are related to CO 2 doubling, and investigate to what extent these stratospheric changes influence the tropospheric zonal mean zonal wind response.…”

“…In general, a warming of the troposphere and a cooling of the stratosphere are observed in these simulations (e.g., Gillett et al, 2003;Sigmond et al, 2004;Bell et al, 2010). The response in the zonal wind, however, reflects the changes in the horizontal temperature gradient, which shows more variability among models.…”

“…A higher stratospheric CO 2 concentration will lead to an enhanced cooling to space (Fels et al, 1980;Butchart et al, 2000;Shindell et al, 2001). CO 2 doubling will cause changes in the radiative cooling, resulting in cooling of the stratosphere that increases with height (e.g., Bell et al, 2010) and hence resulting in a change in the stability and the PV. The relation between the diabatic heating and PV is given by the PV evolution equation as given in, for example, Eq.…”

“…This work is based on simulations with the middleatmosphere version of the ECHAM4 climate model as performed by Sigmond et al (2004) and simulations with the 64-level version of the Hadley Centre Atmosphere Model coupled to a thermodynamic slab ocean model (HadSM3), which is based on the Unified Model (Gillett et al, 2003;Bell et al, 2010). These models will be referred to as the "ECHAM model" and the "UM model", respectively.…”

The nature of Arctic polar vortices in chemistry–climate models

“…For instance, Bell et al (2010) showed, using the Hadley Centre HadSM3-L64 model, that under four times pre-industrial CO 2 concentrations, the vortex would become more highly disturbed with an increased frequency of SSW events. In contrast, Rind et al (1998) reported a decrease in SSW frequency using the GISS global climate/middle atmosphere model with two times pre-industrial CO 2 concentrations.…”