Abstract

Ensemble simulations with a coupled ocean-troposphere-stratosphere model for the pre-industrial era (1860 AD), late twentieth century (1990 AD) greenhouse gas (GHG) concentrations, the SRES scenarios B1, A1B, A2, as well as stabilization experiments up to the Twenty-third century with B1 and A1B scenario GHG concentrations at their values at 2100, have been analyzed with regard to the occurrence of major sudden stratospheric warmings (SSWs). An automated algorithm using 60°N and 10 hPa zonal wind and the temperature gradient between 60°N and the North Pole is used to identify this phenomenon in the large data set. With 1990 CO2 concentrations (352 ppmv), the frequency of simulated SSWs in February and March is comparable to observation, but they are underestimated during November to January. All simulations show an increase in the number of SSWs from the pre-industrial period to the end of the twenty-first century, indicating that the increase of GHG is also reflected in the number of sudden warmings. However, a high variability partially masks the underlying trend. Multi-century averages during the stabilization periods indicate that the increase of SSWs is linear to the applied radiative forcing. A doubling of SSWs occurs when the GHG concentration reaches the level of the A2 scenario at the end of the twenty-first century (836 ppmv). The increase in SSWs in the projections is caused by a combination of increased wave flux from the troposphere and weaker middle atmospheric zonal winds.

Highlights

  • Since the discovery of the first major sudden stratospheric warming in 1952 (Scherhag 1952) the understanding of stratospheric dynamics and the coupling to tropospheric processes has continuously been improved

  • The impact of greenhouse gas (GHG) concentrations on the frequencies of stratospheric warmings (SSWs) is investigated with simulations forced by historical GHG concentrations for 1860–2000 (Huebener et al 2007) and thereafter concentrations according to the special report on emissions scenarios (SRES) B1, A1B and A2 (Nakicenovic 2000)

  • The new algorithm is used to analyze a set of experiments run with a coupled ocean-troposphere-stratosphere GCM, including control simulations and transient simulations with a prescribed increase in GHG concentrations and a stabilization thereafter

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Summary

Introduction

Since the discovery of the first major sudden stratospheric warming (hereafter referred to as SSW) in 1952 (Scherhag 1952) the understanding of stratospheric dynamics and the coupling to tropospheric processes has continuously been improved. High variability is the main problem for deducing robust trends from observations (Labitzke and Kunze 2005) and model studies (Butchart et al 2000; Charlton-Perez et al 2008; Schimanke et al 2011). In consequence of this variability large data sets need to be considered when analyzing long-term changes in the number of SSWs. Two methods have been developed to detect SSWs by automated algorithms in recent years (Limpasuvan et al 2004; Charlton and Polvani 2007).

The model
Experiments
Identification of sudden stratospheric warmings
Present day climate
Pre-industrial climate
Variability of SSWs
Historical period
Scenario simulations
The transient phase
The stabilization period
Idealized experiments
Future SSW characteristics
Multi-century mean values
Mechanism for increase of SSWs due to increasing GHG
Findings
Conclusions and discussion
Full Text
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