THE FREQUENCY AND DYNAMICS OF STRATOSPHERIC SUDDEN WARMINGS IN THE 21ST CENTURY

The dynamical coupling between the stratosphere and troposphere is strongest during extreme events in the stratosphere known as Stratospheric Sudden Warmings (SSWs). Our recent work has focussed on establishing a new climatology of these events, and developing dynamically realistic benchmarks for their simulation in GCMs. Here we use these new tools, together with a state-of-the-art coupled chemistry climate model, to investigate if either the frequency or the character of SSWs will change over the coming century. Our motivation is simple: in order to understand what role the Stratosphere might play in future tropospheric climate, we must first understand and quantify the future stratospheric climate itself.

We investigate two sets of three ensemble member integrations, performed with the GFDL AMTRAC coupled chemistry climate model: the first set is forced with observed SSTs and climate forcings between 1960 and 2000, and the second set is forced with SSTs from a coupled model run forced with trace gas concentrations based on the SRES A1B scenario of the IPCC and equivalent climate forcings. To the best of our knowledge, this is the longest set of integrations used for a study of this kind.

We find an upward trend in SSW frequency of 1 event/decade, and this is statistically significant at the 90% confidence level and is consistent over the two sets of model integrations. Comparison of SSW climatology between the late 20th and 21st centuries in the model shows that the increase is largest toward the end of the winter season. Comparison of the dynamical properties of SSWs in the 20th and 21st centuries shows that the character of SSWs is not altered by the increase in SSW frequency. These results have implications for the impact of future Stratospheric climate on the Troposphere and also on future concentrations of Ozone in the Northern Hemisphere Stratosphere.