Changes in the atmospheric circulation are one of the major sources of uncertainty in climate projections [1], and therefore a major area of scientific research. Atmospheric circulation and composition are strongly interconnected through many feedback processes, for instance climate change through ghg, ozone recovery, etc.

In particular the Brewer-Dobson Circulation (BDC) [2] transports chemical tracers from the tropical troposphere to the polar stratosphere. This transport is a key factor in determining the possible chemical transformations, changes in the source-sink balance, as well as how gases are redistributed on global scale.

The BDC is projected to change, therefore it is crucial to foresee the possible consequences of this change in terms of tracers response and, in turn, of climate feedbacks.

It is not clear how this change will take place. Model simulations consistently indicate a BDC speed-up, as a response to increasing ghg in the troposphere. There is no supporting evidence of it, observations rather indicate a slowdown of the BDC ([3], [4]).

ACCROSS (Atmospheric Composition and Circulation investigated with meteorological Reanalyses, Observational dataset and models for the study of the Stratosphere and its changes) is a 4-years project in conjunction with the University of Liege (ULg) which intends to improve our understanding of the circulation changes in the stratosphere during the past three decades, through the use of observation and model simulation time series of selected long-lived tracers, namely HF, N2O and CH4.

ACCROSS aims at explore the following research aspects:

- Characterize the long term trends and the impact of the stratospheric circulation changes (such those identified by [3]) on the selected tracers exploiting ground based FTIR data, as well as satellite data time series.

- Evaluate and compare the representations of the trends and circulation changes in different meteorological reanalyses (e.g. ERA-interim, MERRA-2). This comparison will use the BASCOE CTM as a transfer tool to model the expected changes in the stratospheric abundances of the selected tracers, and compare them with the observed changes.

- Investigate the ability of a state-of-the-art climate model, WACCM, to simulate the observed changes of the stratospheric circulation. It is not known how well such a climate model would reproduce the observed changes. A successful simulation would allow to identify the cause(s) of those changes thanks to sensitivity tests, while a failure would provide new areas to improve our knowledge about global change.

[1] Sheperd: Atmospheric circulation as a source of uncertainty in climate science projections, Nature Geosci., 7 (10), 703-708, 2014

[2] Butchart, The Brewer-Dobson Circulation, Rev. Geophys., 52, 157-184, 2014

[3] Mahieu et al.: Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes, Nature, 515 (7525), 104-107, 2014

[4] Stiller et al.: Observed temporal evolution of global mean age of stratospheric air for the 2002 to 2010 period, Atmos. Chem. Phys., 12, 3311-3331, 2012