SHARP-BDC

The work carried out so far in SHARP-BDC is characterised by intensive collaboration between the SHARP-BDC groups and with the SHARP consortium as a whole. The work plan has mostly been kept. All topics of SHARP-BDC have been worked on and new and interesting scientific results have been found; significant contributions to all questions raised in the original proposal have been given. There are slight shifts in time regarding the scheduled work plan which is caused by delayed employment of PhD students (FUB) or interruption of the PhD (maternity break, GUF), but which have not significantly affected the progress of the project. Contributions elaborated within SHARP-BDC were considered in international assessment reports, i.e. SPARC CCMVal (2010) and WMO (2011).

Each group carried out interesting research related to the scientific topics discussed in the original proposal (summary see below). Joint activities helped to perform better and more consolidated interpretation of results. Multi-decadal transient simulations with Atmospheric General Circulation Models, climate models and Chemistry-Climate Models together with assembled, consistent long-term observations (especially derived from space-borne- and balloon-based instruments) have been used to investigate atmospheric processes affecting the BDC. Supplementary numerical sensitivity studies with the different models were performed and interpreted to establish cause and effect relationships.

DLR carried out extensive studies on the dynamical forcing of changes in residual meridional circulation with the CCM E39CA. In addition to three transient simulations (REF-B1, SCN-B2c, SCN-B2d), a large set of sensitivity simulations (SEN) was performed that served as basis for the studies. For example, it was shown that the increase in the residual circulation in E39CA is driven by changes in the troposphere due to the indirect effect of changes in sea-surface temperatures (SSTs). Therefore, it can be concluded that processes associated with tropospheric warming are responsible for the increase in tropical upwelling. Enhanced tropical SSTs lead both, to a strengthening of the subtropical jets due to the warming of the tropical troposphere, and to changes in the distribution of convective latent heat release. Work performed at DLR has been published in the PhD-thesis of Hella Garny (November 2010) and by Garny et al. (2011b). A main focus of investigations at FUB has been on future changes of the BDC in the Northern Hemisphere winter season. The FUB future simulation SCN-B2d indicates an overall positive trend in tropical upwelling and upward mass-flux, growing towards the end of the simulation period. In future, a significant strengthening in wave generation and dissipation is found in the changes of the EP-flux vectors and the EP-flux divergence. This intensification can be attributed to significant changes in stationary waves. Transient waves (mostly resolved gravity waves) account for changes especially in the mesosphere. In order to investigate the origin of the trend in BDC strength and the offset between different model configurations, the downward control principle was used by MPI-M to obtain the contributions of different wave types to the extra-tropical part of the BDC. The upward mass flux through the 70 hPa pressure surface, as defined by the difference in mass streamfunction at the turnaround latitudes of the residual vertical velocity was used as a measure of the BDC strength in the tropics. For different vertical extents of the MPI-M model, trends in the lower stratospheric extra-tropical circulation pattern were found to be of the same sign and similar in magnitude for resolved wave drag as well as for unresolved orographic and non-orographic gravity wave drag. The main features of the trend in the extra-tropical circulation are the same also for different resolutions of the full vertical domain in the high-top model. GUF showed that the stratospheric residual circulation may be considered to consist of two branches: A deep branch more strongly associated with planetary waves breaking in the middle to upper stratosphere, and a shallow branch associated with synoptic and planetary scale waves breaking in the subtropical lower stratosphere. They further suggested that when investigating changes in stratospheric transport the effects of changes in residual transport and in mixing need to be considered, as well as it is necessary to distinguish between different regimes of the stratospheric circulation. A complete description of results was published in Birner and Bönisch (2011). The residual transit times derived in Bönisch et al. (2011) based on meteorological reanalysis data (JRA25) indicated that the residual transport in the deep branch of the BDC has not increased over the past 25 years.

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