SHARP-BDC Phase II
So far analyses carried out in SHARP-BDC have concentrated on the role of PW activity. Questions were mainly related to the internal dynamical variability of the stratosphere and the BDC, and its changes in future climate. Analyses of model data indicate that changes in the meridional residual circulation in a changing climate primarily occur in the lower stratosphere. This leads to a year-round positive trend in tropical upwelling. The amount of upwelling and its annual cycle is accounted for by resolved wave forcing that occurs locally in the tropics and sub-tropics. Major wave sources are stationary planetary waves (PWs) in the tropics and transient synoptic scale waves in the extra-tropics. The increase in the residual circulation is driven by changes in the troposphere due to the indirect effect of changes in sea-surface temperatures (SSTs). Prognostic studies for the future indicate an overall positive trend in tropical upwelling growing towards the end of the simulation period (2100). A significant strengthening in wave generation and dissipation is visible in the changes of the EP-flux vectors and the EP-flux divergence. This intensification can be attributed to significant changes in stationary PWs. A significant offset in age of air for different model configurations as well as large differences between model and observational data were found. For different vertical extents of the model, trends in the lower stratospheric extra-tropical circulation pattern were identified to be of the same sign and similar in magnitude. Changes in different tracers and in correlations between tracers can be used to diagnose changes in mixing and in residual transport and also partly to also to distinguish between the two effects.
More analyses are required to get a better understanding of PW activity changes, its origin and consequences. In this connection, discussions about the role of gravity waves (GWs) have arisen. Currently it is discussed that GW drag could interact with resolved (i.e. large-scale) PWs and partially cancel the effect of strengthening of the BDC. There are indications that the acceleration of the BDC can lead to seemingly paradoxical increases in stratospheric age of air depending on the latitude-altitude pathways of air. Furthermore, based on work carried out in connection with SHARP-BDC it is 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. The question to be answered in SHARP-II is how increasing greenhouse gas concentrations and the subsequent changes in climate are influencing stratospheric transport and mixing separately.