Stratosphere–troposphere exchange: A zonal mean perspective of angular momentum

The exchange of axial angular momentum (AM) between stratosphere and troposphere forms an important part of stratosphere–troposphere exchange (STE). Data are used to quantify the related fluxes through an idealized tropopause for specific flux events both for the global tropopause and regional segments. Time mean fluxes are considered as well. These are largest in the tropics and closely tied to mass fluxes. The data accuracy is found to be insufficient for a closure of the momentum budget of the Brewer–Dobson circulation. Positive regional flux events at the tropical tropopause are partly due to a short lived intensification of the Hadley circulation, but are also embedded in a rather long lived circulation anomaly which leads to a gain of positive AM in the lower tropical stratosphere. Moreover, there are large fluxes through the ‘tropopause breaks’. Flux events at midlatitude segments involve also strong horizontal fluxes through the ‘tropopause breaks’, which override an otherwise downward flux from the stratosphere. These events are closely linked to tropical anomalies of AM, so that all ‘regional’ events turn out to be of global character. Data accuracy becomes a problem when fluxes through a global tropopause are investigated where substantial uncertainties are encountered near the ‘tropopause breaks’.All cases discussed exhibit a distinct similarity with deep anomalies of AM in the tropics sandwiched between anomalous AM columns of opposite sign to the north and south. There is always a center of flux convergence or divergence in the lower stratosphere. This suggests that all the events of large scale-angular momentum exchange between stratosphere and troposphere as analysed here follow a distinct dynamic pattern, which appears not to have been described before.