A Trajectory-Based Investigation of Physical and Dynamical Processes That Govern the Temporal Evolution of the Subtropical Jet Streams over Africa

Abstract

ropical, subtropical, and extratropical dynamical processes govern the synoptic-scale evolution of the subtropical jet stream(s) over Africa. However, the relative importance of the respective effects is still under debate and is the focus of this study. Interim ECMWF Re-Analysis (ERA-Interim) data are used to calculate backward trajectories from the subtropical jet over Africa during winter 2005/06. The trajectories allow for studying the jet dynamics from both a potential vorticity (PV) and an angular momentum point of view and for linking the two theoretical frameworks.Three cases of synoptic-scale Rossby wave breaking in the extratropics and subtropics are presented in detail. They illustrate basic flow configurations where (i) the subtropical jet is mainly forced by tropical dynamics, (ii) extratropical forcing contributes substantially to the jet acceleration, and (iii) strong diabatic processes in the subtropics impact the jet.The case study results are then generalized for the entire winter season. The main findings are as follows: (i) Approximately 41% of the trajectories reach the subtropical jet from the deep tropics and for these trajectories the nonconservation of angular momentum M due to eddy forcing leads to a decrease of M by about 5%. (ii) A nonnegligible fraction of roughly 18% of the trajectories reaches the subtropical jet from the extratropics. (iii) Wave breaking is instrumental for bringing extratropical, high-PV air southward. (iv) Diabatic processes in the subtropics have a negligible direct effect on the upper-level PV. This is in contrast to observations from the extratropics and might be the consequence of the small planetary vorticity in the tropics and subtropics.