On the dominance of changes in planetary angular momentum in large scale extra‐tropical flows

Abstract

Considerations of the temporal changes in angular momentum are employed to diagnose air trajectories over large scale distances calculated from the publicly available global data base of NCEP/NCAR. It is shown that outside the Tropics both the total angular momentum and its Lagrangian (material) time‐derivative are dominated by the planetary term, regardless of the direction of the transport, whereas the contribution of the non‐linear term is always negligible. In the Tropics the planetary term and the acceleration terms alternately dominated the angular momentum evolution. These Lagrangian results are reinforced by Eulerian calculations of instantaneous maps of the relative contributions of the various terms in the evolution equation of the angular momentum. The simple equation of angular momentum evolution (compared to the more complex zonal momentum equation) is accurately satisfied by NCEP/NCAR re‐analyzed fields even over Africa, where the data for the reanalysis is scant. The only exceptions to the general dominance of the planetary term over the angular momentum evolution in the extra‐Tropics are along trough‐/ridge‐lines (where the flow changes direction sharply) and at the entrance/exit regions of straight jets, where the flow accelerates/decelerates.