Lagrangian Partial-Inertial Oscillations, and Subtropical andLow-Level Monsoon Jet Streaks

Abstract

Jet streaks (local wind maxima) are prominent features of current systems both in the high and lowtroposphere. As particles in the core of the current flow through them, they experience cyclical velocityoscillations whose characteristics (period, dimensions and form of the wave described by a trajectory) arerelated to the way in which the geostrophic wind varies along the current. Although there is considerablevariation, the modal period is nearly semi-inertial (twice the inertial period). Two special cases are examined. First, jet streaks in the mean winter subtropical jet stream (STJ) aredemonstrated to be approximate semi-inertial oscillations and also planetary waves, a coincidence shownto be unique to the subtropical latitude. STJ waves are distinctive from middle-latitude Rossby waves inthe respects that they are dominated by departures from gradient-wind motion and also have dissimilardivergence distributions. Second, the mean July low-level monsoon jet over the Arabian Sea, with anoscillation set up by large ageostrophic winds east of Africa, is treated in terms of inertial motion dampedby friction, superimposed ow a geostrophic current that decreases eastward. In both examples, comprisingjet streaks that tend to be topographically bound, mean observed features are well represented by analyticallyexpressed trajectories.