AXIAL VELOCITY STREAKS IN THE JET STREAM: AGEOSTROPHIC “INERTIAL” OSCILLATIONS

Abstract

An air particle alternately speeds up and slows down as it passes through local isotach maxima. It is shown that, on the average, the observed period of such speed variations in the core of the jet stream is about twice the period of an inertial oscillation. Periods are somewhat longer for mean anticyclonically curved flow and shorter for cyclonically curved flow. The length of a maximum-velocity streak, measured between successive points of minimum speed, is greatest when the average wind speed in the jet axis is large and is greater at low latitudes than at high latitudes, other things being equal. This behavior is consistent with the hypothesis that, for an air particle in the jet core, the geostrophic- (or gradient-) wind velocity varies at one-half the vector change in the actual-wind velocity. Outside the jet core, where periods are longer due to smaller wind speeds, the geostrophic (or gradient) wind more nearly accommodates to changes in the actual-wind velocity.