THE SUBTROPICAL JET STREAM OF WINTER

Abstract

Daily upper-wind analysis at 200 mb was carried out between the equator and 45N, around the globe, for December 1955 to February 1956 to map the subtropical jet stream. It turned out that this current formed a pattern of three standing long waves which maintained nearly a steady state. This made possible an analysis of the meteorological variables in a coordinate system fixed with respect to the jet stream axis at 200 mb. Monthly mean maps and vertical cross sections of wind, temperature, moisture and mass distribution were computed in this coordinate system. The jet-stream core is situated close to the 200-mb surface and has a maximum speed close to 70 m per sec in the mean integrated around the world; this is mainly due to very high speeds over Africa and Asia. Mean velocities close to or above 160 kn are found in the ridges of the meandering jet axis, and lower speeds are observed in the troughs. Computations of mass circulation across the jet-stream axis revealed (for all three months of winter) a thermally direct system of the order of a few knots in the sense of the classical Hadley circulation. The center of the mass-circulation cell was about 15 deg lat equatorward of the jet axis. Mean structure of the wave is obtained by analysis of fields of temperature, wind and height of constant pressure surfaces. Some simple calculations are made to evaluate the strength of the computed mass circulation in the heat balance and kinetic-energy balance of the atmospheric general circulation. Rossby's hypothesis of constant vorticity transport is examined.