Observational Study of the Tropical Upper Tropospheric Motion Field during the Northern Hemisphere Summer

Abstract

Conventional and commerical aircraft wind observations over the global tropics have been used to prepare daily charts of the field of motion at the 200-mb level for June, July and August 1967. Computations describe the climatology of the field of motion (streamlies and isotachs) and the divergence and vorticity distributions. A number of dynamical computations are made utilizing these data. These include 1) A comparison of the strength of the rotational part of the wind with the total wind. 2) A description of the strength and geometry of the Hadley and Walker circulations. 3) An analysis of the ultralong waves (zonal wavenumbers 1 and 2). The waves, which are quasi-stationary, are shown to carry a large proportion of the total variance of the motion field and to have a very pronounced southwest to northeast tilt. The amplitude and motion of transient ultralong waves, wavenumbers 1 and 2, are studied on polar diagrams. It is shown that these waves are intermittent and rapidly moving. 4) An analysis of the amplitude and motion of shorter waves (wavenumbers 7 and 8 of the zonal spectra of the meridional wind), which are intermittent, have been studied on x?t diagrams. Their amplitudes and speeds of westward propagation vary respectively between 0 and 6 m sec?1 and 4° to 12° longitude day?1. 5) A computation of the energy exchange between the zonal flow and eddies. Quasi-periodic oscillations in time are shown with a period of roughly 3.5 days. The phenomenon is suggested to be a manifestation of pure barotropic type instability of the tropical upper level flows. 6) Computations of meridional fluxes of momentum and kinetic energy. These are shown to be related to the shapes of the quasi-stationery ultralong waves and possibly also to certain shapes of westward moving shorter waves over the Northern Hemisphere tropics.