On the Structure of Variability of the Observed Tropospheric and Stratospheric Zonal-Mean Zonal Wind

Abstract

The latitude-height structure of variability of the monthly-mean zonally-averaged zonal wind (?) is objectively documented for the 9-year period (1980?88) during which both ECMWF and NMC global analyses are available. Modes, resulting from a rotated principal component analysis of the wintertime variability in each dataset, are compared not only with each other but also with those present in a longer dataset (1963?77) of NMC's geostrophically analyzed extratropical winds. In the northern extratropics, there is considerable agreement between the two modern datasets on the structure of wintertime variability: the first two modes, which together account for over 58% of the integrated variance, have largest amplitudes (?3 m s?1) at the tropopause level and little, if any, phase variation with height. The first mode, which explains over 40% of the variance (in the ECMWF, and over 32% in the NMC data), has meridionally a dipole structure centered approximately at the latitude of the subtropical jet?suggestive of small latitudinal shifts of the jet core. The dominant mode of fluctuation in the 14-year NMC's geostrophic wind record, however, has a node at ?40°N, which is suggestive more of ?in place? fluctuations in the jet speed rather than in the ?jet-location.? In the tropics and subtropics, the variability in both 9-year datasets is dominated by a mode that represents fluctuations in the intensity of tropical convection. The time series associated with this mode is rather intriguing. An examination of variability in the winter troposphere/stratosphere in an 8-year (1978/79?1985/86) record of zonal-mean zonal winds, derived from ?NMC/CAC-analyzed? geopotential heights, reveals interesting baroclinic-type modes of variability.