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

Sumant Nigam COLA, Department of Meteorology, University of Maryland, College Park, Maryland

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Abstract

The latitude-height structure of variability of the monthly-mean zonally-averaged zonal wind (Ū) 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.

Abstract

The latitude-height structure of variability of the monthly-mean zonally-averaged zonal wind (Ū) 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.

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