On the Three-Dimensional Structure of the Observed Transient Eddy Statistics of the Northern Hemisphere Wintertime Circulation

Abstract

The three-dimensional structure of temporal mean, variance and covariance fields of the Northern Hemisphere wintertime circulation is documented by making use of twice-daily hemispheric analyses at 10 pressure levels (1000–100 mb) for 11 winters (1965–76). The Northern Hemisphere is partitioned into seven longitudinal sectors which correspond, respectively, to 1) the entrance regions of the two principal jet streams; 2) the two major oceanic storm tracks (areas with pronounced geopotential height variability in the 2.5–6 day period range); 3) the western portions of the North American and Eurasian land masses; and 4) the central part of the Asian continent. Meridional cross sections of various seasonal mean statistics averaged over meridians comprising these individual sectors are displayed. The fields presented in this manner include 1) time-averaged zonal wind and temperature; 2) temporal variance statistics of zonal and meridional wind components, temperature, and geopotential height; 3) meridional fluxes of heat, geopotential energy, westerly momentum and potential vorticity by transient eddies; and 4) poleward component of the time averaged ageostrophic flow. The hemispheric distributions of temperature statistics, meridional heat flux and mean potential vorticity in the vicinity of the tropopause (200 and 300 mb) are also shown. The structure of eddy statistics and the implied energetics in the oceanic storm tracks are in good agreement with the characteristics of developing baroclinic waves. The strongest variability in geopotential height and wind occurs at 300 mb. The geopotential height fluctuations in the 2.5–6 day period range exhibit a secondary maximum at the ground level, so that the 850 mb level corresponds to a relative minimum. Thermal variability is strongest in the middle troposphere and just above the tropopause. The primary maximum for the heat flux statistics is located at 850 mb, with a weaker secondary maximum at 200 mb. The distribution of eddy momentum flux is characterized by strong convergence of westerly momentum into the storm track axis. The western parts of the two major land masses exhibit quite distinct dynamical features in middle latitudes. The upper troposphere is characterized by equatorward heat fluxes, whereas the direction of heat transport in the lower stratosphere is poleward. The momentum fluxes in these regions are strong and poleward at the jet stream level. This configuration of heat and momentum fluxes is consistent with observed strong poleward fluxes of potential vorticity in the vicinity of the tropopause. Strong supergeostrophic winds in the eddies over the western continents are shown to induce conversion from eddy kinetic energy to eddy available potential energy in these regions. The relative importance of the mean flow forcing by the transient eddies is discussed. It is seen that the jet streams in the entrance regions are maintained by strong poleward ageostrophic flows at the tropopause level, while strong equatorward ageostrophic flows over the oceanic storm tracks act to decelerate the jets at these longitudes. The intensity of these local meridional circulations is about an order of magnitude stronger than that of the zonally averaged Ferrel cell in midlatitudes. The local forcing of the mean zonal flow by the Coriolis acceleration accompanying these time-mean ageostrophic flows is shown to be much larger than the forcing due to eddy momentum flux convergence.