An Observational Study of the Northern Hemisphere Wintertime Circulation

Twice-daily synoptic analyses are statistically analyzed for a sample of nine individual winters. Temporal variance and covariance quantities at each individual grid point are partitioned into “low-pass” (approximately 10–90 day) and “band-pass” (approximately 2.5–6 day) components by means of conventional filtering procedures. The time-filtered variance and covariance fields are displayed in terms of hemisphere maps. Included in the analysis are sea level pressure, 300 mb height, 500 mb wind statistics, and 850 mb temperature and poleward heat flux. The most definitive results of the study involve the “band-pass” fluctuations which appear to be associated with developing baroclinic waves. The fields of band-pass 1000, 50 and 300 mb geopotential height, as well as the 500 mb meridional wind component and relative vorticity all exhibit elongated variance maxima coincident with the two major Northern Hemisphere storm tracks, which lie downstream and somewhat poleward of the cores of the Asian and North American jet streams at the tropopause level. The storm tracks are characterized by strong poleward band-pass fluxes of heat at the 850 mb level and strong convergence in the band-pass eddy flux of westerly momentum at the 500 and 300 mb levels. There is no convergence of eddy flux of westerly momentum into the regions upstream of the jet streams, where the strong westerly accelerations are taking place. We are thus led to the conclusion that the jet streams in the time-averaged flow develop as a result of thermally direct (time) mean meridional circulations over eastern Asia and North America. It appears that baroclinic waves, together with their induced thermally indirect (time) mean meridional circulations over the North Pacific and Atlantic, function as a brake on the jet streams. These conclusions are supported by observational evidence concerning the geographical distribution of the ageostrophic component of the time mean flow. The geographical distribution of the variances and covariances of the low-pass filtered data varies widely from one winter to another. At all the levels investigated, the low-pass variability of the geopotential height field is largest over the oceans, downstream from the major storm tracks, whereas the low-pass variability of the lower tropospheric temperature field is largest over the continents, at high latitudes. Evidence of enhanced “lee-slope cyclogenesis” can be seen in the sea level pressure and 850 mb temperature data, but not in the data for the middle and upper troposphere.