An Observational Study of the Northern Hemisphere Extratropical Summertime General Circulation

Abstract

The three-dimensional structure of the extratropical Northern Hemisphere summertime general circulation is documented by making use of twice-daily operational analyses at nine pressure levels (1000?100 mb) for 12 summers (1966?77). Two distinct regimes of standing waves quite unlike the wintertime standing waves appear: a subtropical regime which is clearly a monsoonal response to thermal forcing, and a higher latitude regime in which the standing waves are of smaller scale and much weaker than in winter and tend to be equivalent barotropic in structure with almost no vertical tilt in the mid- and upper troposphere. While the transient eddies shift poleward from their wintertime location and are weaker in summer, their properties and relation to the mean flow are much the same as Blackmon et al. (1977), Lau (1978, 1979a,b) and Lau and Wallace (1979) found for the winter. In both seasons the regions of strongest transient activity generally coincide with the regions where the height scale of the most unstable baroclinic wave derived by Charney (1947) is large compared to the atmospheric scale height. An exception occurs to the north of the Himalayas where the effect of topography severely limits baroclinic instability.