On the Dynamics of the General Atmospheric Circulation

Abstract

The annual flow of angular momentum across latitudes 30-35° effected by north and south currents lying side by side (cyclonic and anticyclonic " eddies ") appears insufficient to balance the angular momentum generated at the surface in lower latitudes. The discrepancy suggests that there must be also a mean poleward drift of air at upper tropospheric and lower stratospheric levels across the latitudes of the mean surface high pressure belt, with a counterdrift which occurs at least in part in near-surface levels. A variety of auxiliary evidence supports the existence of these flows, which are indicated as intermittent or fluctuating components with mean speed over a long period of the order of a few miles per hour and so, at upper levels, only on the margin of direct observation. The origin of the poleward component is associated with the strong anticyclonic shear to equatorward of the jet stream axis. Since these drifts are found in latitudes where the vertical wind shear is greatest, - they constitute an important agency in the transport of angular momentum from its source region in the tropics to the sink region of higher latitudes. This significant contribution is masked in studies of interchange which invoke the assumption of geostrophic wind. The " index " representing the total strength of the tropical easterly surface winds, which fluctuates considerably, relates closely to the rate of generation of angular momentum in the tropics. On the basis of the drifts referred to above, a descriptive theory of the index cycle is developed, which links the fluctuations of index with the fluctuations in strength and in latitude of the jet stream. Quantitative checks are applied to this theory wherever possible and no discrepancy appears, while the normal sequence of synoptic events agrees broadly with that ascribed to the mutual interactions of surface wind strength, jet-stream, and drift.