Abstract
Abstract An approximate differential equation is presented, relating the change in speed of the zonal westerly winds to the contemporary zonal wind-speed and the meridional flow of absolute angular momentum. This equation is tested statistically by means of values of the momentum flow and the zonal wind-speed, computed with the aid of the geostrophic-wind approximation, from pressure and height data extracted from analyzed northern-hemisphere maps. The momentum flow is found to be positively correlated with the contemporary zonal wind-speed, and also with the contemporary change of the zonal wind-speed, in agreement with the approximate equation. The study suggests that the momentum flow may be a useful quantity for forecasting the zonal wind-speed. It also implies that an important part of the momentum flow is accomplished by means of large-scale horizontal eddies, whose forms are not obscured by the use of subjectively analyzed maps nor by the geostrophic-wind approximation.
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