In the area extending from the Middle East to the southern Sudan the pressure distribution generally assumes typical seasonal patterns. Apart from winter (December to February) the main low-pressure system which has a direct control on the weather of the area under consideration is an oscillatory barometric minimum, which during the two transitional seasons (spring—March to May—and autumn—September to November) is centered over the central Sudan, and is referred to as the Sudan monsoon low. In October, which represents average conditions in autumn, for example, this low normally extends to about lat. 16°N, but continues towards north with a small inverted V-shaped arm projecting to the northern Red Sea. As winter starts the low acquires a southerly displacement and by January, which represents average conditions in winter, it becomes situated near the Abyssinian Lake Plateau. On the other hand in late spring and early summer the low moves from the central Sudan across Arabia to Persia, and by July it becomes a part of the Asiatic monsoon low which extends to the N.E. Sudan.
The movements of the Sudan low take place in the form of a series of oscillations the average track of which has two outstanding features, namely: (1) There is a remarkable tendency for the low to be located near tablelands. A similar feature is also observed with secondary depressions travelling over the E. Mediterranean in winter. (2) Outstanding northward oscillations of the low take place when middle-latitudes travelling depressions invade the E. Mediterranean from West.
It has also been observed that air currents, not necessarily obeying pressure-gradient, flow along the general run of the Abyssinian Plateau as a result of convergence into the Sudan low, and along the general run of the Persian Plateau (the Valley of Iraq) as a result of convergence into active depressions near Cyprus in winter. Such air currents have been referred to as “effective currents.”
The writer suggests that the image principle may enter into the explanation of these features. The theory which determines the type of motion produced when fluid is drawn off from the center of a revolving disc of incompressible fluid is made use of; but further mathematical treatment and dynamical considerations have been carried out, assuming the active oscillatory low as a slowly travelling cylinder whose axis represents a two-dimensional sink.