Recent Rainfall Trends in Africa, the Middle East and India

Abstract

IN the Middle East and Africa north of the Sahara the wet season extends from October to May; small amounts of rain also fall in Pakistan and north-west India at that time of year. Winter–spring rainfall in these areas is controlled primarily by divergence patterns in pressure troughs in the mid- and upper-tropospheric circumpolar westerlies. Rainfall is associated with the widespread ascent of warm, moist air from the tropics on the eastern side of the troughs, and also with deep convective instability in the cold air at, and slightly to the rear of, the trough axes, especially over coasts and mountains. It is in winter that the increase in the latitudinal temperature gradient in the northern hemisphere causes the westerly circulation to increase in strength and the westerly troughs to penetrate into sub-tropical latitudes. In summer, the latitudinal temperature gradient is at a minimum, the general circulation is relatively weak, and the westerly troughs are of small amplitude and restricted to middle and high latitudes.