PLUVIAL REGIME

a regular seasonal and annual pattern of rainfall and other precipitation in a specified region or locality. Averaged zonally, regions of excessive precipitation, in which annual precipitation exceeds annual evaporation, exist in the tropics and at latitudes poleward of 40 in both hemispheres. In the subtropics, dominated by high atmospheric pressure, annual evaporation exceeds annual precipitation, at least over the oceans. At high-latitude coastal locations, precipitation tends to peak in winter; in the continental interior and in regions affected by monsoon winds, the maximum usually occurs in summer. Many points near the Equator have two rainy seasons, in response to the northward and southward movement of the equatorial trough with the Sun. The trough separates the trade winds of the two hemispheres and is a zone of strong convergence, rising motion, and heavy precipitation. In paleoclimatology, pluvial regimes are usually associated with periods of unusually heavy precipitation as deduced from geologic evidence, especially in tropical and subtropical latitudes. The best evidence for pluvial phases in the ancient past comes from splendidly preserved lake terraces in present desert areas. Lake Bonneville in western Utah is an excellent example. The level of this lake has fluctuated greatly and rapidly during at least the last 500,000 years, with the latest pluvial stage occurring about 10,000 years ago, when the lake level was 180 metres (600 feet) above the present elevation of the lake bed. Lake Chad, in the southern Sahara, is another notable example of this widespread phenomenon. Most authorities equate pluvial regimes in the subtropics with periods of glacial advance at higher latitudes, though, during these cold periods, global precipitation and evaporation may decrease. Because of the advancing ice sheets, the middle-latitude storm track is shifted Equatorward, thus bringing heavy precipitation to the subtropics, especially in winter. In contrast, recent evidence indicates a correspondence between glacial and interpluvial periods in the tropics. Tropical lowlands were apparently dry during the last glaciation and wet during the postglacial period.