Late quaternary variability of the Arabian Sea monsoon and oxygen minimum zone

Abstract

The Monsoon Among the first Europeans observing the Asiatic monsoon was Alexander the Great during his campaign to the mouth of the Indus (325 B.C.). The oldest known records of the Arabian Sea monsoonal climate, however, are shipping documents, dated about 2300 B.C., which refer to the use of the seasonal changing winds by traders [Warren , 1987]. The monsoon has always played an important role in the live of the sailors inhabiting the coasts of the Arabian Sea, because they depended for their trade on the seasonal change in wind direction. The Arabic word 'mausim', which means season, is also the origin of the term monsoon. The monsoon cycle is caused by the annual shifting position of the Inter Tropical Convergence Zone (ITCZ) which oscillates between approximately the tropics of Capricorn and Cancer, following the region of maximum solar heating. The ITCZ results from the rise of warm humid air, which cools with increasing height causing heavy rainfall in the tropics. The latent heat released during cloud formation and rainfall provides an additional heat source for the ITCZ. The resulting surface air flow towards the ITCZ is deflected by the Coriolis force, causing the characteristic trade wind pattern. The yearly course of the sun causes large temperature differences on the continents, whereas differences in the ocean are relatively small. This differential heating originates from the difference in heat capacity being smaller for land than for sea, and results in the breakdown of the subtropical high pressure cell and the formation of a low pressure cell over the continent during summer. During winter the SUbtropical high pressure cell restores causing a reversal in wind direction. Thus, continents in the tropical region and adjacent ocean experience a semiannual reversal in wind direction, which is termed monsoon. Strength and direction of the monsoon is influenced by the position and size of the continents. The Indian monsoon is particularly strong due to height and position of Himalayas and Tibetan Plateau, and the supply of latent heat from the Indian Ocean. The agriculture in India and China, the two countries in the world with the largest population, depends heavily on the rainfall during the summer monsooon. During the boreal summer heavy monsoonal rains fall in India, southeast Asia and southern China. Locally they are orographically amplified, so that, for example in parts of Assam, in the three monsoon months of June, July and August, precipitation averages more than 1200 em. Variability in the intensity of the monsoonal rains has resulted in extreme drought, with failing crops, or extreme rainfall, resulting in catastrophic floods. For example, the failure in the late Holocene of regular monsoonal rains about 3,500 yr. BP. related to astronomical forced climate change- is thought to have caused the collapse of the flourishing Harrapan culture in the Indus valley [Bryson and Swain, 1981]. Until this decrease in monsoonal rainfall wheat, barley, melons, and perhaps cotton were grown in what is now the Thar desert of Radjasthan [Lamb, 1995].