Evolution and Development of the Indian Monsoon

Abstract

The Indian monsoon is a complex oceanic-atmospheric-coupled mechanism of the tropics that plays a key role in inter-hemispheric heat transfer on Earth. The summer monsoon brings moisture to the highly populated South Asian countries and affects the livelihood of more than a billion people. The intensity of the monsoon significantly influences the ecological diversity and hydrological reservoirs across the South Asian region. However, the intensity of the monsoon greatly varies spatially and temporally, driven by both external and internal forcing factors. Modeling and palaeoclimatic studies indicate several phases of strong and weak summer monsoon rainfall caused by changes in solar insolation, snow accumulation in western Europe, El Niño-Southern Oscillation, North Atlantic Oscillations and sea surface temperature in the Indian and Pacific Oceans. The initiation and strengthening of the Indian monsoon during the middle-late Miocene are sometimes linked with phases of major surface uplift of the Himalayan and/or Tibetan Plateau. The Plio-Pleistocene glaciation prompted a strong winter monsoon and a weak summer monsoon. During the early Holocene, the summer monsoon strengthened and subsequently weakened with two major phases of sudden rainfall reduction at ~8.2 and ~4.2 kyr BP; the latter event caused significant societal impact including the migration of population of the Indus Valley Civilization. In the last millennium, the Indian summer monsoon (ISM) was strong during the Medieval Warm Period (MWP) now designated as Medieval Climate Anomaly (MCA) and Current Warm Period (CWP), punctuated by a weak phase during the Little Ice Age (LIA). Meteorological records indicate an increasing trend in the intensity and frequency of extreme rainfall events in the last few decades leading to widespread floods and droughts. High-resolution climatic records from marine as well as continental archives improve our understanding of Indian monsoon variability and its forcing factors on different time scales.