UC Berkeley

On orbital timescales, higher summer insolation is thought to strengthen the continental monsoon while weakening the maritime monsoon in the Northern hemisphere. Through simulations using the Community Earth System Model, we evaluated the relative influence of perihelion precession and high obliquity in the early Holocene during the Asian summer monsoon. The major finding was that precession dominates the atmospheric heating change over the Tibetan Plateau–Himalayas and Maritime Continent, whereas obliquity is responsible for the heating change over the equatorial Indian Ocean. Thus, precession and obliquity can play contrasting roles in driving the monsoons on orbital timescales. In late spring–early summer, interior Asian continental heating drives the South and East Asian monsoons. The broad-scale monsoonal circulation further expands zonally in July–August, corresponding to the development of summer monsoons in West Africa and the subtropical Western North Pacific (WNP) as well as a sizable increase in convection over the equatorial Indian Ocean. Tropical and oceanic heating becomes crucial in late summer. Over South Asia–Indian Ocean (50°E−110°E), the precession maximum intensifies the monsoonal Hadley cell (heating with an inland/highland origin), which is opposite to the meridional circulation change induced by high obliquity (heating with a tropical origin). The existence of the Tibetan Plateau–Himalayas intensifies the precessional impact. During the late-summer phase of the monsoon season, the effect of obliquity on tropical heating can be substantial. In addition to competing with Asian continental heating, obliquity-enhanced heating over the equatorial Indian Ocean also has a Walker-type circulation impact, resulting in suppression of precession-enhanced heating over the Maritime Continent.