The Asian monsoon is one of the most vigorous climatic phenomena on Earth and also one of the most societally important. The monsoon drives vital seasonal rainstorms that water crops and forests as well as damaging typhoons and floods. In a typical year, 80%–85% of the rain in the affected regions, often totaling 1.5–2.5 meters, falls during the summer monsoon season. On longer timescales, scientists have often cited the evolution of this seasonal wind flow over millions of years as one major cause of past changes in the environments, biosphere, and oceanography of this region, which includes the Indian subcontinent, Southeast Asia, China, Korea, and Japan. Many important questions remain about the monsoon and its effects, however, such as how and why it has changed in intensity through geologic time.
This past January, scientists gathered at AGU’s headquarters in Washington, D.C., for a Chapman Conference to discuss the evolution of the Asian monsoon. The review was timely, given that the first stages of research based on samples collected during a 2014–2016 series of scientific ocean drilling expeditions by the International Ocean Discovery Program (IODP) had just been completed. These expeditions provided, for the first time, relatively continuous records of the evolving oceanographic and climate conditions, as well as the erosion and weathering of the continent that spanned millions of years from all the major basins around South and East Asia. These records contributed to our understanding of the monsoon by allowing the long-term development of climate to be compared with the evolution of the solid Earth and surface processes over million-year timescales.
The monsoon can be thought of as a mobile, wide belt of low pressure akin to the equatorial Intertropical Convergence Zone, where the trade winds of the Northern and Southern Hemispheres come together. Thus, in addition to the ocean drilling research results, participants at the Chapman Conference reviewed an array of atmospheric models that have attempted to explain the intensity of the circulation of the Asian monsoon.
Attendees discussed interpretations of the effects of tectonic and climatic influences, erosion, and planetary-scale factors related to the monsoon on plant and animal life, precipitation levels, and carbon sequestration. They also addressed issues of a more basic nature: One outcome of the meeting was an expressed desire for a consensus of what Earth scientists mean when they talk about the Asian monsoon. Although some researchers focus on the intensity and duration of summer rainfall, others address seasonal wind systems or periods of upwelling and productivity in the oceans.
The monsoons of Asia comprise a dry, cold winter phase and a wet, warm summer phase. During winter, cold, dry winds blow out of the continent, driven by an atmospheric high-pressure system located in Siberia. In summer, moisture is delivered from the Indian Ocean to the Indian subcontinent, Mainland Southeast Asia, and SW China in the case of the South Asian monsoon (SAM). This flow is driven by a low-pressure system in northern India. In the East Asian monsoon (EAM), the moisture derives from the Pacific Ocean and South China Sea and is transported to central and northern China, as well as to the Korean Peninsula and the Japanese islands. The Asian monsoons mirror a similar antiphased and less intense seasonal climate system affecting northern Australia and New Guinea. The summer wet season progressively advances across each region but typically commences in April–May, finishing in September–October.
When Continents Collide
Uplift of the Tibetan Plateau and the Himalayan mountains, driven by the collision of the Indian subcontinent with the Eurasian plate starting some 55 million years ago, has exerted significant influences on numerous physical phenomena across the region, including profound effects on the weather. Conference attendees presented several examples of these effects. For example, the topographic barrier of the Himalayas strengthens the SAM, but it reduces rainfall across the Arabian Sea. The growth of the Tibetan Plateau, meanwhile, may have a bigger impact in East Asia.
Even distant tectonic changes have contributed to the evolution of the Asian monsoon, producing widely varying climate conditions in different regions. The closure of the Tethyan oceanic gateway, for example, which separated the Mediterranean Sea from the Indian Ocean about 14 million years ago (Ma), is seen as critical to the establishment of the Somali Jet, a strong air current that flows along the northeastern African coast and affects the monsoon by limiting the advance of the summer rain front. And uplift in Siberia and Mongolia over the past 10 million years may have affected the winter monsoon.
Seasonal Cycles and Climate Effects
Earth’s rotation and large-scale climate effects also influence the monsoon, which in turn affects the evolution and spread of various life-forms. Marine salinity records spanning 400,000 years in the East China Sea show climate-driven differences in the SAM and EAM tuned to slow changes in Earth’s orbit and the tilt of its axis (orbital forcing), but they give little evidence for direct forcing from the amount of sunlight a region receives (insolation). Oxygen isotope data in foraminifers can be corrected for global isotope changes and local temperature to isolate the effects of freshwater dilution from the Yangtze River, a relatively simple rainfall proxy. This adjusted record of discharge implies that the EAM is more sensitive to greenhouse gas concentrations and high-latitude ice sheet forcing than solar heating of the continent, as previously proposed.
Southern Hemisphere climate appears to have guided the more tropical SAM because much of the moisture to the SAM comes from the Southern Hemisphere. Beginning about 13 Ma, the SAM winds began to strengthen, a phenomenon that is probably linked to the establishment of the East Antarctic ice sheet. The presence of this ice sheet strengthened the interhemispheric temperature gradient, leading to intensification of atmospheric and ocean circulation. This change was followed by cooling of the Indian Ocean from 8 to 6 Ma. Together these effects reduced rainfall along the Himalayas but increased rainfall in peninsular India, the southern South China Sea, and Australia. Freshening of the Arabian Sea at that time is linked to cooling and to Northern Hemisphere ice sheet expansion. Runoff from the Ganges-Brahmaputra and Irrawaddy Rivers, which is correlated with precipitation levels, increased after 5.5 Ma as the regional late Miocene climate warmed and increased precipitation following a period of drying between 8 and 6 Ma.
More recently, major changes occurred in monsoon cyclicity through the Mid-Pleistocene Transition, a significant shift in the behavior of glacial cycles that occurred 1.25–0.7 Ma. During this time, cyclicity changed from 41,000-year cycles related to Earth’s obliquity (the planet’s tilt on its axis) to 100,000-year cycles related to precession (the imaginary circles that Earth’s axis traces as it moves). It is clear that the Australian monsoon, which is linked to the Asian monsoon, follows a cycle that is driven more strongly by insolation, compared with the SAM and EAM. The Australian monsoon and the SAM follow cycles that are antiphased: One cycle reaches its maximum rainfall and wind speed at the same time the other reaches its minimum.
Paleoprecipitation reconstructions based on leaf wax records indicate that the Himalayas were exposed to a wet monsoon through most of the Miocene (23–8 Ma), and precipitation levels were then relatively stable through the Pliocene until the beginning of the Pleistocene about 2.6 Ma.
Just as the uplift of the Himalayas and the Tibetan Plateau influenced the monsoons, the strength of the monsoons influenced the erosion of the Himalayas. The amount of carbonate minerals washed into the Bengal Fan, a submerged river delta in the Bay of Bengal, from weathered rocks upstream peaks at 5–13 Ma in the stratigraphic record, a further suggestion that the monsoon was especially strong during this period.
This discharge of carbonate-rich material coincides with erosion of the Tethyan Himalayas and initial unroofing (tectonic and erosional removal of overlying units) of the Lesser Himalayas after 8 Ma. The Tethyan Himalayas, lightly metamorphosed sedimentary rocks that were deposited on the northern margin of India before it collided with Eurasia, are generally exposed in the rain shadow north of the highest ranges of the Greater Himalayas. The Lesser Himalayas are weakly metamorphosed rocks that structurally underlie the high-grade rocks of the Greater Himalayas and are mostly exposed in the foothills on the southern flank of the range. Widespread erosion of the Lesser Himalayas occurred later, after 3 Ma in the western Indus Basin compared to farther east.
The switch in the dominant sources of erosion from northern to southern areas likely reflects the raising of the High Himalayan barrier due to the ongoing tectonic collision of India and Asia, especially the break off of the subducting lithospheric slab. The lack of focused erosion in the High Himalayas promoted propagation of the Himalayan front southward after 10 Ma. Dating of detrital zircons in the eastern Nicobar Fan (a lobe of the Bengal Fan) indicates that its sources of eroded material did not shift geographically as much as the sources of the main Bengal Fan did. The Nicobar Fan shows the influence of tectonically driven basin inversion as well as erosion of primary sources in supplying sediment to the deep sea.
Although we often associate heavy rainfall and flooding with increases in erosion, drying of the Indian peninsula also led to enhanced erosion on various timescales. A loss of stabilizing plant cover largely drove this erosion, implying that erosion does not have a simple linear relationship with precipitation.
How does erosion affect Earth’s carbon cycle? Attendees at the Chapman Conference raised doubts about whether chemical weathering of Himalaya-derived sediment controlled global climate through carbon dioxide drawdown because of a lack of a clear trend to increased weathering reconstructed from the new drilled record and linked to the global cooling that occurred during the Miocene and Pliocene epochs.
Instead, burial of organic matter, including woody debris, especially on the Bengal Fan, appears to have had global significance in drawing down carbon dioxide through the Miocene and Pliocene. During this period, erosion of the Himalayas accelerated as the ranges rose following progressive tearing and break off of the dense Indian lithosphere that had thrust below the southern margin of Eurasia after the start of the Indian collision, largely from west to east between 24 and 12 Ma. The same uplift may have intensified the SAM and caused erosion to move south from the Tethyan Himalayas to the High Himalayas and Lesser Himalayas.
Tackling Unanswered Questions
Untangling these interacting influences remains a work very much in progress. Chapman Conference attendees identified several key future research goals. These goals include recovery of a complete record of erosion through the Paleogene (66–23 Ma); key areas include the Murray Ridge in the Arabian Sea, the Bengal Fan, and the Red River delta. This time period is important because of recent suggestions that the monsoon may have strengthened much earlier than generally proposed, around 36 Ma. Furthermore, if the formation of the Greater Himalayas after 23 Ma was climatically triggered, then an erosional record spanning their birth is required to test this hypothesis.
In general, if we are to make progress in quantifying erosion in the sedimentary record, then we must understand the 3D structure of the submarine fans through seismic surveys linked to ocean drilling. At higher resolution, we need to clarify sediment budgets over thousands of years (Milankovitch cycle scale) to resolve the links between tectonics and climate and to develop new models beyond the “channel flow” model that links surface processes and tectonics.
Concerning reconstruction of past continental environments, the research community aims to produce a regional Miocene vegetation cover map and to determine feedbacks between vegetation and monsoon climate.
Meanwhile, key goals in paleoceanographic research include more focus on winter monsoon reconstructions, an orbital-scale record of the monsoon extending back to the Middle Miocene Climatic Optimum (one of Earth’s most recent prolonged warming events), and a way to individually distinguish runoff effects from precipitation effects.
And perhaps future conferences will produce a consensus definition of “monsoon.”
We thank Heather Nalley, Amy Bocian, and Judy Dalie from AGU for their organizational support. We also thank the U.S. Science Support Program (USSSP) and IODP France and IODP Germany for partial financial support.
Peter D. Clift ([email protected]), Department of Geology and Geophysics, Louisiana State University, Baton Rouge; Ann Holbourn, Institute of Geosciences, Christian-Albrechts Universität zu Kiel, Kiel, Germany; Christian France-Lanord, Centre de Recherches Pétrographiques et Géochimiques, Centre National de la Recherche Scientifique–Université de Lorraine, Vandoeuvre les Nancy, France; and Hongbo Zheng, Research Center for Earth System Science, Yunnan University, Kunming, China