Glacial fluctuations of the Indian monsoon and their relationship with North Atlantic climate: new data and modelling experiments

Abstract

Abstract. Several paleoclimate records such as from Chinese loess, speleothems or upwelling indicators in marine sediments present large variations of the Asian monsoon system during the last glaciation. Here, we present a new record from the northern Andaman Sea (core MD77-176) which shows the variations of the hydrological cycle of the Bay of Bengal. The high-resolution record of surface water δ18O dominantly reflects salinity changes and displays large millennial-scale oscillations over the period 40 000 to 11 000 yr BP. Their timing and sequence suggests that events of high (resp. low) salinity in the Bay of Bengal, i.e. weak (resp. strong) Indian monsoon, correspond to cold (resp. warm) events in the North Atlantic and Arctic, as documented by the Greenland ice core record. We use the IPSL_CM4 Atmosphere-Ocean coupled General Circulation Model to study the processes that could explain the teleconnection between the Indian monsoon and the North Atlantic climate. We first analyse a numerical experiment in which such a rapid event in the North Atlantic is obtained under glacial conditions by increasing the freshwater flux in the North Atlantic, which results in a reduction of the intensity of the Atlantic meridional overturning circulation. This freshwater hosing results in a weakening of the Indian monsoon rainfall and circulation. The changes in the continental runoff and local hydrological cycle are responsible for an increase in salinity in the Bay of Bengal. This therefore compares favourably with the new sea water δ18O record presented here and the hypothesis of synchronous cold North Atlantic and weak Indian monsoon events. Additional sensitivity experiments are produced with the LMDZ atmospheric model to analyse the teleconnection mechanisms between the North Atlantic and the Indian monsoon. The changes over the tropical Atlantic are shown to be essential in triggering perturbations of the subtropical jet over Africa and Eurasia, that in turn affect the intensity of the Indian monsoon. These relationships are also found to be valid in additional coupled model simulations in which the Atlantic meridional overturning circulation (AMOC) is forced to resume.

North Atlantic forcing of tropical Indian Ocean climate

“…10). A similar circulation anomaly over Africa and the Arabian Sea was recently found in freshwater hosing experiments by Marzin et al 3 and, by means of atmosphere-only sensitivity experiments, could be attributed to anomalous sea surface temperature forcing in the tropical Atlantic ("tropical pathway"). Unrelated to the Indian summer monsoon an equivalent barotropic wave train propagating south-eastward from the northern North Atlantic source region is also found in the Heinrich Stadial winter circulation anomaly (Extended Data Fig.…”

“…As part of the Hadley cell reorganization during Heinrich stadials, the Indian summer monsoon weakens. This weakening has been recently attributed to anomalous SST forcing in the tropical Atlantic that affects the Indian monsoon via a tropical atmospheric pathway 3 . Our model results suggest a rapid response of the Indian monsoon to North Atlantic abrupt events through both a stationary Rossby wave-train teleconnection that originates in the northern North Atlantic and a tropical atmospheric pathway (Methods).…”

Phase relationships between orbital forcing and the composition of air trapped in Antarctic ice cores

“…As evidenced over Terminations I and II and over the last 240 ka, δ 18 O atm variations are closely related to the dynamic of the low-latitude hydrological cycle (Wang et al, 2008;Severinghaus et al, 2009;Landais et al, 2007Landais et al, , 2010Landais et al, , 2013Cheng et al, 2009). Monsoons are influenced by orbital forcing, with a strong imprint of precession (Wang et al, 2008;Braconnot et al, 2008), but also by the millennial-scale variability (Wang et al, 2001;Marzin et al, 2013). The Heinrich event 1 is for instance associated with a weak monsoon interval (e.g.…”

Evolution of the Indian summer monsoon during the interval 32.7–11.4cal. ka BP: Evidence from the Baoxiu peat, Yunnan, southwest China

“…The abrupt climate changes seen in our records correspond closely to the wet periods recorded in stalagmite d 18 O data from Brazil (Wang et al, , 2007, which also correspond to Heinrich events in the North Atlantic (Bond et al, 1997). Many studies have found that ISM and EASM precipitation patterns are related to variations in North Atlantic climate (Cosford et al, 2008;Deplazes et al, 2013;Marzin et al, 2013;Tierney and Pausata, 2015). The mechanism could explain the millennial-scale teleconnection between the North Atlantic, Asian monsoon region, and South America.…”