Glacial fluctuations of the Indian monsoon and their relationship with North Atlantic abrupt climate change: new data and climate experiments
1Laboratoire des Sciences du Climat et de l'Environnement/IPSL, CEA-CNRS-UVSQ – UMR8212, CE Saclay, l'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
2Université de Sfax, Faculté des Sciences, Laboratoire GEOGLOB, BP 802, 3038 Sfax, Tunisia
*now at: Met Office, Exeter, UK
Abstract. Several paleoclimate records such as Chinese loess and speleothem sequences or upwelling indicators present large variations of the Asian monsoon system during the last glaciation. Here, a unique record in the northern Andaman Sea (core MD77-176) is used to reconstruct the variations of the hydrological cycle of the Bay of Bengal. The high resolution salinity record displays large millennial scale oscillations over the period 40 000 to 11 000 yr BP that are synchronous with the Greenland ice core record of changes in polar air temperature during the last glaciations. 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. We use the IPSL_CM4 model to study the processes that can explain the relationship between the Indian monsoon and the North Atlantic climate. A modelling experiment represents such a rapid event in the North Atlantic under glacial conditions by increasing the freshwater flux in the North Atlantic and reducing 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 the changes in salinity of the Bay of Bengal in the model. This is a favourable comparison with the new salinity record presented here. 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 Eurasia that in turn affect the intensity of the Indian monsoon.