Review of speleothem records of the late Holocene: Indian summer monsoon variability & interplay between the solar and oceanic forcing

Abstract

Indian summer monsoon (ISM) variability in the late Holocene (last 4000 years) is of great interest as the arid phase of ISM and cultivation of kharif (rainy season) crops started in this period. The present study aims to revisit the available speleothem records to study ISM rainfall variations and possible controlling factors in different Indian regions viz., core monsoon zone (CMZ), peninsular, the Himalaya, and the Bay of Bengal in the last 4000 years. The late Holocene encompasses notable global warm periods viz., Medieval Climate Anomaly, Roman Warm Period (RWP) & Minoan Warm Period, and cold periods viz., Dark Age cold period and Little Ice Age (LIA). ISM rainfall declines until the middle of RWP in all the regions. The decline is linked with the southward migration of the Intertropical convergence zone caused by the northern Hemisphere summer insolation. Though the Solar insolation follows the same trend, all the records show large variability with the disappearance of the declining trend around the middle of RWP. This change coincided with the reversal in the north Atlantic Sea surface temperature (SST) trend, northern hemisphere temperature, and abrupt changes in the southeastern Indian Ocean SST. This period also witnessed high El-Nino Southern Oscillation activities. This synergy of ISM variations in the characterized zones falls apart during the pre-LIA with a slight increasing (decreasing) trend in CMZ and Bay of Bengal (peninsular and Himalayan). The spatially variable response of ISM rainfall in recent centuries might be caused by the interaction between internal variability and external forcing and also by anthropogenic-induced warming and aerosol forcing.