Deglacial-Holocene variability in Somali upwelling and its influence on Indian summer monsoon precipitation

Somali upwelling and Indian summer monsoon precipitation are caused by the southwest monsoonal winds during boreal summer. Recent studies observed that an increase in Somali upwelling can reduce the amount of precipitation over Indian sub-continent by reducing the moisture source area. To understand the paleo-relationship between Somali upwelling and Indian summer monsoon precipitation, we have reconstructed the Somali upwelling history during the last 18.5 ka using biogenic silica flux estimated from a sediment core retrieved from the western Arabian Sea. The reconstructed record demonstrates periodic changes in the Somali upwelling strength during the past 18.5 ka. Low biogenic silica fluxes observed at the bottom of the sediment core suggest weak upwelling during the last glacial period (18.5-15 ka BP). Post glacial onset of southwest monsoon is marked by strengthened upwelling during the Bølling-Allerød period (15-13 ka BP). The reduced biogenic silica flux at Younger Dryas (13-11 ka BP) indicates reduced upwelling strength. Intensification of the Somali upwelling at the beginning of the Holocene and a decline at 8 ka BP are observed. Increases in upwelling strength recorded since 8 ka BP suggest strengthening of the southwest monsoon during the latter part of the Holocene. Linking these upwelling variations with Indian summer monsoon precipitation records, a major shift has been observed in the relationship between the Somali upwelling strength and Indian summer monsoon precipitation from positive to negative during the pre-Holocene to the Holocene. The observed shift is attributed to the variation in the strength of southwest monsoon due to latitudinal shift of the Intertropical Convergence Zone (ITCZ) associated with changes in moisture source region.