Variations of the Somali upwelling since 18.5 ka BP and its relationship with southwest monsoon rainfall

Abstract

Abstract. Somali upwelling history has been reconstructed for the last 18.5 ka BP based on biogenic silica fluxes estimated from a sediment core retrieved from the western Arabian Sea. Surface winds along the east African coast during the southwest monsoon (SWM) cause the Somali upwelling; thus, the intensity of this upwelling has been related to the variability of the SWM. Biogenic silica flux variation suggests periodic weakening and strengthening of the Somali upwelling. Weakened upwelling during the 18.5–15 ka BP period and strengthened upwelling during the Bølling–Allerød (15–12.9 ka BP) suggest the onset of the SWM. The Younger Dryas (12.9–11.7 ka BP) is marked by reduced upwelling strength, with an intensification of the Somali upwelling observed at the beginning of the Holocene and a further decline at 8 ka BP. The increase in the upwelling strength recorded since 8 ka BP suggests SWM strengthening during the latter part of the Holocene. A comparison of upwelling variations with the SWM precipitation record demonstrates a reversal in the relationship between the strength of the Somali upwelling and SWM rainfall at the beginning of the Holocene. This observed shift has been attributed to the variation in the SWM strength due to the latitudinal shift of the intertropical convergence zone (ITCZ) associated with changes in moisture sources.