Sub‐Annual to Interannual Arabian Sea Upwelling, Sea Surface Temperature, and Indian Monsoon Rainfall Reconstructed Using Congruent Micrometer‐Scale Climate Proxies

Abstract

AbstractThe Indian monsoon system impacts over 1 billion people, and rainfall from its southwest monsoon is critical for agriculture. Cool sea surface temperatures (SSTs) in the Arabian Sea have been associated with reduced monsoon rainfall and vice versa, although this relationship is difficult to examine due to scarce and short instrumental records. We utilize laminated Pakistan Margin sediments to reconstruct southwest monsoon rainfall and Arabian Sea SSTs at sub‐annual to multi‐annual resolution for the twentieth century. We applied paired sub‐mm mapping techniques to intact sediment core surfaces to measure elemental abundances and lipid biomarkers. The first principal component of the elemental results (PC1) explains 47% of the elemental variance and is driven by detrital lithogenic elements K, Ti, and Fe. PC1 is positively correlated with homogeneous India June‐September rainfall departures from 1903 to 1928 and 1949–1970 on a multi‐annual scale. Correlations in the remaining timespans are either negative or absent, likely driven by a combination of dam and barrage construction on the Hub and Indus Rivers and negative rainfall departures. We propose PC1 as a proxy for southwest monsoon rainfall on multi‐annual timescales prior to ∼1930. A dampened annual SST cycle is resolved with the alkenone paleothermometer, which also captures the interannual‐decadal trends of twentieth century Arabian Sea SSTs. We propose the crenarchaeol‐caldarchaeol tetraether index, sourced from planktonic Thaumarchaeota, as a delayed upwelling proxy for the western Arabian Sea. Our reconstructions corroborate the established relationship of reduced Arabian Sea upwelling leading to warmer SSTs and increased monsoon rainfall, and vice versa.