A dipole pattern of orbital-scale precipitation oxygen isotope variation in North African monsoon region and the driving mechanism

Abstract. Variations in North African (NAF) monsoon have pronounced socioeconomic impacts on many Western African countries. The orbital-scale NAF monsoon variation is reconstructed using speleothem δ¹⁸O (δ¹⁸O_c) records from caves. However, the climate meaning of these records is not well-known. δ¹⁸O_c in tropical monsoon regions is primarily determined by precipitation δ¹⁸O (δ¹⁸O_p). Here, we use the isotope-enabled Community Earth System Model (iCESM) to investigate the climate interpretation of δ¹⁸O_c in the NAF monsoon region on the orbital timescale. Our analysis shows that both δ¹⁸O_p and NAF monsoon rainfall exhibit a distinct precession signal. Enhanced Northern Hemisphere summer insolation (NHSI) intensifies land-sea thermal contrast, leading to strengthened monsoon circulation and increased NAF monsoon rainfall. However, the δ¹⁸O_p presents a spatially dipole pattern, with depletion in the southern NAF and enrichment in the north. Tagging experiments further reveal that, while depletion in the south is driven by increased rainfall in upstream regions, enrichment in the north results from shifts in moisture sources, with a reduced contribution from distant sources and an increased influence of local sources. This work provides new insights into understanding the climatic meaning of isotopic records in NAF region and underscores the complex spatial features of isotopic signals compared to rainfall responses.