doi: 10.1073/pnas.2107720119. Epub 2022 Mar 1.
Higher sea surface temperature in the Indian Ocean during the Last Interglacial weakened the South Asian monsoon
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- PMID:35238640
- PMCID: PMC8915836
- DOI: 10.1073/pnas.2107720119
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Higher sea surface temperature in the Indian Ocean during the Last Interglacial weakened the South Asian monsoon
Yiming V Wang et al. Proc Natl Acad Sci U S A..
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Abstract
SignificanceUnderstanding the drivers of South Asian monsoon intensity is pivotal for improving climate forecasting under global warming scenarios. Solar insolation is assumed to be the dominant driver of monsoon variability in warm climate regimes, but this has not been verified by proxy data. We report a South Asian monsoon rainfall record spanning the last ∼130 kyr in the Ganges-Brahmaputra-Meghna river catchment. Our multiproxy data reveal that the South Asian monsoon was weaker during the Last Interglacial (130 to 115 ka)-despite higher insolation-than during the Holocene (11.6 ka to present), thus questioning the widely accepted model assumption. Our work implies that Indian Ocean warming may increase the occurrence of severe monsoon failures in South Asia.
Keywords: Bay of Bengal; Indian summer monsoon; compound specific isotopes; paleohydroclimate and paleoenvironment; sedimentary leaf wax.
Conflict of interest statement
The authors declare no competing interest.
Figures
Modern rainfall variability over South Asia. SST (red; 2 °C isotherms), amount of precipitation (green; mm per month), and surface wind direction and strength (arrows with speed proportional to vector length) over the Indian Ocean, India, and Southeast Asia for (A) January and (B) July. The position of marine sediment core SO 188-17286-1 (this study) is indicated by a yellow star, and the G-B-M catchment is marked by a dashed red line. The position of marine sediment core GeoB16602 from the South China Sea (16) is indicated by a red dot. Large pink and blue arrows indicate monsoonal wind directions for the summer and winter seasons, respectively. Monthly SST and precipitation as well as surface wind speed and strength for the 925-hPa pressure level were derived from National Centers for Environmental Prediction reanalysis data (http://iridl.ldeo.columbia.edu ). Black circles mark weather stations (1, Allahabad; 2, Barisal; 3, Chuadanga; 4, Dhaka (Savar); 5, Dinajpur; 6, New Delhi; 7, Satkhira; 8, Shillong; and 9, Sylhet) in the G-B-M catchment where the International Atomic Energy Agency has collected meteorological data (48).
Climate records derived from sediment core 17286-1 in comparison with other high- and low-latitude climate proxy records: (A) Antarctic ice cores composite CO2 concentration (53, 54). (B) δ13C (mean ± SE) ofn-C29 andn-C31 and concentration-weighted average δ13C based on all four homologs (n-C27,n-C29,n-C31, andn-C33) from sediment core 17286-1 as a proxy for vegetation (C3 vs. C4 plants) changes (this study). (C) Ice volume–corrected δD (mean ± SE) ofn-C29 andn-C31 and concentration-weighted average δDivc based on all four homologs (n-C27,n-C29,n-C31, andn-C33) from sediment core 17286-1 as a proxy for rainfall amount (this study). The dashed black line represents summer (JJA) mean insolation at 0 to 20°N (55). (D) Ice volume–corrected planktonic foraminiferal δ18O (δ18Osw-ivc; red) as a proxy for local sea water salinity compared to boreal summer insolation as in Fig. 2C (27). (E) δ18O record from Chinese stalagmites (56), reflecting past EASM changes. (F) North Greenland Ice Core Project (NGRIP) ice core δ18O record (57), reflecting climate variability and particular Heinrich events in the Northern Atlantic realm. Gray bars mark the YD, HE1, and the LGM as reflected in the NGRIP ice core δ18O record as well as their correlatives in the Asian monsoon proxy records. Age control points for sediment core 17286-1 are also indicated, including radiocarbon dates (black lines) and tie points (blue circles) derived from tuning of the benthic foraminifera δ18O record to the LS16 global δ18Obenthic stack (47). Red and blue intervals at the bottom delineate MISs, separating warm and cold climate periods, respectively.
Detailed comparison of GAM results from paleoclimate proxy records in sediment core 17286-1 and climate forcing during the Holocene and the Last Interglacial. (A) Summer (JJA) mean insolation at 0 to 20°N (55). (B) δDivc ofn-C29 andn-C31 and GAM results with 95% confidence intervals. (C) concentration-weighted δDivc based on all four homologs (n-C27,n-C29,n-C31, andn-C33) and respective GAM results with 95% confidence interval. (D) Freshwater runoff inferred from δ18Osw-ivc (27). (E) Vegetation trends as inferred fromn-alkane δ13C (mean ± SE) and GAM results forn-C29 andn-C31 with 95% confidence intervals. (F) Concentration-weighted δ13C based on all four homologs (n-C27,n-C29,n-C31, andn-C33) and GAM results with 95% confidence interval. (G) UK’37-based SST record of sediment core 17286-1 (27). (H) Amount-weighted meann-alkane δD record (adjusted for ice volume and temperature effect) from South China Sea sediment core GeoB16602 (16). (I) Antarctic ice core composite CO2 concentration (54, 59). (J) Eurasia ice volume relative to present day (21). The gray bars mark the Holocene (11.6 ka to present) and the Last Interglacial (130 to 115 ka), as well as the YD and HE1.
Comparison of SST records from the equatorial and tropical Indian Ocean during the Holocene and the Last Interglacial. (A) Map of the ISM domain with average July precipitation [WorldClim1 30 arc sec gridded precipitation data (71)] and the locations of sediment core 17286 (yellow star) and other regional SST records (white dots) that are displayed below. (B) UK’37-based SST record of sediment core 17286-1 (27). Marine sediment SST records based on either UK’37 orG. ruber Mg/Ca from tropical or equatorial Indian Ocean arranged in a west to east sequence: (C) UK’37-based SST record of sediment core RC09-1664 (72). (D) UK’37-based SST record of sediment core MD85668 (73). (E) UK’37-based SST record of sediment core MD86674 (73). (F) UK’37-based SST record of sediment core TY93-929 (74). (G) UK’37-based SST record of sediment core MD900963 (75). (H)G. ruber Mg/Ca-based SST record of sediment core SK157/4 (76). (I) UK’37-based SST record of sediment core GeoB10038-4 (77). (J)G. ruber Mg/Ca-based SST record of sediment core GeoB10038-4 (77). (K) UK’37-based SST records of sediment core SO139-74KL (78).
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