Centennial-scale teleconnection between North Atlantic sea surface temperatures and the Indian summer monsoon during the Holocene
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Proxy records have shown that abrupt changes in the Indian summer monsoon (ISM) are closely linked to cold events in the North Atlantic at centennial timescales during the Holocene. However, mechanisms for these co-occurring phenomena are not fully understood. This study uses simulation results from a coupled atmosphere–ocean–sea-ice general circulation model forced by astronomical variations to investigate how summer (June, July, August and September) North Atlantic sea surface temperatures (SSTs) may have influenced the ISM at centennial timescales during the Holocene (9.5–0 ka BP). Our analyses identified an intimate relationship between the North Atlantic tripole SST (NATS) mode and the ISM. The NATS mode can affect the ISM in several ways. First, air–sea interactions over the tropical Atlantic can induce negative tropospheric temperature (TT) anomalies over the Indian Ocean, resulting in a strengthened meridional TT gradient favorable to a prolonged monsoonal rainy season. Second, a positive NATS mode tends to induce closed zonal vertical circulation over the tropical Atlantic, North Africa and the tropical Indian Ocean, creating anomalous convergence over India, and hence an enhanced ISM. Third, westerly surface wind anomalies, related to the NATS mode and coursing over the Arabian Sea, can increase moisture delivery to the monsoon region, causing enhanced rainfall in India. This mechanism resembles a decadal-scale mechanism that operates in the present-day climate. We also compared the Atlantic multidecadal oscillation (AMO), an alternative North Atlantic SST mode, with the NATS mode to determine their relationships to the ISM. A Holocene transient simulation indicates that the AMO’s trend has diverged from that of the ISM since 5.5 ka BP, due to inverse SST trends over the tropical and extratropical North Atlantic. This latter trend leads to a much weaker relationship between the AMO and the ISM, relative to that observed between the NATS mode and the ISM. We therefore suggest that the centennial relationship between the North Atlantic SSTs and the ISM during the Holocene differs from the decadal to multidecadal relationship they share in the present-day climate system.
KeywordsNorth Atlantic tripole SST variability Atlantic multidecadal oscillation Indian summer monsoon Holocene
This research was jointly supported by the National Natural Science Foundation of China (NSFC) (41275071, 41130102) and the Fundamental Research Funds for the Central Universities (lzujbky-2015-218). LJ acknowledges support from a Deutsche Forschungsgemeinschaft (DFG) grant through the Cluster of Excellence’s Future Ocean initiative (EXC 80/1). Modeling experiments using the KCM were performed at the Kiel University Computer Center, Kiel, Germany. W. Park and L. Mojib of GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel and B. Schneider of Christian-Albrechts-Universität zu Kiel are kindly thanked for their help and thoughtful discussions on KCM simulation methods. The authors also appreciate the reviewers’ constructive comments and suggestions on an earlier version of this paper.
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