State of the tropical Pacific Ocean and its enhanced impact on precipitation over East Asia during marine isotopic stage 13
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Multiple terrestrial records suggest that marine isotopic stage 13 (MIS-13), an interglacial period approximately 0.5 million years ago, had the strongest East Asian summer monsoon (EASM) of the last one million years. This is unexpected given that, compared to other interglacials, MIS-13 was globally cooler with a lower CO2 concentration. We use two coupled atmosphere–ocean general circulation models, the Hadley Centre Coupled Model, version 3 (HadCM3) and Community Climate System Model, version 3.0 (CCSM3), to simulate the climate of MIS-13 forced with different insolation and greenhouse gas concentrations relative to the pre-industrial (PrI) situation. Both models confirm a stronger EASM during MIS-13 compared to PrI. Here we specially focus on analyzing the impact of the tropical Pacific Ocean on the EASM. Our simulations suggest that the mean climatic state in the tropical Pacific during MIS-13 was La Niña-like and that associated teleconnections with the extra-tropics favored increased precipitation over the EASM. As compared to PrI, it is found that the summer (June–July–August) sea surface temperature (SST) is warmer in the eastern tropical Pacific Ocean and colder to the west. In concert with previous studies, we show that colder summer SSTs in the central tropical Pacific during MIS-13 promotes an upper-level teleconnection between the tropical Pacific Ocean and EASM. It also contributes to the strengthening of the northern Pacific subtropical high and, therefore, the transport of more moisture into the EASM. We suggest that the reduced east–west SST difference in the tropical Pacific in summer helps to maintain the teleconnection between the tropical Pacific and EASM. The correlation between tropical Pacific SSTs and the EASM was higher in our MIS-13 simulations, further supporting the enhancement of their relationship. It is found that the pure impact of El Niño Southern Oscillation on EASM precipitation increases by up to 30 % in MIS-13 for HadCM3 while it is minor for CCSM3. Better constraining the spatio-temporal variability of tropical Pacific SST during the interglacials may thus help explain the anomalously strong EASM during MIS-13 which has been observed from geological records.
KeywordsPaleoclimate modeling MIS-13 ENSO Teleconnection East Asian summer monsoon
This work and M. P. Karami were supported by the European Research Council Advanced Grant EMIS (No 227348 of the Programme ‘Ideas’). Q. Z. Yin is supported by the Belgian National Fund for Scientific Research (F.R.S.-FNRS). H. Muri is supported by the Research Council of Norway (Grant agreement 229760). We are grateful to the reviewers for their constructive comments and suggestions. We thank Dr. Fred Kucharski, Dr. Carlos Almeida, Gauillame Lenoir and Dr. Tobias Bayr for helpful discussions. Access to computer facilities was facilitated through sponsorship from S. A. Electrabel, Belgium. We are also grateful to CISM staff at Université catholique de Louvain for their technical support.
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