[M-IS06] Evolution and variability of the Tropical Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS06-06] Monsoon intensity is not the dominant control of glacial-interglacial SST latitudinal patterns in the South and East China Sea

Alicia Hou^1,2、*Sze Ling Ho¹、Ru-Yun Tung¹、Shih-Yun Lin¹、Masanobu Yamamoto³、Kuo-Fang Huang⁴、Yuan-Pin Chang⁵、Min-Te Chen⁶ (1.Institute of Oceanography, National Taiwan University、2.EPOC, University of Bordeaux、3.Faculty of Environmental Earth Science, Hokkaido University、4.Institute of Earth Sciences, Academia Sinica、5.Department of Oceanography, National Sun Yat-Sen University、6.Institute of Earth Sciences, Center of Excellence for the Oceans & Center of Excellence for Ocean Engineering, National Taiwan Ocean University)

キーワード：East Asian Winter Monsoon, Sea surface temperature, Mg/Ca, UK'37, TEX86

In regions dominated by monsoon systems, such as the South China Sea (SCS), the annual cycle of sea surface temperature (SST) is strongly controlled by the seasonally reversing monsoon winds. As such, paleoclimate studies from the SCS have used proxy-based SST reconstructions to diagnose the strength of the East Asian Winter Monsoon (EAWM) during past climate intervals. During the Last Glacial Maximum (LGM), a strengthening of the latitudinal SST gradient from this region has been used to support the conventional view that the EAWM was enhanced. However, due to the biological origin of SST proxies, the derived climate information may be shaped by the ecology of the proxy producing organism. Furthermore, the sedimentation process may exert additional influence on the proxy signal preserved in the sedimentary archive. Here, we used published and newly generated multiproxy data (Mg/Ca, U^K'₃₇ and TEX₈₆) from four sites oriented along a latitudinal transect extending from the southern SCS into the East China Sea to assess whether the proxy-derived latitudinal gradient is a suitable metric for capturing past shifts in the intensity of the EAWM. By comparing proxy-derived late-Holocene SST reconstructions to modern climatological SSTs, we clearly demonstrate that at the two subtropical sites, which experience the strongest seasonal variations and are most sensitive to EAWM forcing, all of the examined proxies generate SSTs that are warm season biased. As such, the proxy-reconstructed late-Holocene latitudinal SST pattern deviates substantially from the winter climatological pattern. Based on this, we conclude that the proxy-reconstructed latitudinal SST gradient does not reflect winter conditions in this region and is likely not a sensitive indicator of changes in EAWM intensity. During the LGM, proxy-derived SSTs suggest that cooling ranged from ~2-4°C and generally increased with latitude, which is consistent with the Paleoclimate modeling Intercomparison Project (PMIP) ensemble mean pattern but differs from the patterns derived from data assimilation products. Based on our previous inference that the EAWM is not the dominant driver of SST changes in our study region, we suggest that the increase in cooling with latitude is primarily due to greenhouse forcing. Thus, our results demonstrate that even in monsoon regions, glacial-interglacial SST shifts may not directly reflect shifts in monsoon strength. Our study also reinforces the importance of ground-truthing proxy-based approaches using modern data.