[M-IS02] Evolution and variability of the Asian Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS02-08] Hemispherically asymmetric trade wind changes in the Indo-Pacific region drive subsurface temperature evolution over the past ~25 kyr

*Alicia Meng Xiao Hou¹、Sze Ling Ho¹、Ren Yi Ooi¹、AiLin Chen¹、Kuo-Fang Huang²、Yuan-Pin Chang³、Min-Te Chen⁴、Chuan-Chou Shen¹、Yu-Heng Tseng¹、Yi-Chun Kuo¹ (1.National Taiwan University、2.Academia Sinica、3.National Sun Yat-Sen University、4.National Taiwan Ocean University)

キーワード：Indo-Pacific, Trade winds, Intertropical convergence zone, Last Glacial Maximum, subsurface ocean temperature, foraminifera

Indo-Pacific upper ocean thermal fluctuations are typically used to infer past changes in the Asian-Australian monsoons, El Niño-Southern Oscillation, Pacific Ocean circulation, and Western Pacific Warm Pool (WPWP) heat content. To understand how oceanic and atmospheric circulation systems in this region respond to drastic changes in global climate, it is useful to determine the mechanisms responsible for upper ocean temperature changes during past transitions between glacial-interglacial states. In this study, we present surface and subsurface temperature records spanning the last ~25 kyr based on mixed-layer and subsurface dwelling planktic foraminifera from marine sediment cores retrieved from three sites located in the Okinawa Trough, northern South China Sea (SCS), and off the coast of Sumatra. Modern climatological data from these sites indicate that subsurface temperatures are closely related to seasonal shifts in trade wind strength. We thus propose that long-term subsurface temperature changes at these sites may reflect glacial-interglacial variations in the intensity of the northeast (NE) and southeast (SE) trade winds. Interestingly, our temperature reconstructions from all three study sites indicate that different species of subsurface foraminifera from the same core reveal vastly different long-term subsurface temperature patterns. We suggest that this may be a result of potential offsets in habitat depth (i.e., lower mixed-layer dwelling vs. upper thermocline dwelling) and/or differences in habitat depth range (i.e., narrow vs. wide range) between different species. The primary subsurface temperature records discussed here are based on the upper-thermocline dwelling species, Neogloboquadrina dutertrei, which has a broad vertical habitat range that captures the mean upper-thermocline temperature signal. Our surface and subsurface ocean temperature records from all three sites reveal divergent trends over the last ~25 kyr; while the surface layer was coldest during the last glacial maximum (LGM) and gradually warmed towards the late-Holocene, subsurface temperatures were warmest during the LGM and late-Holocene and coldest during the mid-Holocene. This pattern of glacial-interglacial subsurface temperature evolution differs considerably from reconstructions from the WPWP. We attribute this dissimilarity to the enhanced subsurface layer sensitivity to wind-driven processes at our study localities and propose that hemispherically asymmetric trade wind changes over the last glacial-interglacial cycle produced the observed subsurface temperature changes at our sites. Coeval warming during the LGM at all three sites was driven by strengthened (weakened) NE (SE) trade winds which enhanced vertical mixing in the Okinawa Trough and northern SCS and reduced coastal upwelling off the coast of Sumatra. Subsurface cooling at all three sites during the mid-Holocene was caused by a weakening (strengthening) of the NE (SE) trade winds which diminished vertical mixing in the Okinawa Trough and northern SCS and intensified upwelling off the coast of Sumatra. The inferred trade wind changes based on our subsurface temperature records are supported by results from the AWI Earth System Model which indicate stronger (weaker) NE (SE) trade winds during the LGM and weaker (stronger) NE (SE) trade winds during the mid-Holocene. Since variability in the strength of the trade winds are typically associated with migrations of the intertropical convergence zone (ITCZ), our results suggest that in the western Pacific, the ITCZ was displaced northward during the LGM and late-Holocene and southward during the mid-Holocene. Thus, the present study demonstrates an unprecedented link between subsurface temperature evolution in the Indo-Pacific oceans and latitudinal displacements of the ITCZ over the last glacial-interglacial cycle.