Correspondence of ENSO-like processes to the thermocline planktic foraminifera in the East China Sea over the last 400 ka

The El Niño-Southern Oscillation (ENSO) is a profound climatic and oceanographic phenomenon arising from the thermal contrast between the western and eastern Pacific Ocean, and has far-reaching influences on ocean heat distribution, monsoonal rainfall and thermal structure of the water column in the Pacific Ocean. The East China Sea (ECS) is a marginal sea in the western Pacific, significantly influenced by the Kuroshio Current (KC), which originates from the West Pacific Warm Pool. The warm and saline water transported by KC determines the surface-to-subsurface oceanographic conditions, including the thermocline structure of the ECS. Downcore variability of the thermocline-sensitive planktic foraminifera over ~400 ka at IODP Site U1429 has been considered to decipher the linkage of ENSO-like processes and the thermocline structure in the ECS. Singular Value Decomposition (SVD) analysis was applied to major thermocline planktic foraminiferal ( Neogloboquadrina dutertrei, Globoconella inflata , and Pulleniatina obliquiloculata ) abundance data to extract three modes representing three distinct palaeoclimate signals. Trend-synchronisation tests were carried out to study the linear/non-linear relationship between obtained SVD modes, KC strength, SST, Salinity, Insolation variability and paleo-ENSO Proxy. SVD Mode 1 corresponds to KC subsurface intrusion, Mode 2 represents KC-induced seasonal upwelling, and Mode 3 is found to be linked with the residual signal of ENSO-like processes in the ECS. This study reveals that stronger KC during interglacial phases (MIS 9, 7, and 5) corresponds to La Niña-like conditions, which deepens the thermocline as indicated by thermocline species P. obliquiloculata . The intensification of La Niña-like conditions in the ECS occurred during the last 150 ka, with no effect of local insolation on the thermocline depths in the ECS.