Planktonic Foraminifera from Ocean Drilling Program Site 1143 in the southern South China Sea were analyzed in order to reconstruct changes in the structure of the upper ocean during the late Pliocene (3.6-2.1 Ma). The gradual decline of the Globigerinoides sacculifer/Globigerinoides ruber ratio from 3.27 Ma to 2.55 Ma indicates a successive decrease of sea surface temperature. The correspondence between sea surface temperature and benthic delta (super 18) O records suggests that the cooling was probably caused by the strengthening of the East Asian winter monsoon, which in turn is associated with progressive intensification of the Northern Hemisphere glaciation. At the same time, the stepwise increase in the relative abundance of mixed-layer species in the planktonic foraminiferal assemblage suggests a deepening of the thermocline. A comparison with records from the northern South China Sea reveals a significant increase in the southern-northern gradient of the depth of the thermocline, implying the final formation or further development of the "Western Pacific Warm Pool". The rapid decrease of SST and the thermocline depth at Site 1143 at about 3.2 Ma suggest a causal relationship between the Northern Hemisphere glaciation and the "Western Pacific Warm Pool".