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The East Asian winter monsoon (EAWM) significantly impacts living conditions in a large part of Asia, and therefore, it is important to understand its major driving mechanisms. Winter sea surface temperature (SSTW) and circulation in the southern Okinawa Trough are today both primarily controlled by the EAWM. Here we present a new SSTW reconstruction for the last millennium based on a diatom record from sediment core MD05‐2908, from the continental slope of the southern Okinawa Trough off northeastern Taiwan. Our reconstruction indicates that SSTW varied between 14.1 and 19.6°C over the past 1,000 years. Changes in SSTW in the southern Okinawa Trough correspond closely to the index of warm winters based on historical documents from the East Asian monsoon domain. This implies that our SSTW record can be used to reconstruct EAWM variability during the last millennium. Comparisons with the reconstructed winter Arctic Oscillation (AO, developed from historical snow anomaly events in Eastern Asia) and Arctic sea ice cover reveal a significant positive correlation between the EAWM and AO during the time interval from 1000–1400 Common Era (C.E.), coinciding with reduced sea ice cover. However, there is no significant correlation with increased sea ice cover during the interval from 1400 to 1700 C.E. This suggests that the reduction in Arctic sea ice may periodically have played a role in strengthening the relationship between the EAWM and the AO during the last millennium and that the current and future reduction in Arctic sea ice may have significant consequences for the EAWM.

Palaeoceanographic changes over the last 6800 years are documented in a diatom record from core MD05–2908 from the southern Okinawa Trough. Changes in diatom components are used to reflect variations in the strength of the Kuroshio Current and the influence of coastal waters, and these changes are compared with records of the East Asian summer monsoon, which exerts an important influence on regional climate and oceanography. A decreasing trend in the contribution of coastal diatoms during the last 6800 years is related to the weakening of the East Asian summer monsoon during the Holocene in response to diminishing insolation and a southward shift in the intertropical convergence zone. Relatively high abundances of the Kuroshio Current indicator species during the intervals 2600–2200 and 4800–3600 cal a BP, and lower abundances during the intervals 3200–2800 and 2200–1300 cal a BP, imply that there is no consistent weakening of the Kuroshio Current during the interval 4500–3000 cal a BP − the Pulleniatina Minimum Event, which is a widespread feature in the East China Sea and the South China Sea. A major reduction in coastal diatoms at 3200 cal a BP and a slight decrease in the Kuroshio Current indicator species during the interval 3200–2800 cal a BP indicate that a palaeoceanographic shift occurred in the southern Okinawa Trough, which coincided with a Northern Hemisphere climatic cooling, denoted the Neoglaciation in Europe. In addition, changes in the freshwater species component of the record are suggested mainly to result from variations in flood frequency in north‐eastern Taiwan caused by typhoons. A dramatic increase in the frequency of flood events during the last millennium may have been caused by relatively strong El Niño‐Southern Oscillation activity, compared with the more stable period between 6800 and 1000 cal a BP, which experienced a low frequency of flood events.

We analyzed sediment diatoms from core MD05-2908 to infer climate and paleoenvironmental changes in the southern Okinawa Trough (SOT) over the past 1000 years. Because the study area is located in the East Asia monsoon area and beneath the main axis of Kuroshio Current, the climatic and hydrographical conditions are strongly influenced by both of these factors. The species used as environmental indicators, including the Kuroshio Current species (KC species) and freshwater species, were investigated in this paper. Changes in the abundance of the two groups of species revealed significant variations in water temperature and hydrography in the SOT during the Medieval Warm Period (MWP) and the Little Ice Age (LIA). From 950-1500 AD, the abundance of the KC species increased fluctuantly, while the freshwater species decreased, showing that the influence of the Kuroshio Current was intensified at that interval and the precipitation of the study area was relatively low. The KC species decreased remarkably and was maintained at a low abundance during the interval of 1500-1900 AD, which suggests that the impact of the Kuroshio Current on the SOT weakened during the period corresponding to the LIA. Moreover, the high abundance of the freshwater species at the same interval indicates a distinct increase in precipitation in northeastern Taiwan, which may be correlated to the south-detention of the rainfall belt in China caused by the southward migration of the western Pacific subtropical high.