Variation of the winter monsoon in South China Sea over the past 183 years: Evidence from oxygen isotopes in coral

Oxygen isotope (δ18O) data in winter months in living coral Porites lutea collected from the Xisha Islands, South China Sea is significantly correlated to the measured winter monsoon velocity (WMV) with a correlation coefficient of 0.63. Based on 40-year (1961–2000) instrumental data, a transfer function between the WMV and the winter δ18O is established: WMV = 2.819 δ18O + 19.615 (N = 40, p < 0.0001). Based on the calculated WMV from δ18O over the past 183 years, the sequence of winter monsoon velocity (WMV) from year 1818 to 2000 in the South China Sea is re-constructed. The sequence can be divided into three stages: the first stage shows a decreasing trend of 0.009 m/s·yr from 1818 to 1954, the second stage indicates an increasing trend of 0.011 m/s·yr from 1955 to 1976, and the last stage shows a decreasing trend of 0.026 m/s·yr from 1977 to 2000. The maximum reduction in winter monsoon velocity over the last 24 years (1977–2000) is approximately 20% of the average over the last 183 years (1918–2000). The variation of wind velocity shows two complete cycles over the past 183 years. In particular, the strongest and weakest winter monsoon velocities in the last two centuries occurred in the 1830s and 1940s, respectively. The variation in winter monsoon velocity in the 20th century is closely linked to the surface temperature of the South China Sea, as well as air temperature over continental China. In addition, the winter monsoon was weak during two warm periods, i.e. the 1940s and 1980s. Statistical analysis of the winter monsoon velocity anomaly as well as the El Niño (warm event) and La Niña (cold event) phenomena indicates that 70% of El Niño events correspond to the weakness of the winter monsoon.