The Kuroshio Current enters the South China Sea via the Luzon Strait and exerts significant effects on circulation and water mass properties of this marine basin. However, limited by the few on-site observations, multi-decadal variations in Luzon Strait Transport remain uncertain.

Recently, a research team led by Prof. Hu Dunxin and Prof. Sun Weidong from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) and their collaborators used coral isotopic proxy from Xiaoliuqiu Island along the Kuroshio intrusion route to reconstruct Luzon Strait Transport and investigated its interannual and decadal variability.

The study was published in Palaeogeography, Palaeoclimatology, Palaeoecology on Feb. 18.

Stable δ¹³C values of coral can record the δ¹³C variations of dissolved inorganic carbon in seawater. Coral δ¹⁸O values are controlled by both sea surface temperature and local seawater δ¹⁸O values, the latter of which is related to the hydrological balance (i.e., precipitation minus evaporation and ocean advection) and sea surface salinity.

The δ¹³C values of dissolved inorganic carbon and temperature of the South China Sea subsurface seawater are much lower than those of surface seawater, whereas the salinity is higher than that of the surface seawater.

The researchers found that increased (reduced) Luzon Strait Transport was expected to generate strong (weak) upwelling, resulting in low (high) δ¹³C values of dissolved inorganic carbon and sea surface temperature, and high (low) sea surface salinity, as recorded in the coral.

They reconstructed the interannual variability in the upper and surface layers of Luzon Strait Transport during 1977–2003 using the coral δ¹⁸O record. Coral δ¹⁸O data indicated that Luzon Strait Transport has weakened since the 1970s.

The change of Luzon Strait Transport was associated with the northward (southward) migration of the North Equatorial Current bifurcation latitude: a northward (southward) shift of the North Equatorial Current bifurcation latitude weakened (strengthened) upstream Kuroshio transport, favoring an increase (decrease) in the Kuroshio intrusion.

“The information recorded by coral provides an opportunity to study ocean current in the case that historical records are lacking,” said Li Xiaohua, first author of the study.