Research papersThe impact of monsoon winds and mesoscale eddies on thermohaline structures and circulation patterns in the northern South China Sea

•EOF method is applied to separate the impacts of monsoon winds and eddies.•Seasonality is attributed to the monsoon southeast of southeastern Vietnam.•Case studies are carried out to demonstrate the hydrographic structures of eddies.•The general circulation pattern in the NSCS is restored by removing the eddy impact.•Quantitative assessment of the impact of monsoon winds and eddies is carried out.

We deployed 5 pressure-recording inverted echo sounders (PIES) along a section in the northern South China Sea (NSCS), and estimated well the distributions of temperature, salinity and velocity across the section. Applying the empirical orthogonal function (EOF) method, we found that variability of the estimates is dominated by two modes: one named the seasonal mode affecting strongly on the hydrographic distribution with explained variability of temperature/salinity by 62.9/72.2%; the other named the eddy mode, corresponding to the arrival of mesoscale eddies, affecting strongly on the circulation pattern with explained variability of velocity by 63.2%. Temporal variation of the seasonal mode is highly correlated with the monsoon winds southeast of Vietnam, suggesting a nonlocal forcing mechanism. Case studies looking at the structures and evolutions of three captured eddies, whose impacts were well quantified by the eddy mode. The monsoon (eddies) significantly affects temperature, salinity and velocity shallower than 635 m (860 m), 160 m (150 m) and 1055 m (920 m), respectively. The monsoon (eddies) can induce maximum temperature, salinity and velocity anomalies up to −1.6 to 2.1 °C (−2.5 to 2.2 °C), −0.11 to 0.14 psu (−0.13 to 0.27 psu) and −0.31 to 0.46 m/s (−0.40 to 0.38 m/s), respectively. Mean volume transport (VT) across the section is 1.0 Sv (1 Sv=1×106 m3 s−1, positive to the northeast). Seasonal VT (with eddy impacts removed) is −4.6 Sv, 11.4 Sv, −5.1 Sv and −4.1 Sv for spring, summer, autumn and winter, respectively.