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Subsurface mesoscale eddies observed in the northeastern South China Sea: dynamic features and water mass transport


A train of subsurface mesoscale eddies (SMEs) consisting of two cyclones and two anticyclones was observed in the northeastern South China Sea (NESCS) in 2015 by a mooring array. In contrast to the widely reported surface-intensified eddies, the SMEs had weak surface signals but showed maximum velocity at ∼370 m with a magnitude of 17.2 cm s-1. The SMEs generally propagated westward with a speed of ∼4.3 cm s-1, which resulted in a distinct ∼120 day-period oscillations in the moored time series. Based on the concurrent velocity, temperature, and salinity from the mooring array, three-dimensional structures of the SMEs were constructed, which were then used to quantify water mass transports induced by them. The results revealed that all these SMEs were vertically tilted with an influence depth exceeding 1000 m. Water mass analysis suggested that the cyclonic and anticyclonic SMEs trapped the northwest Pacific water and the NESCS local water, respectively. The cyclones transported 1.00±0.25 Sv North Pacific Intermediate Water westward into the NESCS during the two-year observation period, accounting for 61.7% of the observed volume transport through the Luzon Strait between 25.8–27.4σ0. Furthermore, it also showed that both the trapping and stirring effects of the SMEs induced an eastward heat transport across the Luzon Strait but the role of the former was much more important than the latter. The present results suggested that the SMEs near the Luzon Strait may provide a novel route for the intermediate-layer water exchange between the NESCS and Pacific.