Upper vertical structures and mixed layer depth in the shelf of the northern South China Sea

Vertical structures of the water column, especially the surface mixed layer and the pycnocline layer, are important in the mediation of vertical transportation of oceanic matter, momentum, heat and salt. Field observations were conducted to investigate vertical structures and mixed layer depth (MLD) in the shelf sea of the northern South China Sea, where the Pearl River plume enters with distinct extents in different seasons. A total of six cruises were performed in spring, summer and autumn from 2006 to 2016. Four different types of vertical structures, A-α, A-β, B and C types, were identified in the upper water layer. The A-α type was characterized by a gradual increase in density with depth, while the A-β type was characterized by an increase in density in a very thin surface layer (<10 m) with a sub-layer of density uniformity. The B type was defined as an almost uniform layer over the surface water column. The C type was defined as an entirely uniform layer throughout the whole water column. Both the A-α and A-β types occurred in spring and summer. The B type appeared in all three seasons, and the C type mostly appeared in spring and autumn. The MLD was shallower in summer and deeper in autumn. The A-α type could change to the B type under strong wind energy. Two 24-h mooring surveys, one within the Pearl River plume and the other beyond the plume zone, were conducted to examine the MLD temporal variations. The A-α and B types prevailed in and beyond the plume zone, respectively. The A-α type resulted from the strong stratification formed by freshwater and sea water, while the B type was mainly induced by air-sea interface buoyancy fluxes. We proposed that the A-α type should be considered in studies of the mixing and transporting of matter, salt, heat and energy in all the global costal seas.