Mapping dataset of dissolved oxygen and carbonate system showing autumnal subsurface acidification in the central North Yellow Sea in 2017

The North Yellow Sea (NYS) is a shallow-water and semi-enclosed marginal sea of the western North Pacific. Its climatic variations are primarily dominated by the East Asian Monsoon. The rain-bearing southwest monsoon lasts from June to September, while the strong northeast monsoon prevails in winter, from November to March of the following year. In relatively warm seasons, the hydrography across the NYS is characterized by a pronounced cold pool of typically 5−11°C that is overlain by 10−20 m of warm water. It is regarded as the remnant of winter cooling and known as the North Yellow Sea cold water mass, where metabolic CO₂ accumulates every year from spring to autumn. In winter, the NYS surface water tends to be quickly cooled and then submerges into the water column, leading to ventilation over the whole water column of ~80 m. The wintertime air–sea CO₂ equilibration time in the well-mixed NYS water column has been estimated to be 25−100 d, depending on wind speeds.In 2017, three central offshore cruises were carried out in the North Yellow Sea in September, October and late December. During these cruises, depth profiles of temperature and salinity (practical salinity scale of 1978) were acquired with calibrated conductivity-temperature-depth/pressure (CTD) recorders (SBE911 plus or SBE19 plus, Sea-Bird Scientific, USA) onboard the R/V <i>Dongfanghong</i> 2 (10−12 September 2017 and from 30 December 2017 to 2 January 2018) and the R/V <i>Beidou</i> (13−16 October 2017). Water samples for dissolved oxygen (DO) and carbonate parameter analyses were obtained at two to four different depths (including the sea surface and bottom layer) using rosette samplers fitted with 12-L Niskin bottles (General Oceanics, USA), which were mounted with the CTD recorders.<br>Sampling surveys were separately supported by the National Natural Science Foundation of China via Ship-time Sharing projects during the Open Research Cruises conducted in the Bohai and Yellow Seas (onboard R/V <i>Dongfanghong 2</i>) in September and December 2017 (Cruise no. NORC2017-01), and by the Yellow Sea Fisheries Research Institute, the Chinese Academy of Fishery Sciences, via the monitoring project of fishery resources and habitat environment in the Yellow Sea (onboard R/V<i> Beidou</i>, October 2017).