Climatology of physical hydrographic and biological characteristics of the Northern South China Sea Shelf-sea (NoSoCS) and adjacent waters: Observations from satellite remote sensing

Abstract

The climatology of the physical hydrographic and biological characteristics of the Northern South China Sea Shelf-sea (NoSoCS) are examined by using remotely sensed surface wind vectors, sea-surface temperatures (SST), surface chlorophyll-a concentrations (Chl_a), and vertically integrated net primary production (PPeu), which can be generally validated by field observations. The annual average Chl_a and PPeu in the NoSoCS are 1.1±0.1mgm−3 and 1.13±0.07g-Cm−2d−1, respectively. The values of Chl_a and PPeu in the inner shelf are about 7 and 3 times those in the outer shelf, respectively. Seasonally, in monthly averages, there are two distinct maxima, one in December/January and the other in July, at the height of the two monsoonal seasons, and two minima, in April/May and September during the two inter-monsoonal seasons, in both Chl_a and PPeu. The maxima in Chl_a are of comparable magnitude. However, the winter maximum in PPeu is about 30% larger than the summer maximum. These seasonal patterns are unique and are distinctly different not only from those generally found in the polar, temperate and tropical waters but also from those reported in the adjoining open South China Sea (SCS) where a summer maximum is absent. In the open SCS, monthly average Chl_a is negatively correlated to monthly average SST and positively correlated to monthly average wind speed. These correlations are consistent with the elevation of primary production in the winter as a result of the enhancement of vertical mixing by the combination of surface cooling and the stronger wind as hypothesized previously. In contrast, in the NoSoCS, monthly average Chl_a is not significantly correlated to monthly average SST and is only weakly correlated to monthly average wind speed, suggesting that other controlling processes may have come into play. These processes may include: (a) the terrestrial input of nutrients, especially through the discharge from the Pearl River, which reaches a maximum in the summer; (b) summer coastal upwelling off the Hainan Island and Dongshan and year-round upwelling off the Taiwan Bank; and (c) the enhanced vertical mixing by the action of internal waves along the outer shelf-upper slope. The influence of the discharge from the Pearl River is indicated by a tongue of water with elevated Chl_a that extends from the Pearl River mouth northeastward into the middle shelf in the summer. This water is also characterized by a positive correlation between monthly average Chl_a and SST together with no correlation or a slightly negative correlation between monthly average Chl_a and wind speed in individual pixels. Upwelling off the Taiwan Bank and the Hainan Bank are clearly depicted by an elevated Chl_a in the former and a depressed SST in the latter during the summer months. The temporal variations in the areal coverage of the upwelling water in the former are small, while the coverage in the latter reaches a maximum in July. The effect of the action of internal waves may be detected as a depression in SST and an elevation in Chl_a along the outer shelf-upper slope. The signals are especially noticeable at the Dongsha Plateau.