Impact of the Kuroshio intrusion on the nutrient inventory in the upper northern South China Sea: insights from an isopycnal mixing model

Abstract

Based on four cruises covering a seasonal cycle in 2009–2011, we examined the impact of the Kuroshio intrusion, featured by extremely oligotrophic waters, on the nutrient inventory in the central northern South China Sea (NSCS). The nutrient inventory in the upper 100 m of the water column in the study area ranged from &sim;200 to &sim;290 mmol m⁻² for N + N (nitrate plus nitrite), from &sim;13 to &sim;24 mmol m⁻² for soluble reactive phosphate and from &sim;210 to &sim;430 mmol m⁻² for silicic acid. The nutrient inventory showed a clear seasonal pattern with the highest value appearing in summer, while the N + N inventory in spring and winter had a reduction of &sim;13 and &sim;30%, respectively, relative to that in summer. To quantify the extent of the Kuroshio intrusion, an isopycnal mixing model was adopted to derive the proportional contribution of water masses from the SCS proper and the Kuroshio along individual isopycnal surfaces. The derived mixing ratio along the isopycnal plane was then employed to predict the genuine gradients of nutrients under the assumption of no biogeochemical alteration. These predicted nutrient concentrations, denoted as <i>N</i>_m, are solely determined by water mass mixing. Results showed that the nutrient inventory in the upper 100 m of the NSCS was overall negatively correlated to the Kuroshio water fraction, suggesting that the Kuroshio intrusion significantly influenced the nutrient distribution in the SCS and its seasonal variation. The difference between the observed nutrient concentrations and their corresponding <i>N</i>_m allowed us to further quantify the nutrient removal/addition associated with the biogeochemical processes on top of the water mass mixing. We revealed that the nutrients in the upper 100 m of the water column had a net consumption in both winter and spring but a net addition in fall

Seasonal variations of sea–air CO&lt;sub&gt;2&lt;/sub&gt; fluxes in the largest tropical marginal sea (South China Sea) based on multiple-year underway measurements

“…1) associated with large riverine inputs. The SCS is also featured by dynamic exchange with the western Pacific Ocean via an upper part exchange with the Kuroshio and overflow at depth (Chen et al, 2001;Dai et al, 2013;Du et al, 2013). At this dynamic interface, mesoscale processes such as eddies are frequently observed.…”

“…The northern SCS also exchanges with the Kuroshio via the Luzon Strait of 2000 m depth. Although the SCS shelf systems are fed by two of the world's major rivers (the Mekong and Pearl rivers) and some smaller rivers featuring either tropical or subtropical watersheds, the majority of the SCS proper is typically oligotrophic with low productivity (Ning et al, 2004;Du et al, 2013). In our study, we focused on four selected physical-biogeochemical domains in the SCS proper (Fig.…”

Inter-shelf nutrient transport from the East China Sea as a major nutrient source supporting winter primary production on the northeast South China Sea shelf

“…Notably, the PP in the NSCS in winter is maintained at similar levels to those in Chen and Chen, 2006;Wang et al, 2012). During this time period, the warm and oligotrophic Kuroshio surface water intrudes through the Luzon Strait and occupies a large area of the NSCS basin (Shaw, 1991;Hu et al, 2000), which significantly reduces the nutrient inventory in the upper ocean (Du et al, 2013). The TWS is ∼ 180 km wide with an average depth of ∼ 60 m (Hong et al, 2011).…”

“…It is noted that the NESCS shelf is strongly impacted by the intrusion of the Kuroshio water of extremely low nutrients (Du et al, 2013) in its surface water, which could displace or dilute the nutrient-laden water of CCC. On the other hand, certain diapycnal mixing processes, such as tidal current, and/or internal waves might be able to transport nutrients from the Kuroshio subsurface water to the upper water column, which are yet to be quantified.…”

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing – an overview and synthesis

“…Aside from affecting the basin-wide upwelling , the intrusion of the Kuroshio into the SCS has more direct consequences as demonstrated by Du et al (2013). They used an isopycnal mixing model to quantify the extent of the Kuroshio intrusion and its impact on the nutrient inventory in the northern SCS.…”