The Wind Effect on Biogeochemistry in Eddy Cores in the Northern South China Sea

Chun Hoe Chow,Jing Yi Chen,Wei-Chang Wu,Ning Fan,Chin-Chang Hung,Ya-Tang Chien,Yung-Yen Shih

doi:10.3389/fmars.2021.717576

Abstract

Cyclonic and anticyclonic eddies are usually characterized by upwelling and downwelling, respectively, which are induced by eddy pumping near their core. Using a repeated expendable bathythermograph transect (XBT) and Argo floats, and by cruise experiments, we determined that not all eddies in the northern South China Sea (NSCS) were accompanied by eddy pumping. The weakening of background thermocline was attributed to the strengthening of eddy pumping, affected by (1) wind-induced meridional Sverdrup transports and (2) Kuroshio intrusion into the NSCS. Higher particulate organic carbon (POC) fluxes (&gt; 100 mg-C m−2 day−1) were found near the eddy cores with significant eddy pumping (defined by a depth change of 22°C isotherm near the thermocline for over 10 m), although the satellite-estimated POC fluxes were inconsistent with the in-situ POC fluxes. nitrogen limitation transition and high POC flux were even found near the core of a smaller mesoscale (diameter &lt; 100 km) cyclonic eddy in May 2014, during the weakening of the background thermocline in the NSCS. This finding provides evidence that small mesoscale eddies can efficiently provide nutrients to the subsurface, and that they can remove carbon from the euphotic zone. This is important for global warming, which generally strengthens upper ocean stratification.

Highlights

The Southeast Asia monsoon largely determines the ocean circulation in the South China Sea (SCS), the largest semi-closed marginal sea in the northwest Pacific
During the southwest (SW) monsoon, ocean circulation is characterized by two basin-wide gyres in the upper layer of the SCS: one is cyclonic in the northern South China Sea (NSCS), and the other is anticyclonic in the southern South China Sea (SSCS) (Su, 2004)
What was the condition required for significant eddy pumping near the core of eddies in the NSCS? What was the biogeochemical response to eddies with and without eddy pumping? To answer these scientific questions, we studied the variability in the ocean vertical structure, nutrients, and particulate organic carbon (POC) flux corresponding to eddies in the NSCS, where the temperature-salinity (TS) properties of seawater are largely affected by ocean currents from its boundary, such as the Kuroshio intrusion

Summary

Introduction

The Southeast Asia monsoon largely determines the ocean circulation in the South China Sea (SCS), the largest semi-closed marginal sea in the northwest Pacific. This section represents the significant and insignificant eddy pumping observed via XBTs, Argo floats, and cruises, FIGURE 5 | (A,B) Sea-level anomaly (SLA), sea surface temperature (SST), (C,D) density anomalies, and (E,F) N2 anomalies observed along 116.17◦E within 18◦N and 19◦N from May 20 to 28, 2014 (left) and along 120.33◦E within 22◦N and 22◦N from April 14 to 17, 2013 (right).

Results

Conclusion