Surface Chlorophyll-A Fronts in the Yellow and Bohai Seas Based on Satellite Data

Lu Xia,Yueqi Wang,Hao Liu,Lei Lin

doi:10.3390/jmse9111301

Abstract

Chlorophyll fronts are important to monitor and map the oceanic front, especially in the season when sea surface temperature (SST) fronts weaken. In this study, surface chlorophyll-a (chl-a) fronts in the Yellow and Bohai seas were characterized for the first time using satellite data. Five distinct chl-a fronts (i.e., the Bohai Strait, Shandong Peninsula, Jiangsu, Liaodong Peninsula, and Korean Peninsula fronts) were observed in summer along the 40 m isobaths and faded in other seasons. Notably, these fronts coincided with SST fronts. Strong chl-a fronts emerged during summer due to chl-a blooms in eutrophic coastal waters paired with surface chl-a fading in strongly stratified offshore waters and coastal physical fronts. Although SST fronts were strong during winter, light limitation and strong vertical mixing in offshore waters led to low chl-a in both coastal and offshore waters, suppressing chl-a front formation. Both chl-a and SST fronts coincided with steep seabed slopes (slope ratio > 1), suggesting that seabed slope may be an indicator of oceanic front location.

Highlights

An oceanic front is a narrow zone of intensified horizontal gradients of water properties that separates different water masses (e.g., [1,2]).Fronts are a common marine phenomenon occurring in coastal, shelf, and pelagic waters, and play an important role in hydrodynamics and the transport of heat, salt, and nutrients, influencing ecological processes, sedimentation, and ocean-atmosphere interaction (e.g., [2,3,4,5]).Sea surface temperature (SST) fronts are common and widely-studied physical fronts (e.g., [6,7])
Lateral gradients can be detected in surface chlorophyll-a concentration, which serves as a proxy for phytoplankton biomass [8]
Several studies on the SST fronts in the Yellow and Bohai seas have been conducted using satellite data (e.g., [6,17,27,28,29]). These studies suggest that strong sea surface cooling and topography act in conjunction with the shear between the cool coastal current and the Yellow Sea warm current to induce strong SST coastal fronts during the winter

Summary

Introduction

An oceanic front is a narrow zone of intensified horizontal gradients of water properties (i.e., physical, chemical, or biological) that separates different water masses (e.g., [1,2]). Sea surface temperature (SST) fronts are common and widely-studied physical fronts (e.g., [6,7]). Chl-a fronts are often used to represent biological fronts, which can be defined as transition regions that separate high-productivity from low-productivity waters and establish the boundaries between different nutrient concentrations. Chl-a fronts are important to monitor and map the oceanic front, especially in the summer season when the SST front weakens or disappears [2]. Legeckis et al (2002) used the SeaWiFS chl-a to map the Loop Current Front in summer to cover gaps in the GOES SST dataset [12]. Based on surface chl-a data, Hu et al (2003) studied front formation mechanisms around Hainan

Methods

Results

Discussion

Conclusion