The features of the coastal fronts in the Eastern Guangdong coastal waters during the downwelling-favorable wind period

Chaoyu Yang,Haibin Ye

doi:10.1038/s41598-021-89649-8

Abstract

A coastal front was detected in the eastern Guangdong (EGD) coastal waters during a downwelling-favorable wind period by using the diffuse attenuation coefficient at 490 nm (Kd(490)). Long-term satellite data, meteorological data and hydrographic data collected from 2003 to 2017 were jointly utilized to analyze the environmental factors affecting coastal fronts. The intensities of the coastal fronts were found to be associated with the downwelling intensity. The monthly mean Kd(490) anomalies in shallow coastal waters less than 25 m deep along the EGD coast and the monthly mean Ekman pumping velocities retrieved by the ERA5 dataset were negatively correlated, with a Pearson correlation of − 0.71. The fronts started in October, became weaker and gradually disappeared after January, extending southwestward from the southeastern coast of Guangdong Province to the Wanshan Archipelago in the South China Sea (SCS). The cross-frontal differences in the mean Kd(490) values could reach 3.7 m−1. Noticeable peaks were found in the meridional distribution of the mean Kd(490) values at 22.5°N and 22.2°N and in the zonal distribution of the mean Kd(490) values at 114.7°E and 114.4°E. The peaks tended to narrow as the latitude increased. The average coastal surface currents obtained from the global Hybrid Coordinate Ocean Model (HYCOM) showed that waters with high nutrient and sediment contents in the Fujian and Zhejiang coastal areas in the southern part of the East China Sea could flow into the SCS. The directions and lengths of the fronts were found to be associated with the flow advection.

Highlights

Coastal fronts are principal hydrographic features that usually occur in regions where the velocity, density, temperature, or salinity gradient is extremely high or a significant discontinuity e xists[1,2]
Kd(490) anomalies in shallow coastal waters less than 25 m deep along the eastern Guangdong (EGD) coast and the monthly mean Ekman pumping velocities retrieved from the ERA5 datasets was − 0.71
The intensity of the fronts was affected by the secondary circulation induced by this downwelling region

Summary

Introduction

Coastal fronts are principal hydrographic features that usually occur in regions where the velocity, density, temperature, or salinity gradient is extremely high or a significant discontinuity e xists[1,2]. Downwelling is one of the most significant dynamic phenomena in the NSCS It has a great influence on the nearshore circulation along the eastern Guangdong (EGD) coast. The EGD coastal fronts that occur in the EGD coastal waters have a pronounced influence on the hydrologic characteristics of the region but are important for algal blooms, pollutant dispersion, seafloor sediments, and ship navigation in the NSCS. With the development of remote sensing techniques, satellite data have become an effective data source for studying the frequency and region of coastal front occurrences in EGD coastal waters. The purpose of the study is to analyze the locations, occurrence times, durations, intensities and convergence properties of coastal fronts in the EGD coastal region by applying remote sensing techniques. Ocean color data and meteorological and hydrographic data were jointly utilized to illustrate the most important factors influencing the coastal fronts

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