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Global Warming Weakens the Ocean Front and Phytoplankton Blooms in the Luzon Strait Over the Past 40 years

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Journal of Geophysical Research: Biogeosciences
Authors:
Qibin Lao at Guangdong Ocean University
Qibin Lao
Sihai Liu at Guangdong Ocean University
Sihai Liu
Chao Wang
Chao Wang
Chao Wang
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Fajin Chen at Guangdong Ocean University
Fajin Chen
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Abstract and Figures

The Luzon Strait is a channel where warmer Kuroshio water from the Pacific Ocean intrudes into the South China Sea (SCS). Under climate change impact, the temperature in marginal seas, including the SCS, rises faster than in open oceans. We speculated that the variation of frontal intensity and its eco‐environmental impact in the Luzon Strait may be different from coastal waters, whose frontal intensity is increasing, thus stimulating phytoplankton growth. To confirm this speculation, 40‐year satellite, and multiple data sources were analyzed in the Luzon Strait. The results showed that strong frontal intensity (front coverage of over 60%) and higher Chlorophyll a content occurred simultaneously in the Luzon Strait during the winter monsoon period. Phytoplankton blooms were enhanced during El Niño years because the stronger Kuroshio intrusion generated stronger fronts and intensified local upwelling in the Luzon Strait. On an interannual scale, the frontal intensity and phytoplankton growth exhibited a significantly decreasing trend in the Luzon Strait over the past 40 years, since the faster warming in the SCS reduced the temperature difference between the Pacific Ocean and the SCS. Warming and weakening fronts reduced the mixed layer depth to the oligotrophic layer, thus limiting the phytoplankton growth. This study confirmed that faster temperature rises in marginal seas reduced the frontal intensity and phytoplankton growth in the strait between oceans and marginal seas.
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1. Introduction
Ocean fronts are formed by the transitions between different water masses (Guo etal.,2017; Lévy etal.,2018).
The ocean front has high gradients of temperature, density, and nutrients and is one of the most important regions
for the exchange of ocean heat, gas (i.e., oxygen and greenhouse gases), energy, and material, as well as the
distribution of biological resources (Behrens etal.,2021; Wang etal.,2021), playing an important role in global
climate change (Chapman etal.,2020; Ferrari, 2011). More importantly, ocean fronts act as oases in the ocean
because of their convergence effect that converges nutrients and other substances from different water masses,
which is favorable for phytoplankton growth and thus contributes significantly to regional and global ecosystem
productivity (Farach-Espinoza etal.,2022; Ito etal.,2023; Li etal.,2022; Woodson & Litvin,2015). Therefore,
ocean fronts play an important role in marine ecology (Feuilloley etal., 2020; Prants, 2022), such as fishing
grounds (Oh etal.,2020; Prants,2022; Woodson & Litvin,2015).
Many physical dynamics drive the formation of ocean fronts, including the Coriolis force, local wind stress, heat
(warming and cooling of sea-surface water), and freshwater input from rivers (Behrens etal., 2021; Chapman
etal., 2020; Shi et al.,2015; Wang etal., 2021). Fronts are boundaries between two sides with distinct water
Abstract The Luzon Strait is a channel where warmer Kuroshio water from the Pacific Ocean intrudes into
the South China Sea (SCS). Under climate change impact, the temperature in marginal seas, including the SCS,
rises faster than in open oceans. We speculated that the variation of frontal intensity and its eco-environmental
impact in the Luzon Strait may be different from coastal waters, whose frontal intensity is increasing, thus
stimulating phytoplankton growth. To confirm this speculation, 40-year satellite, and multiple data sources
were analyzed in the Luzon Strait. The results showed that strong frontal intensity (front coverage of over 60%)
and higher Chlorophyll a content occurred simultaneously in the Luzon Strait during the winter monsoon
period. Phytoplankton blooms were enhanced during El Niño years because the stronger Kuroshio intrusion
generated stronger fronts and intensified local upwelling in the Luzon Strait. On an interannual scale, the frontal
intensity and phytoplankton growth exhibited a significantly decreasing trend in the Luzon Strait over the past
40years, since the faster warming in the SCS reduced the temperature difference between the Pacific Ocean
and the SCS. Warming and weakening fronts reduced the mixed layer depth to the oligotrophic layer, thus
limiting the phytoplankton growth. This study confirmed that faster temperature rises in marginal seas reduced
the frontal intensity and phytoplankton growth in the strait between oceans and marginal seas.
Plain Language Summary Ocean fronts are the boundaries between two sides with distinct water
mass properties and play an extremely important role in marine eco-environment and global climate change.
Studies have shown that thermal fronts are enhancing, thus stimulating chlorophyll growth in the coastal waters
due to the intensification of global warming. However, the variation of frontal intensity in the open oceans may
be different because warming in marginal seas rises faster than in oceans. To confirm this speculation, 40-year
satellite, and multiple data sources were analyzed in the Luzon Strait, a deep conduit between the Pacific Ocean
and the South China Sea (SCS). This study found that the frontal intensity and phytoplankton growth exhibited
a significantly decreasing trend in the Luzon Strait over the past 40years, since the faster warming in the SCS
reduced the temperature difference between the Pacific Ocean and the SCS. Warming and weakening fronts
reduced the mixed layer depth to the oligotrophic layer, thus limiting the phytoplankton growth.
LAO ETAL.
© 2023. American Geophysical Union.
All Rights Reserved.
Global Warming Weakens the Ocean Front and Phytoplankton
Blooms in the Luzon Strait Over the Past 40years
Qibin Lao1,2 , Sihai Liu1,2, Chao Wang1,2,3, and Fajin Chen1,2,3
1College of Ocean and Meteorology, Guangdong Ocean University, Zhanjiang, China, 2College of Chemistry and
Environmental Science, Guangdong Ocean University, Zhanjiang, China, 3Key Laboratory of Climate, Resources and
Environment in Continental Shelf Sea and Deep Sea of Department of Education of Guangdong Province, Guangdong Ocean
University, Zhanjiang, China
Key Points:
• Strong ocean front and higher
Chlorophyll a (Chl-a) content
occurred simultaneously in winter
• Phytoplankton blooms enhanced in El
Niño years due to stronger Kuroshio
intrusion
• Global warming decreased the frontal
intensity and Chl-a level over the past
40years
Correspondence to:
F. Chen,
fjchen04@163.com
Citation:
Lao, Q., Liu, S., Wang, C., & Chen, F.
(2023). Global warming weakens the
ocean front and phytoplankton blooms in
the Luzon Strait over the past 40years.
Journal of Geophysical Research:
Biogeosciences, 128, e2023JG007726.
https://doi.org/10.1029/2023JG007726
Received 25 JUL 2023
Accepted 21 NOV 2023
Author Contributions:
Conceptualization: Qibin Lao, Fajin Chen
Data curation: Qibin Lao, Sihai Liu
Formal analysis: Qibin Lao, Sihai Liu
Funding acquisition: Fajin Chen
Methodology: Qibin Lao, Sihai Liu
Project Administration: Fajin Chen
Supervision: Fajin Chen
Visualization: Qibin Lao
Writing – original draft: Qibin Lao
Writing – review & editing: Qibin Lao,
Chao Wang, Fajin Chen
10.1029/2023JG007726
RESEARCH ARTICLE
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... Chlorophyll-a (Chl-a) is a crucial photosynthetic pigment produced by algae during photosynthesis, making it an effective indicator for monitoring water eutrophication [13]. In recent years, factors such as climate change [14] and excessive aquaculture activities resulting from human activities [15][16][17] have led to elevated eutrophication [18][19][20]. Consequently, there has been a proliferation of algae in inland water bodies, leading to an increase in Chl-a concentration [21,22]. Hence, continuous monitoring of Chl-a concentration in fish ponds in the GBA and studies of water eutrophication are vital for comprehending the current ecological conditions of Chinese aquaculture ponds. ...
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