External Dynamic Mechanisms Controlling the Periodic Offshore Blooms in Beibu Gulf

Abstract

Since offshore waters are less affected by human activities and nutrient‐rich water masses, existing theories on periodic offshore blooms (POB) consider that the POB is proportional to the intensity of ocean fronts (nutrient supply from enhancing vertical mixing), ignoring external nutrient supply and external forcing (climatic oscillations). This study proposes an external dynamic mechanism of the POB on the basis of field observations and long‐term satellite remote sensing data (1981–2022) in Beibu Gulf, which is influenced by remarkable external forcing. A strong thermal front with an inverted‐V structure occurred in the central gulf every winter due to the strong wind stress. The intensity of the front on the east side is weaker due to the intrusion of the west‐Guangdong coastal current (WGCC). However, nutrient supply variation resulted in the POB's different intensity in different areas of the inverted‐V front. Chlorophyll a concentrations in the eastern (nutrients supplied by WGCC) and northern (nutrients supplied by vertical mixing) were obviously higher than that in the western front (limited nutrient supply) in winter. On an interannual scale, the intensity of POB in La Niña years is remarkably stronger than in El Niño years due to the stronger WGCC supplying more nutrients in La Niña. This study suggests that the intensity and range of POB are not proportional to the frontal intensity in the gulf, but are directly driven by the internal forcing (fronts and nutrient supply from WGCC), which is controlled by the external forcing.

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

“…However, under global warming, the global SST shows an increasing trend, with a faster warming rate in marginal seas (Cai et al, 2017;Rhein et al, 2013;Yao et al, 2020), which can affect the activity of ocean fronts (Lao, Liu, et al, 2023). Warming SST has enhanced the intensity of the thermal front in coastal waters, which increases the possibility of large-scale algal blooms (even red tides) (Lao, Liu, et al, 2023). As the warming rate in marginal seas is faster than that in open oceans (Cai et al, 2017;Rhein et al, 2013;Yao et al, 2020), we speculate that the variation of frontal intensity and its eco-environmental impact in the Luzon Strait may differ from coastal waters.…”

“…Generally, wind stress curl in coastal waters is negatively correlated with SST, and strong winds cause cooling in the upper ocean (Lao, Lu, et al, 2023), which is favorable for the generation of thermal fronts (Chelton et al, 2007;Jing et al, 2015;Letelier et al, 2009). Frontal activity is also affected by the El Niño-Southern Oscillation (ENSO), which can enhance or inhibit the formation of ocean fronts by affecting the SST (Hopkins et al, 2010;Lao, Liu, et al, 2023;Wang et al, 2021). With global warming, the SST gradually increases in coastal waters (Cai et al, 2017;Yao et al, 2020), which affects frontal activity (Feuilloley et al, 2020;Lao, Liu, et al, 2023).…”

“…Frontal activity is also affected by the El Niño-Southern Oscillation (ENSO), which can enhance or inhibit the formation of ocean fronts by affecting the SST (Hopkins et al, 2010;Lao, Liu, et al, 2023;Wang et al, 2021). With global warming, the SST gradually increases in coastal waters (Cai et al, 2017;Yao et al, 2020), which affects frontal activity (Feuilloley et al, 2020;Lao, Liu, et al, 2023). Recently, ocean fronts are enhancing in the coastal waters due to the continuous warming of the ocean under global warming, thus increasing the probability of coastal blooms (Lao, Liu, et al, 2023).…”

“…With global warming, the SST gradually increases in coastal waters (Cai et al, 2017;Yao et al, 2020), which affects frontal activity (Feuilloley et al, 2020;Lao, Liu, et al, 2023). Recently, ocean fronts are enhancing in the coastal waters due to the continuous warming of the ocean under global warming, thus increasing the probability of coastal blooms (Lao, Liu, et al, 2023). However, the intensity of the ocean front and its eco-environmental response to climate change in the open ocean remains unclear.…”

“…However, under global warming, the global SST shows an increasing trend, with a faster warming rate in marginal seas (Cai et al, 2017;Rhein et al, 2013;Yao et al, 2020), which can affect the activity of ocean fronts (Lao, Liu, et al, 2023). Warming SST has enhanced the intensity of the thermal front in coastal waters, which increases the possibility of large-scale algal blooms (even red tides) (Lao, Liu, et al, 2023).…”