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Under global warming, the South China Sea (SCS) is experiencing increasingly severe marine heatwaves (MHWs), with impacts on marine ecosystems such as coral reefs and marine pastures becoming more evident. The numerous anticyclonic eddies (AEs) distributed in the SCS are important drivers of MHW generation and development, yet their impacts on MHWs are still not fully understood. In this study, the vertical structures of various types of MHWs inside the AEs and in the background field were mapped and compared, and we found that AEs of varying amplitudes have distinct impacts on the vertical structures of MHWs. MHWs inside the AEs can be divided into two categories: subsurface-reversed MHWs and subsurface-intensified MHWs. The former is manifested as anomalous cooling in the subsurface, driven by the uplift of thermocline due to the inhibition of downward mixing. The latter is characterized by anomalous warming in the subsurface, resulting from strong vertical warm-water subsidence induced by large-amplitude AEs. This process may penetrate the thermocline and produce maximum warming anomalies in the layer beneath the region of greatest temperature gradient change. Our research reveals characteristics of various vertical structures of MHWs in the SCS, attributing their differences to the stable water layer's different response to varying intensities of vertical heat conduction, and deepening people's understanding of the impact of AEs in the SCS on the vertical structure of MHWs. [ABSTRACT FROM AUTHOR]

Wintertime thermal front west of Luzon Island (WTFWLI) plays a vital role in local marine biogeochemical environment, geostrophic thermal advection of Luzon cold eddy (LCE) was once recognized as the causation of WTFWLI. However, current investigation based on multi‐source data revealed poor spatial correspondence between WTFWL and LCE over an interannual scale during 1993–2022, highlighting the necessity of revisiting the mechanism of WTFWLI. Our analysis shows that formation of WTFWLI was mainly caused by a northward warm tongue caused by geostrophic thermal advection of a northward current west of Luzon Island. The northward warm tongue encounters cold water west of Luzon Island and forms the WTFWLI at the boundary of cold and warm water. Our study provides a new explanation for the formation mechanism of WTFWLI, which will help deepen people's understanding of thermal front dynamics. Plain Language Summary: Wintertime thermal front west of Luzon Island (WTFWLI) plays an important role in sea‐air interaction and marine ecosystem, previous studies pointed out that the formation of WTFWLI was related with the Luzon cold eddy (LCE). Warm (cold) water was located in the southeast (northwest) part of the LCE due to geostrophic thermal advection, then leading to the formation of WTFWL at the boundary of cold and warm water. However, current investigation shows that there was poor spatial correspondence between WTFWLI and LCE in winter from 1993 to 2022, which is inconsistent with previous conclusions. Further analysis shows that formation of WTFWLI was mainly caused by a northward warm tongue caused by geostrophic thermal advection of a northward current west of Luzon Island. The northward warm tongue encounters cold water west of Luzon Island and forms the WTFWLI at their boundary. This study provides a new insight into the formation mechanism of WTFWLI, which will raise people's awareness of thermal front dynamics. Key Points: A new insight was provided into the formation mechanism of thermal front west of Luzon Island in winterThe thermal front west of Luzon Island was mainly caused by the encounter of a northward warm tongue and cold waterThe northward warm tongue was caused by a northward current west of Luzon Island [ABSTRACT FROM AUTHOR]

Ocean chlorophyll‐a concentration is a reliable indicator of phytoplankton biomass that plays an important role in controlling the marine ecosystem. Here, we investigated the interannual variations in sea surface chlorophyll‐a concentration (SSC) in the Yellow Sea and underlying mechanisms with 22 yr (1998–2019) of satellite ocean color observations. Results indicated that SSC showed a positive trend (0.039 mg m−3 yr−1 on average) during 1998–2010 and a negative trend (−0.058 mg m−3 yr−1 on average) during 2011–2019. Similar SSC trends occurred in the four seasons. These trends had larger magnitudes in inshore waters than in offshore waters. SSC variations were primarily driven by rainfall and anthropogenic nutrient emissions with a possibly secondary contribution from sea surface temperature (SST). During 1998–2010, increased rainfall and anthropogenic nutrient emissions largely enhanced the terrestrial nutrient inputs into the Bohai Sea and Yellow Sea. Coincidentally, decreased SST might have led to a weakened stratification in summer, favoring upward turbulent diffusion of deeper waters rich in nutrients and chlorophyll‐a. These processes resulted in significant increases of surface nutrient concentrations and SSC. During 2011–2019, the opposite processes occurred to generate decreased SSC. The current study highlights the change of SSC trends in the Yellow Sea in the first two decades of the 21st century and physical processes at work, enriching our understanding of ocean chlorophyll‐a dynamics in the global shelf seas. Plain Language Summary: The Yellow Sea is a shallow marginal sea of the western Pacific Ocean. Combined effects of the complicated hydrodynamic conditions and human activities make the Yellow Sea one of the important marine ecosystems in the world, which contributes greatly to the socioeconomic development of bordering coastal countries, namely China, North Korea and South Korea. Previous studies indicated that increased anthropogenic nutrient emissions have predominantly caused a positive trend in sea surface chlorophyll‐a concentration (which is usually used as a reliable indicator of phytoplankton biomass in the marine ecosystem) in the Yellow Sea. In the current study, based on satellite ocean color observations, we found that sea surface chlorophyll‐a concentration in the Yellow Sea showed a positive trend during 1998–2010 and a negative trend during 2011–2019. These variations were primarily driven by rainfall and anthropogenic nutrient emissions with a possibly secondary contribution from sea surface temperature. The current study highlights the important role of the natural factors (primarily rainfall) in causing the change of sea surface chlorophyll‐a concentration trends in the Yellow Sea in the first two decades of the 21st century. Key Points: Surface chlorophyll‐a concentration in the Yellow Sea had significant interannual variation during 1998–2019Surface chlorophyll‐a concentration showed a positive trend during 1998–2010 and a negative trend during 2011–2019The trends were primarily driven by rainfall and anthropogenic nutrient emissions with a possible contribution from surface temperature [ABSTRACT FROM AUTHOR]

Characterized by a sharp thermocline and large cold‐water mass below, the Yellow Sea (YS) in summer is a shelf sea that is one of the most vulnerable to typhoons in the world. With observations and high‐resolution numerical simulations, we investigated the three‐dimensional temperature changes and underlying dynamics during the passage of Typhoon Bavi in August 2020, which was representative of northward‐moving typhoons in the eastern YS. Bavi's cyclonic strong winds caused intense turbulent mixing and Ekman divergence. The intense vertical mixing generated spatially coherent temperature cooling in the surface layer and partly caused temperature warming in the subsurface layer on both sides of the typhoon track. The largest surface cooling and subsurface warming both occurred on the right side of the typhoon track. Wind‐induced Ekman divergence generated two overturning circulation systems zonally across the YS, consisting of upper layer shoreward currents, coastal downwelling, lower layer seaward currents and upwelling on the typhoon track. Downwelling and seaward currents caused temperature warming below the thermocline, especially in the bottom layer. Upwelling shoaled the thermocline, resulting in subsurface cooling on the typhoon track. Additionally, Ekman divergence produced sea surface height minima on the typhoon track, causing strong southward barotropic currents to the left, which subsequently drove strong temperature cooling above the southern bottom slope of the South YS and fast southward movement of the YS Cold Water Mass. Further numerical experiments indicated that the magnitudes of the above changes increased with increasing maximum wind speeds and/or decreasing moving speeds of typhoons. Plain Language Summary: Typhoons are the most severe synoptic events on Earth, imposing large influences on physical‐biogeochemical environments in the global oceans and seas. As a marginal shelf sea of the Northwestern Pacific Ocean, the Yellow Sea is characterized by a sharp thermocline and large cold‐water mass in summer. It experienced 1–2 typhoons on average each year in past decades and is projected to undergo more frequent typhoons in a future warming climate. These typhoons have quite different tracks, and moving northward/northeastward in the eastern Yellow Sea is the most common track. Using observations and high‐resolution numerical simulations, we investigated in detail the three‐dimensional temperature changes and underlying dynamics in the Yellow Sea caused by Bavi in August 2020, which was representative of northward‐moving typhoons. The important novel results of the current study are that typhoons such as Bavi can cause significant three‐dimensional circulation changes, which further result in dramatic temperature changes below the surface layer and fast southward movement of the Yellow Sea Cold Water Mass. The current study enriches our knowledge of oceanic responses to typhoon passage over the Yellow Sea, which has one of the most important large marine ecosystems in the world. Key Points: Typhoon Bavi caused significant three‐dimensional changes in temperature and circulation in the Yellow Sea in August 2020Temperature experienced spatially coherent cooling in the surface layer but region‐specific changes in the subsurface and bottom layersThe underlying dynamics involved enhanced vertical mixing and current advection forced by cyclonic winds and the meridional coastline [ABSTRACT FROM AUTHOR]

Hypoxia has been observed worldwide and causes great damage to marine ecosystems. In the northeastern mariculture areas of the Shandong Peninsula, summer oxygen depletion and even hypoxia have been observed in the bottom layer, but the underlying dynamics are still poorly understood. In the current study, the physical controls of variations in bottom layer oxygen concentrations in summer and their interannual variations in the northeastern mariculture areas of the Shandong Peninsula were first investigated using in situ observations collected during 2016–2020. Over the past 5 years, the dissolved oxygen (DO) concentrations of the bottom layer have ranged from 0.8 to 13.0 mg L−1 and exhibited a robust annual cycle, with the lowest value occurring in August. Hypoxia events covering several days were observed intermittently in the summers of 2016 and 2017. Summer oxygen depletion was caused by the combined effects of ocean stratification and enhanced biogeochemical oxygen consumption. At the interannual time scale, stratification and bottom layer DO concentrations showed synchronous variations and both peaked in 2018. The underlying mechanism was related to bottom layer currents, which were created by southerly monsoon winds. Strong/weak southerly surface winds induced a strong/weak bottom shoreward advection of cold, oxygen‐rich water and resulted in strong/weak stratification and high/low DO concentrations. The results provide a good reference for coastal mariculture management in the Shandong Province and enrich our understanding of the global distribution of coastal hypoxia. Plain Language Summary: Dissolved oxygen (DO) is necessary to sustain the life of all fish and invertebrates. When the DO concentration falls below 2.0 mg L−1 (≈61 µmol kg−1), hypoxia causes great damage to marine ecosystems. Low DO concentrations in summer and even hypoxia are observed in the bottom layer of northeastern mariculture areas in the Shandong Peninsula, which is mainly farmed for precious seafood such as sea cucumbers, abalones, and scallops that are less mobile and vulnerable to hypoxia. In the current study, the physical controls of bottom layer oxygen variations in summer and their interannual variations in the northeastern mariculture areas of the Shandong Peninsula were first investigated using in situ observations during 2016–2020. The results indicate that bottom layer oxygen depletion in summer was caused by the combined effects of stratification and biogeochemical oxygen consumption. In addition, stratification and bottom layer DO concentrations showed synchronous interannual variations. The underlying mechanism was related to the bottom layer current, which was created by southerly monsoon winds. The results provide a good reference for coastal mariculture management in the Shandong Province and enrich our understanding of the global distribution of coastal hypoxia. Key Points: Summer low dissolved oxygen and even hypoxia was observed in the bottom layer of northeastern mariculture areas of the Shandong PeninsulaBottom layer oxygen depletion in summer was caused by the combined effects of stratification and biogeochemical oxygen consumptionBottom shoreward advection caused by southerly winds controlled synchronous interannual variations in stratification and the oxygen budget [ABSTRACT FROM AUTHOR]

Hydrodynamic conditions in the Bohai Sea (BS) play an important role in regulating variabilities in the regional climate and marine ecosystem. In the current study, wintertime current variations in the BS were investigated using continuous in situ observations at two sites in the western BS (S1) and southeastern BS (S2) from November/December 2020 to March 2021. The results indicated that residual currents and their variations were both stronger at S1 than at S2. Residual currents had notable vertical variations, with nearly opposite directions in the upper and lower layers. Upper layer residual currents were mostly southeastward at both sites, while lower layer residual currents were basically west-northwestward at S2 following the topography in the deep trough in the South BS but were north-northwestward at S1 up the slope to the coast. Upper layer residual currents were strongly subsurface-intensified and experienced significant intraseasonal variations. Subsurface currents in the upper layer shifted from being weak in December 2020 to strong in January–February 2021, especially in the western BS. Surface currents were directly driven by surface winds following the Ekman layer dynamics in shallow waters. Weak/strong subsurface currents occurred during/after sustained strong sea surface winds, implying that they experienced more complex dynamics than surface currents. This study provides new understanding to the wintertime circulation pattern in the BS. It is helpful for fully understanding the characteristics and underlying dynamics of the BS circulation variations in winter and their impacts on biogeochemical processes. • Observed currents flowed in opposite directions in the upper and lower layers. • Waters from the Yellow Sea entered the Bohai Sea only in the lower layer. • Currents in upper layer were subsurface-intensified and stronger than in lower layer. • Weak/strong subsurface currents occurred during/after sustained strong winds. • One possible mechanism was related to PV input by sea surface wind stress curl. [ABSTRACT FROM AUTHOR]

Chlorophyll-a (Chl-a) concentration is an important indicator of the state of marine ecosystems. In the current study, the interannual-decadal variation in the Bohai Sea's surface Chl-a concentration and the underlying dynamics are investigated based on 16 years (August 2002–February 2019) of satellite ocean colour observations. The Chl-a concentration shows a spatially coherent increasing trend from 2003 to 2011 and a decreasing trend from 2012 to 2018 in both the Bohai and North Yellow Seas. Both decadal trends are clearer in the Bohai Sea than in the Yellow Sea and in coastal waters than in offshore waters. The decreasing trend is more obvious than the increasing trend. The low-frequency variations are mainly controlled by sea surface temperature (SST) and rainfall variations and eventually related to the Pacific Decadal Oscillation (PDO). During positive PDO phases, warm SST anomalies exist in the Bohai and Yellow Seas and negative rainfall anomalies exist over the sea and surrounding lands. These anomalies would decrease the surface layer dissolved inorganic nitrogen concentration through suppressing the upward turbulent nutrient diffusion and reducing land-sourced nutrient fluxes into the ocean, leading to negative Chl-a concentration anomalies. During negative PDO phases, the opposite situation occurs. Human activities may make a secondary contribution through enhancing/reducing the anthropogenic nitrogen emission before/after 2011. This study emphasizes the natural climate variability in modulating low-frequency variations in surface Chl-a concentrations in the Bohai and Yellow Seas in recent decades and is useful for marine resource planning and marine ecological management in the Bohai Sea in the future. • Surface chlorophyll-a concentration in the Bohai Sea has interannual-decadal variation. • Spatially coherent upward trend over 2003–2011 and downward trend over 2012–2018. • Downward trends are more obvious than upward trends. • Chlorophyll-a concentration variation is related to the Pacific Decadal Oscillation. [ABSTRACT FROM AUTHOR]