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The hourly L2-level chlorophyll-a (CHL-a) concentration spatial energy spectra of GOCI-II from 2021 to 2023 are employed to investigate the characteristics of the CHL-a spatial energy spectrum slopes in three regions of the East China Sea, namely nearshore, offshore, and open ocean. The seasonal trends of the spatial energy spectrum slopes are also examined for the nearshore and offshore regions. It is observed that the slopes of the CHL-a spatial energy spectrum are −2 at scales larger than 5 km, whereas at smaller scales, they are −5/3, −1, and −0.3 from the nearshore region to the open sea, respectively. On the larger scales, the spatial energy spectrum slopes are consistent with surface quasi-geostrophic (sQG) theory, but this is not the case on smaller scales. An insufficient regional CHL-a concentration leads to a flattening of the slope at the smaller scales. On the submesoscale, the slope of the nearshore CHL-a concentration spatial energy spectrum is steeper in summer and flatter in winter, a pattern that contrasts with changes observed offshore. This seasonal variation is attributed to the southward flow of ZheMin Coastal Current (ZMCC) during winter, which carries freshwater and enhances the horizontal buoyancy gradient in the nearshore region. [ABSTRACT FROM AUTHOR]

Using satellite-observed data and reanalysis data, we studied the spatiotemporal variation characteristics and dynamic mechanisms of thermal fronts in the Beibu Gulf (TFIBG). TFIBG occur in December, reach their strongest point in January in the following year, and then gradually weaken until they completely disappear in May. Their formation is related to the bathymetry of the Beibu Gulf. In winter, the seawater in shallow-water areas (deep-water areas) cools down more (less), and Ekman currents concurrently transport warm water from the central basin of the Beibu Gulf to the west coast, which results in the formation of a thermal front at the junction of cold and warm water. The interannual variation in TFIBG intensity is related to the northeast monsoon. The strengthened (weakened) Ekman current caused by the northeast monsoon transports more (less) warm water from the central basin of the Beibu Gulf to the west coast, forming a strong (weak) thermal front at the junction of cold and warm water on an interannual scale. The upward trend of TFIBG intensity may be related to the regional heterogeneity of climate warming. This research systematically studied TFIBG, which will help improve people's understanding of the thermal front in the South China Sea (SCS). [ABSTRACT FROM AUTHOR]

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]

The latest satellite in the Landsat series, Landsat-9, was successfully launched on 27 September 2021, equipped with the Operational Land Imager-2 (OLI-2) sensor, continuing the legacy of OLI/Landsat-8. To evaluate the uncertainties in water surface reflectance derived from OLI-2, this study conducts a comprehensive performance assessment of six atmospheric correction (AC) methods—DSF, C2RCC, iCOR, L2gen (NIR-SWIR1), L2gen (NIR-SWIR2), and Polymer—using in-situ measurements from 14 global sites, including 13 AERONET-OC stations and 1 MOBY station, collected between 2021 and 2023. Error analysis shows that L2gen (NIR-SWIR1) (RMSE ≤ 0.0017 sr−1, SA = 6.33°) and L2gen (NIR-SWIR2) (RMSE ≤ 0.0019 sr−1, SA = 6.38°) provide the best results across four visible bands, demonstrating stable performance across different optical water types (OWTs) ranging from clear to turbid water. Following these are C2RCC (RMSE ≤ 0.0030 sr−1, SA = 5.74°) and Polymer (RMSE ≤ 0.0027 sr−1, SA = 7.76°), with DSF (RMSE ≤ 0.0058 sr−1, SA = 11.33°) and iCOR (RMSE ≤ 0.0051 sr−1, SA = 12.96°) showing the poorest results. By comparing the uncertainty and consistency of Landsat-9 (OLI-2) with Sentinel-2A/B (MSI) and S-NPP/NOAA20 (VIIRS), results show that OLI-2 has similar uncertainties to MSI and VIIRS in the blue, blue-green, and green bands, with RMSE differences within 0.0002 sr−1. In the red band, the OLI-2 uncertainties are lower than those of MSI but higher than those of VIIRS, with an RMSE difference of about 0.0004 sr−1. Overall, OLI-2 data processed using L2gen provide reliable surface reflectance and show high consistency with MSI and VIIRS, making it suitable for integrating multi-satellite observations to enhance global coastal water color monitoring. [ABSTRACT FROM AUTHOR]

Computer vision in marine ranching enables real-time monitoring of underwater resources. Detecting fish presents challenges due to varying water turbidity and lighting, affecting color consistency. We propose a Foreground Region Convolutional Neural Network (FR-CNN) that combines unsupervised and supervised methods. It introduces an adaptive multiscale regression Gaussian background model to distinguish fish from noise at different scales. Probability density functions integrate spatiotemporal information for object detection, addressing illumination and water quality shifts. FR-CNN achieves 95% mAP with IoU of 0.5, reducing errors from open-source datasets. It updates anchor boxes automatically on local datasets, enhancing object detection accuracy in long-term monitoring. The results analyze fish species behaviors in relation to environmental conditions, validating the method's practicality. [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]

Using satellite observation and reanalysis data, we studied the cold water phenomenon in the sea area of the Xisha Islands in autumn from 2003 to 2022. The cold water phenomenon refers to a colder water body relative to its surrounding area, which can partially offset the impact of global warming, thereby contributing to the protection and restoration of marine ecosystems. The cold water with a central position close to the Xisha Islands is distributed in a nearly circular area, and exhibits seasonal and interannual variations. It usually occurs at the end of September, with its intensity rapidly increasing and reaching its peak on October 1, then gradually weakening, and finally disappearing by November 6. The maximum intensity of the cold water phenomenon can reach 1.6°C. This phenomenon notably observed in October 2006, 2009, 2013 and 2022, with intensities reaching 0.9°C, 1.1°C, 1.1°C, and 0.4°C, respectively. The dynamic mechanism of its formation can be described as follows: in autumn of some years, the northeast monsoon occurs in the northern part of the South China Sea (SCS), while the southwest monsoon still prevails in the southern part of the SCS, which causes the generation of strong positive wind stress curl (WSC) near the Xisha sea area. The positive WSC causes cyclonic circulation and further causes upwelling through Ekman pumping, which transports low‐temperature water from the seabed to the sea surface and thus leads to the occurrence of the cold water phenomenon in the Xisha sea area. Plain Language Summary: The Xisha sea area has numerous coral reefs and marine ranching sites, which form an important part of the marine ecosystem. Under global climate warming, extreme high temperature events frequently lead to coral reef bleaching and marine ranching degradation. The cold water phenomenon can partially offset the impact of climate warming and reduce the frequency and intensity of marine ecological disasters. We found that there are substantial cold water phenomena in the Xisha sea area during autumn in some years, and the formation mechanism can be attributed to upwelling caused by the East Asian monsoon through an ocean suction process. The cold water phenomenon has seasonal and interannual variations and can cause phytoplankton blooms. The discovery of the cold water phenomenon has important scientific significance for the protection and restoration of coral reefs and marine ranching in the Xisha sea area. Key Points: The cold water phenomenon in the Xisha sea area was reported for the first timeThe cold water in the Xisha sea area exhibited seasonal and interannual variationsThe formation of the cold water can be attributed to upwelling caused by wind stress curl through Ekman pumping in the Xisha sea area [ABSTRACT FROM AUTHOR]

Off the east shore of China, thermal front plays a crucial role in modulating coastal biogeochemical environment. Based on multi‐source observational and reanalysis products, decadal variation and regulatory mechanism of Zhejiang‐Fujian coastal front (ZFCF) were investigated. The intensity of ZFCF showed a two‐stage inversed varying trend over past four decades: a downtrend from 1982 to 2002 followed by an uptrend from 2002 to 2022. During 1982–2002, sea surface temperature (SST) experienced more pronounced warming in inshore areas than offshore due to weakening of the southward coastal current, and this primarily led to the downtrend of ZFCF. Within 2002–2022, opposite SST trends were observed on two sides of ZFCF, that is, the ZFCF kept intensifying due to the cooperation of cooling inshore and warming offshore. The enhanced coastal current driven by stronger monsoon and increased Yangtze River discharge cooled the inshore area, then the cooling was further facilitated by intensified heat loss in the upper layer of the plume. Meanwhile, the onshore shift of Kuroshio led to strengthened intrusion of warm water into the East China Sea, heating the offshore side of ZFCF and thus intensifying the front. Plain Language Summary: Oceanic fronts delineate the boundaries of different water masses and, in coastal regions, front zones are closely associated with biological habitats and hydrodynamics. Located along the East China Sea (ECS), the Zhejiang‐Fujian coastal front (ZFCF) is a region of high biological production, and the famous Zhoushan fishing ground is in that front zone. In this study, we found the ZFCF showed significant decadal variations based on 40 years of satellite observations of sea surface temperature: the intensity of ZFCF exhibits a negative trend during 1982–2002 and a positive trend during 2002–2022. These processes were primarily driven by the combined effects of the terrigenous runoff, coastal current and the Kuroshio Current, with the latter two factors closely related to the variabilities of local winds. The current study provides critical insights into the long‐term change of the ZFCF and we aim to better understand the coastal environments in the ECS. Key Points: Zhejiang‐Fujian coastal thermal front kept weakening within 1982–2002 while intensifying during 2002–2022Weakened coastal current dominated the downtrend of front intensity in the context of global warmingEnhanced coastal current and increased runoff together with stronger westward intrusion of Kuroshio governed the uptrend of front intensity [ABSTRACT FROM AUTHOR]

North Pacific subtropical mode water (NPSTMW) is formed as a low-stratification water mass in the wintertime mixed layer south of the Kuroshio Extension (KE). In a recent period of 2018–2021, the KE jet was in a persistent stable dynamic state. But based on analysis of Argo observation, the mean volume of NPSTMW in the ventilation region dropped anomalously by ∼ 21 % during 2018–2021 relative to 2012–2015, when the KE jet was likewise stable. Moreover, the NPSTMW volume in a denser density range (approximately σθ>25.2 kg m -3) has started to decrease since 2018. The decreases in the NPSTMW subduction and formation rate are associated with anomalously shallow wintertime mixed-layer depth (MLD) and weak heat loss in the NPSTMW formation region. The decrease in air–sea heat exchange acts to weaken the vertical mixing and decrease the MLD, resulting in the weakening of subduction. The interannual variations in the air–sea heat exchange and wintertime MLD reflect the variability in the overlying atmosphere, which is correlated with a Pacific Decadal Oscillation (PDO) shift in 2018–2021. When the PDO shifted from its positive phase to a negative phase in the analysis period, the effects of local wind stress anomalies seemed to play an evident role in driving the variability in NPSTMW on interannual timescales. The MLD and heat loss change during the cold season in 2018–2021 were strongly coupled with the poleward shift of the westerlies – which cause weaker wintertime wind and easterly wind anomalies over the NPSTMW formation region. The declines in heat loss and southward Ekman transport, owing to the wind stress anomalies, further prohibit upper-ocean convection and mixed-layer deepening and cooling. Additionally, the insufficient development of wintertime MLD in 2018–2021 may also be associated with the significantly intensified preconditioning of near-surface stratification (< 150 m depth) due to the persistent near-surface warming and the weak vertical entrainment process in winter. [ABSTRACT FROM AUTHOR]

North Pacific subtropical mode water (NPSTMW) is formed as the low stratification water mass in the wintertime mixed layer south of the Kuroshio Extension (KE). In a recent period of 2018–2021, the KE jet is in a persisting stable dynamic state. But based on analysis of Argo observation, the mean volume of NPSTMW in ventilation region drop anomalously by ~21 % during 2018–2021 relative to 2012–2015 when the KE jet is likewise stable. Moreover, the NPSTMW volume in denser density range (approximately σθ >25.2 kg m-3) starts to decrease but the volume of lighter density of the NPSTMW increases since 2018. The decreasing of the NPSTMW subduction and formation rate are associated with anomalously shallow wintertime mixed layer depth (MLD) and weak heat loss in the NPSTMW formation region. The interannual variations of the NPSTMW subduction and formation reflect the variability of the overlying atmosphere which is correlated with Pacific decadal oscillation (PDO) shift in 2018–2021. When the PDO shifts from its positive to a negative phase in analysis period, the effects of local wind stress anomalies seem to play an evident role in driving the variability of NPSTMW on interannual time scales. The MLD and heat loss change during the cold season in 2018–2021 are strongly coupled with the poleward shift of the westerlies-which cause the weaker wintertime wind and the easterly wind anomalies over the NPSTMW formation region. The declines of heat loss and southward Ekman transport, owing to the wind stress anomalies, further prohibit the upper-ocean convection and mixed layer deepening and cooling. Additionally, the insufficient development of wintertime MLD in 2018–2021 may also be correlated with the significantly intensified preconditioning of near surface stratification (<150 m depth) due to the persisting near surface warming. [ABSTRACT FROM AUTHOR]