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The ocean surface mixed layer plays a crucial role as an entry or exit point for heat, salt, momentum, and nutrients from the surface to the deep ocean. In this study, we introduce a framework to assess the evolution of the mixed layer depth (MLD) for realistic forcings and preconditioning conditions. Our approach involves a physically‐based parameter space defined by three dimensionless numbers: λs representing the relative contribution of the buoyancy flux and the wind stress at the air‐sea interface, Rh the Richardson number which characterizes the stability of the water column relative to the wind shear, and f/Nh which characterizes the importance of the Earth's rotation (ratio of the Coriolis frequency f and the pycnocline stratification Nh). Four MLD evolution regimes ("restratification," "stable," "deepening," and "strong deepening") are defined based on the values of the normalized temporal evolution of the MLD. We evaluate the 3D parameter space in the context of 1D simulations and we find that considering only the two dimensions (λs, Rh) is the best choice of 2D projection of this 3D parameter space. We then demonstrate the utility of this two‐dimensional λs − Rh parameter space to compare 3D realistic ocean simulations: we discuss the impact of the horizontal resolution (1°, 1/12°, or 1/60°) and the Gent‐McWilliams parameterization on MLD evolution regimes. Finally, a proof of concept of using observational data as a truth indicates how the parameter space could be used for model calibration. Plain Language Summary: Vertical mixing of water near the ocean surface occurs when cold air temperatures create dense cold water at the surface that tends to sink in the ocean or when a strong wind induces turbulence at the ocean surface. These processes mix heat and salt and create a layer at the top of the ocean that has a uniform temperature and salinity and that is called the "mixed layer." This mixed layer plays a fundamental role in the Earth climate system, and the representation of its evolution in ocean models hence needs to be assessed. For this purpose, we propose to map the mixed layer evolution in a three‐dimensional space where the first axis is related to the wind and the surface heat flux, the second axis to the stability of the water column, and the third axis to the Earth's rotation. We show that this tool performs statistically well and we present how to use it in the context of realistic ocean models. Key Points: A parameter space is proposed to assess the evolution of the mixed layer depth for realistic forcings and preconditioning conditionsAn evaluation of a collection of 1D simulations shows a statistically good performance of the parameter spaceTwo applications demonstrate the utility of the parameter space for assessing and comparing realistic 3D simulations [ABSTRACT FROM AUTHOR]

Based on the Ocean Reanalysis System version 5 (ORAS5) and the fifth-generation reanalysis datasets derived from European Centre for Medium-Range Weather Forecasts (ERA5), we investigate the different impacts of the central Pacific (CP) El Niño and the eastern Pacific (EP) El Niño on the Southern Ocean (SO) mixed layer depth (MLD) during austral winter. The MLD response to the EP El Niño shows a dipole pattern in the South Pacific, namely the MLD dipole, which is the leading El Niño-induced MLD variability in the SO. The tropical Pacific warm sea surface temperature anomaly (SSTA) signal associated with the EP El Niño excites a Rossby wave train propagating southeastward and then enhances the Amundsen Sea low (ASL). This results in an anomalous cyclone over the Amundsen Sea. As a result, the anomalous southerly wind to the west of this anomalous cyclone advects colder and drier air into the southeast of New Zealand, leading to surface cooling through less total surface heat flux, especially surface sensible heat (SH) flux and latent heat (LH) flux, and thus contributing to the mix layer (ML) deepening. The east of the anomalous cyclone brings warmer and wetter air to the southwest of Chile, but the total heat flux anomaly shows no significant change. The warm air promotes the sea ice melting and maintains fresh water, which strengthens stratification. This results in a shallower MLD. During the CP El Niño, the response of MLD shows a separate negative MLD anomaly center in the central South Pacific. The Rossby wave train triggered by the warm SSTA in the central Pacific Ocean spreads to the Amundsen Sea, which weakens the ASL. Therefore, the anomalous anticyclone dominates the Amundsen Sea. Consequently, the anomalous northerly wind to the west of anomalous anticyclone advects warmer and wetter air into the central and southern Pacific, causing surface warming through increased SH, LH, and longwave radiation flux, and thus contributing to the ML shoaling. However, to the east of the anomalous anticyclone, there is no statistically significant impact on the MLD. [ABSTRACT FROM AUTHOR]

Sonar serves as a critical submarine detection apparatus for naval vessels, with its detection range forming the foundation of its overall performance in underwater surveillance. The Luzon Strait, in the eastern part of the South China Sea, presents a complex hydrographic setting that profoundly influences sonar performance, necessitating mastery of the detection range variation for enhanced anti-submarine operational efficiency. This study employs the Bellhop acoustic propagation model to estimate the transmission loss. Subsequently, a detection probability integration approach is applied to determine the sonar detection range in the Luzon Strait from 2019 to 2023, which is then subjected to statistical analysis. The findings indicate the following. (1) During the summer and autumn, the shallow mixed layer fails to generate a surface duct, resulting in shorter detection ranges that are primarily dependent on the water depth. In the Shallow Water Zone (<150 m), frequent interactions between sound waves and the sea boundaries lead to considerable acoustic energy attenuation, maintaining a short detection range. In the Intermediate Depth Zone (150–2500 m), sound rays retain adequate energy post-seabed reflection, extending the sonar detection to 5–8 km. Beyond 2500 m, the diminishing reflective energy restricts the range to 2–5 km. (2) Conversely, in the winter and spring, the formation of a surface duct becomes the predominant determinant of the detection range, capable of exceeding 10 km, overshadowing the influence of the water depth. [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) in the South China Sea (SCS) have prolonged impacts on local ecosystems and economies, and accurate projection of MHWs under future global warming is crucial for the high-quality development of local society. The future change in the spatial pattern of MHWs, however, is not clear despite the well-known overall intensification of MHWs. Here we find that the Coupled Model Intercomparison Project Phase 6 (CMIP6) models can effectively capture the main distribution of observed SCS MHWs, showing a uniform distribution of frequency and a "north high-south low" distribution of mean intensity and cumulative intensity. However, it is worth noting that the simulated center of long MHW duration is shifted to the southern SCS compared to the central SCS in observations. Under the Shared Socioeconomic Pathway 1–2.6 (SSP126) scenario, the increase in MHW cumulative intensity exhibits a double-center structure in the northern coastal region and southern SCS. This is primarily attributed to the large increase in frequency and mean intensity in the north, and an increase in duration in the south. Both the SSP245 and SSP585 scenarios project similar patterns of MHW intensification, but with larger magnitudes. The climatological distribution of the mixed layer depth (MLD), which is deeper in the south and shallower in the north, contributes to the spatial distribution of SCS MHW changes. The strong seasonal-mean sea surface temperature (SST) warming in the northern SCS, caused by enhanced solar radiation, also contributes to the intensification of MHW frequency and mean intensity in the northern region. [ABSTRACT FROM AUTHOR]

Surface waves have a considerable effect on vertical mixing in the upper ocean. In the past two decades, the vertical mixing induced through nonbreaking surface waves has been used in ocean and climate models to improve the simulation of the upper ocean. Thus far, several nonbreaking wave-induced mixing parameterization schemes have been proposed; however, no quantitative comparison has been performed among them. In this paper, a one-dimensional ocean model was used to compare the performances of five schemes, including those of Qiao et al. (Q), Hu and Wang (HW), Huang and Qiao (HQ), Pleskachevsky et al. (P), and Ghantous and Babanin (GB). Similar to previous studies, all of these schemes can decrease the simulated sea surface temperature (SST), increase the subsurface temperature, and deepen the mixed layer, thereby alleviating the common thermal deviation problem of the ocean model for upper ocean simulation. Among these schemes, the HQ scheme exhibited the weakest wave-induced mixing effect, and the HW scheme exhibited the strongest effect; the other three schemes exhibited roughly the same effect. In particular, the Q and P schemes exhibited nearly the same effect. In the simulation based on observations from the Ocean Weather Station Papa, the HQ scheme exhibited the best performance, followed by the Q scheme. In the experiment with the HQ scheme, the root-mean-square deviation of the simulated SST from the observations was 0.43°C, and the mixed layer depth (MLD) was 2.0 m. As a contrast, the deviations of the SST and MLD reached 1.25°C and 8.4 m, respectively, in the experiment without wave-induced mixing. [ABSTRACT FROM AUTHOR]

Abstract The ocean surface mixed layer plays a crucial role as an entry or exit point for heat, salt, momentum, and nutrients from the surface to the deep ocean. In this study, we introduce a framework to assess the evolution of the mixed layer depth (MLD) for realistic forcings and preconditioning conditions. Our approach involves a physically‐based parameter space defined by three dimensionless numbers: λs representing the relative contribution of the buoyancy flux and the wind stress at the air‐sea interface, Rh the Richardson number which characterizes the stability of the water column relative to the wind shear, and f/Nh which characterizes the importance of the Earth's rotation (ratio of the Coriolis frequency f and the pycnocline stratification Nh). Four MLD evolution regimes (“restratification,” “stable,” “deepening,” and “strong deepening”) are defined based on the values of the normalized temporal evolution of the MLD. We evaluate the 3D parameter space in the context of 1D simulations and we find that considering only the two dimensions (λs, Rh) is the best choice of 2D projection of this 3D parameter space. We then demonstrate the utility of this two‐dimensional λs − Rh parameter space to compare 3D realistic ocean simulations: we discuss the impact of the horizontal resolution (1°, 1/12°, or 1/60°) and the Gent‐McWilliams parameterization on MLD evolution regimes. Finally, a proof of concept of using observational data as a truth indicates how the parameter space could be used for model calibration.

ABSTRACTEstimating the ocean mixed layer depth (MLD) is crucial for studying the atmosphere-ocean interaction and global climate change. Satellite observations can accurately estimate the MLD over large scales, effectively overcoming the limitation of sparse in situ observations and reducing uncertainty caused by estimation based on in situ and reanalysis data. However, combining multisource satellite observations to accurately estimate the global MLD is still extremely challenging. This study proposed a novel Residual Convolutional Gate Recurrent Unit (ResConvGRU) neural networks, to accurately estimate global MLD along with multisource remote sensing data and Argo gridded data. With the inherent spatiotemporal nonlinearity and dependence of the ocean dynamic process, the proposed method is effective in spatiotemporal feature learning by considering temporal dependence and capturing more spatial features of the ocean observation data. The performance metrics show that the proposed ResConvGRU outperforms other well-used machine learning models, with a global determination coefficient (R2) and a global root mean squared error (RMSE) of 0.886 and 17.83 m, respectively. Overall, the new deep learning approach proposed is more robust and advantageous in data-driven spatiotemporal modeling for retrieving ocean MLD at the global scale, and significantly improves the estimation accuracy of MLD from remote sensing observations.

Blue holes are part of complex water systems formed by tropical wetlands, karst aquifers, and different coastal environments. In the southeastern Yucatán Peninsula, three submerged blue holes (i.e., Lool ja', Ch'och-ja', and Taam-ja') are located in the shallow tropical estuary of Chetumal Bay. This study examines the morphological features of the three blue holes and explores the seasonal variability of their physicochemical parameters through quarterly field measurements taken between 2021 and 2022. Additionally, this study represents the first documentation of the Lool-ja' blue hole, which has the largest surface area and volume of the karst structures analyzed. Temperature and salinity profiles along the water column were measured together with surface pH and DO at the blue hole center and within its vicinity (~ 500 m apart). Time series of water temperature, conductivity, and water level were also measured between June and December 2021. Morphological features of the blue holes were explored through georeferenced echo-sounding and 3D-modelling. Seasonal variations were observed on the mixed water layer depth (MLD). During the rainy (dry) season, the surficial mixed water mass became deeper (shallower). Variability in the MLD was proportional to the area occupied by the mouth of the blue hole (r = 0.813). Further homogenized layers accompanied by density interfaces were observed at different depths inside the blue holes. Finally, temperature and salinity diagrams revealed the possible interseasonal exchange of water in the Lool-ja' and Ch'och-ja' blue holes with possible intrusion from marine sources. [ABSTRACT FROM AUTHOR]

Temporal and spatial variations in the ocean surface mixed layer are important for the climate and ecological systems. During 1980–2019, the Southern Indian Ocean (SIO) mixed layer depth (MLD) displays a basin‐wide shoaling trend that is absent in the other basins within 40°S–40°N. The SIO MLD shoaling is mostly prominent in austral winter with deep climatology MLD, substantially weakening the MLD seasonality. Moreover, the SIO MLD changes are primarily caused by a southward shift of the subtropical anticyclonic winds and hence ocean gyre, associated with a strengthening of the Southern Annular Mode, in recent decades for both winter and summer. However, the poleward‐shifted subtropical ocean circulation preferentially shoals the SIO MLD in winter when the meridional MLD gradient is sharp but not in summer when the gradient is flat. This highlights the distinct subtropical MLD response to meridional mitigation in winds due to different background oceanic conditions across seasons. Plain Language Summary: The ocean surface mixed layer (ML) is a well‐mixed layer with uniform physical and chemical properties and is key for the ocean in exchanging materials and energy with the atmosphere. The present study shows that during 1980–2019, the Southern Indian Ocean (SIO) ML depth (MLD) displays a basin‐wide decreasing trend, which is absent in the other basins within 40°S–40°N. The SIO MLD shoaling primarily appears in austral winter when the climatology ML is deep but is insignificant in summer, substantially weakening the MLD seasonality. The SIO MLD changes are primarily explained by the ocean dynamical adjustment driven by the surface zonal wind changes. Specifically, the strengthened Southern Annular Mode in recent decades drives southward shifts of the subtropical anticyclonic winds and ocean gyre year‐round. However, the poleward‐shifted ocean gyre preferentially decreases the SIO winter MLD as the meridional MLD gradient is sharp and thus efficiently reduces the deep ML water converging from the Southern Ocean into the SIO. In contrast, the SIO MLD displays negligible change in summer when its meridional gradient is flat. The results highlight that despite under nearly identical southward‐shifted subtropical winds, the winter and summer MLD responses are distinct due to different background oceanic conditions. Key Points: The seasonality of the Southern Indian Ocean surface mixed layer (ML) depth prominently weakens during 1980–2019The weakened seasonality mainly results from a pronounced winter ML shoalingThe southward shift of the subtropical ocean gyre driven by the strengthened Southern Annular Mode dominates the ML shoaling [ABSTRACT FROM AUTHOR]

Mixed Layer Depth (MLD) plays an important role in various aspects of oceanography. MLD has a characteristic parameter value that is uniform with depth. MLD has an important role for local, regional, and global phenomena. Indonesia, which is surrounded by the East Indian Ocean, will be directly influenced by the dynamics of MLD. This study aimed to analyze seasonal variability and MLD between years. Mixed layer depth data from the ARMOR3D Dataset Copernicus-Marine Environment Monitoring Service was used for MLD analysis with a threshold of 0.2°C for temperature. Wavelet analysis showed that MLD variability in the eastern Indian Ocean spans from intra-seasonal to interannual scales. Time series analysis showed a complex relationship between MLD and SST in the annual and interannual periods which indicates a different process. The MLD monthly climatology at point 90E, 0 showed the depth of mixed layers is deeper during the east monsoon (JJA-SON) ranging from 50-65 m compared to the west monsoon (DJF-MAM) which has a range of 20-40 m. Spatially the MLD in the south of the equator is deeper than in the north. Interannually, MLD is heavily influenced by the Indian Ocean Dipole. MLD depth is deeper in nIOD with a maximum depth in the range of 100 m compared to pIOD. MLD with maximum depth in the strong nIOD phase is around the equator and the pIOD phase is south of the equator. The study also showed that inter-annual variability in regions around the mainland showed a stronger response. [ABSTRACT FROM AUTHOR]

The ocean's mixed layer depth (MLD) plays an important role in understanding climate dynamics, especially during extreme weather occurrences like hurricanes. This study investigates the effects of Hurricane Katrina (2005) on the MLD in the Gulf of Mexico, using the Delft3D modeling system. By integrating hydrodynamics and wave dynamics modules, we simulate the ocean's response to extreme weather, focusing on temperature, salinity and MLD variations. Our analysis reveals significant cooling and mixing induced by Katrina, resulting in spatial and temporal fluctuations in temperature (~±4 °C) and salinity (~±1.5 ppt). The MLD is estimated using a simple threshold method, revealing a substantial deepening to ~120 m on 29–30 August during Hurricane Katrina in the middle of the northern Gulf of Mexico, compared to an average MLD of ~20–40 m during pre-storm conditions. It took about 18 days to recover to ~84% of the pre-storm level after Katrina. Compared to the stand-alone FLOW model, the coupled FLOW+WAVE model yields a deeper MLD of ~5%. The MLD recovery and wave effect on the MLD provide insights from various scientific, environmental and operational perspectives, offering a valuable basis for ocean management, planning and applications, particularly during extreme weather events. [ABSTRACT FROM AUTHOR]

El Niño–Southern Oscillation (ENSO) exhibits a distinctive phase-locking characteristic, first expressed during its onset in boreal spring, developing during summer and autumn, reaching its peak towards winter, and decaying over the next spring. Several studies have demonstrated that this feature arises as a result of seasonal variation in the growth rate of ENSO as expressed by the sea surface temperature (SST). The bias towards simulating the phase locking of ENSO by many state-of-the-art climate models is also attributed to the unrealistic depiction of the growth rate. In this study, the seasonal variation of SST growth rate in the Niño-3.4 region (5°S–5°N, 120°–170°W) is estimated in detail based on the mixed layer heat budget equation and recharge oscillator model during 1981–2020. It is suggested that the consideration of a variable mixed layer depth is essential to its diagnostic process. The estimated growth rate has a remarkable seasonal cycle with minimum rates occurring in spring and maximum rates evident in autumn. More specifically, the growth rate derived from the meridional advection (surface heat flux) is positive (negative) throughout the year. Vertical diffusion generally makes a negative contribution to the evolution of growth rate and the magnitude of vertical entrainment represents the smallest contributor. Analysis indicates that the zonal advective feedback is regulated by the meridional immigration of the intertropical convergence zone, which approaches its southernmost extent in February and progresses to its northernmost location in September, and dominates the seasonal variation of the SST growth rate. [ABSTRACT FROM AUTHOR]

Abstract Mixed Layer Depth (MLD) plays an important role in various aspects of oceanography. MLD has a characteristic parameter value that is uniform with depth. MLD has an important role for local, regional, and global phenomena. Indonesia, which is surrounded by the East Indian Ocean, will be directly influenced by the dynamics of MLD. This study aimed to analyze seasonal variability and MLD between years. Mixed layer depth data from the ARMOR3D Dataset Copernicus-Marine Environment Monitoring Service was used for MLD analysis with a threshold of 0.2oC for temperature. Wavelet analysis showed that MLD variability in the eastern Indian Ocean spans from intra-seasonal to interannual scales. Time series analysis showed a complex relationship between MLD and SST in the annual and interannual periods which indicates a different process. The MLD monthly climatology at point 90E, 0 showed the depth of mixed layers is deeper during the east monsoon (JJA-SON) ranging from 50-65 m compared to the west monsoon (DJF-MAM) which has a range of 20-40 m. Spatially the MLD in the south of the equator is deeper than in the north. Interannually, MLD is heavily influenced by the Indian Ocean Dipole. MLD depth is deeper in nIOD with a maximum depth in the range of 100 m compared to pIOD. MLD with maximum depth in the strong nIOD phase is around the equator and the pIOD phase is south of the equator. The study also showed that inter-annual variability in regions around the mainland showed a stronger response. Highlight Researh • ARMOR3D data has fairly good accuracy as evidenced by data validation • In general, EIO is influenced by intra-seasonal, seasonal and inter-annual variability • The shallowing and deepening of the MLD are strongly correlated with the wind speed associated with the annual cycle • IOD has a strong role in MLD which looks significantly different, especially when nIOD and pIOD

An obvious trend shift in the annual mean and winter mixed layer depth (MLD) in the Antarctic Circumpolar Current (ACC) region was detected during the 1960–2021 period. Shallowing trends stopped in mid-1980s, followed by a period of weak trends. The MLD deepening trend difference between the two periods were mainly distributed in the western areas in the Drake Passage, the areas north to Victoria Land and Wilkes Land, and the central parts of the South Indian sector. The newly formed ocean current shear due to the meridional shift of the ACC flow axis between the two periods is the dominant driver for the MLD trends shift distributed in the western areas in the Drake Passage and the central parts of the South Indian sector. The saltier trends in the regions north to Victoria Land and Wilkes Land could be responsible for the strengthening mixing processes in this region. [ABSTRACT FROM AUTHOR]

Antarctic coastal polynyas are biological hotspots in the Southern Ocean that support the abundance of high-trophic-level predators and are important for carbon cycling in the high-latitude oceans. In this study, we examined the interannual variation of summertime phytoplankton biomass in the Marguerite Bay polynya (MBP) in the western Antarctic Peninsula area, and linked such variability to the Southern Annular Mode (SAM) that dominated the southern hemisphere extratropical climate variability. Combining satellite data, atmosphere reanalysis products and numerical simulations, we found that the interannual variation of summer chlorophyll-a (Chl-a) concentration in the MBP is significantly and negatively correlated with the spring SAM index, and weakly correlated with the summer SAM index. The negative relation between summer Chl-a and spring SAM is due to weaker spring vertical mixing under a more positive SAM condition, which would inhibit the supply of iron from deep layers into the surface euphotic layer. The negative relation between spring mixing and spring SAM results from greater precipitation rate over the MBP region in positive SAM phase, which leads to lower salinity in the ocean surface layer. The coupled physical-biological mechanisms between SAM and phytoplankton biomass revealed in this study is important for us to predict the future variations of phytoplankton biomasses in Antarctic polynyas under climate change. [ABSTRACT FROM AUTHOR]

Predictions of tropical cyclone (TC) activity have been a topic of recurrent interest and research in the past. Here we utilize reanalysis datasets of sea surface temperature (SST) and mixed layer depth (MLD) to build a statistical seasonal forecasting model that produces outlooks of expected TC counts in the region of the Southwest Pacific (SWP). Nevertheless, the model applicability can be extended to other regions and basins. A novel TC predictor index is developed at the daily scale and used to obtain an objective classification of synoptic weather patterns. This classification has been performed by clustering the daily index predictor fields, previously transformed into principal components, using a K‐mean algorithm. As a result, 49 daily weather types (DWTs) are presented which inform about the mean representative features and spatial patterns of both predictor and predictand variables. Thus, statistical relationships between TC activity and nonlinear combinations of predictor variables are found to assign daily rates of expected TCs. The cluster‐based model is calibrated from 1982 to 2019 and validated by recent TC season observations, demonstrating the operational application using ensembles of long‐term predictions in the Southwest Pacific. Results have shown which synoptic types of SST and MLD are favourable to cyclogenesis and activity, with additional information related to concurrent sea level pressure and precipitation synoptic patterns, as well as seasonal and interannual climate variability. [ABSTRACT FROM AUTHOR]

Abstract Temporal and spatial variations in the ocean surface mixed layer are important for the climate and ecological systems. During 1980–2019, the Southern Indian Ocean (SIO) mixed layer depth (MLD) displays a basin‐wide shoaling trend that is absent in the other basins within 40°S–40°N. The SIO MLD shoaling is mostly prominent in austral winter with deep climatology MLD, substantially weakening the MLD seasonality. Moreover, the SIO MLD changes are primarily caused by a southward shift of the subtropical anticyclonic winds and hence ocean gyre, associated with a strengthening of the Southern Annular Mode, in recent decades for both winter and summer. However, the poleward‐shifted subtropical ocean circulation preferentially shoals the SIO MLD in winter when the meridional MLD gradient is sharp but not in summer when the gradient is flat. This highlights the distinct subtropical MLD response to meridional mitigation in winds due to different background oceanic conditions across seasons.

Sonar serves as a critical submarine detection apparatus for naval vessels, with its detection range forming the foundation of its overall performance in underwater surveillance. The Luzon Strait, in the eastern part of the South China Sea, presents a complex hydrographic setting that profoundly influences sonar performance, necessitating mastery of the detection range variation for enhanced anti-submarine operational efficiency. This study employs the Bellhop acoustic propagation model to estimate the transmission loss. Subsequently, a detection probability integration approach is applied to determine the sonar detection range in the Luzon Strait from 2019 to 2023, which is then subjected to statistical analysis. The findings indicate the following. (1) During the summer and autumn, the shallow mixed layer fails to generate a surface duct, resulting in shorter detection ranges that are primarily dependent on the water depth. In the Shallow Water Zone (

The statistical characteristics and mechanism of the South Atlantic Ocean Dipole (SAOD) from 1980 to 2021 are analysed using observational datasets. The spatial pattern of the sea surface temperature anomaly (SSTA) during SAOD is a dipole pattern oriented in the northeast‐southwest direction, and the intensity of the SSTA in the northeast pole (NEP) is stronger than that in the southwest pole (SWP). SAOD has a decadal variability of about 12 years during 1980–2007, along with obvious seasonal phase‐locking, with the anomaly pattern developing in boreal spring (March–May), peaking in summer (June–August) and decaying in autumn (September–November). For a positive SAOD event (positive SSTA in the NEP, negative in the SWP), positive SSTA in the NEP grows due to a decrease in wind speed and thus in latent heat flux loss in boreal spring, as well as an increase in shortwave radiation flux from boreal spring to summer. However, southwesterly wind anomalies drive cold water from high latitudes to the SWP in boreal spring and summer, coupled with strong wind speed anomalies enhancing the loss of latent heat flux, which contributes to a negative SSTA in the SWP. In addition, the intensity of the SSTA in the SWP is weaker than that in the NEP because of the contribution of smaller shortwave radiation flux, sensible heat flux and larger mixed layer depth in the SWP in summer. For a negative SAOD mode (negative SSTA in the NEP, positive in the SWP), the wind, shortwave radiation flux, sensible heat flux and mixed layer depth anomalies are the opposite of those under a positive SAOD event. [ABSTRACT FROM AUTHOR]

Oceanic general circulation models (OGCMs) are important tools used to investigate mechanisms for ocean climate variability and predict the ocean change in the future. However, in most current ocean models, the impact of sea surface waves as one of the most significant dynamic processes in the upper ocean is absent. In this study, the Stokes drift and the vertical mixing induced by nonbreaking surface waves derived from the wave model (WAVEWATCH III) are incorporated into a Climate System Ocean Model, and their effects on an ocean climate simulation are analyzed. Numerical experiments show that both physical processes can improve the simulation of sea surface temperature (SST) and mixed layer depth (MLD) in the Southern Hemisphere. The introduction of Stokes drift effectively reduces the subsurface warm bias in the equatorial tropics, which is caused by the weakening of vertical mixing in the equatorial region. The nonbreaking surface wave mainly reduces the temperature bias in the Southern Ocean by enhancing mixing in the upper ocean. For the MLD, the Stokes drift mainly improves the simulation of the winter MLD, and the nonbreaking surface wave improves the summer MLD. For MLD south of 40° S in summer, the introduction of nonbreaking surface waves resulted in a reduction of 11.86 m in MLD bias and 7.8 m in root mean square errors (RMSEs), respectively. For winter subtropical MLD in the Southern Hemisphere, considering the Stokes drift, the MLD bias and RMSEs were reduced by 2.49 and 5.39 m, respectively. Adding these two physical processes simultaneously provides the best simulation performance for the structure of the upper layer. The introduction of sea surface waves effectively modulates the vertical mixing of the upper ocean and then improves the simulation of the MLD. Thus, sea surface waves are very important for ocean simulation, so we will further couple a sea waves model in the Chinese Academy of Sciences Earth System Model (CAS-ESM) as part of their default model component. [ABSTRACT FROM AUTHOR]

The article presents the discussion on PAR estimates indicate that a variety of physiological and ecological conditions can control the onset of spring blooms. Topics include satellite observations have started to report optimal light intensities for phytoplankton blooming in various physiological and ecological environments; and investigating the optimal light intensities required for phytoplankton spring bloom development in the East Sea (ES).

Mansor, K.N.A.A.K.; Roseli, N.H.; Ali, F.S.M., and Akhir, M.F.M., 2023. Physical properties of seawater in Malacca Strait (Southeast Asia) during monsoon seasons. Journal of Coastal Research, 39(5), 921–932. Charlotte (North Carolina), ISSN 0749-0208. Malacca Strait (MS) is a narrow passage in Southeast Asia mainly influenced by the Asian monsoon system. As a busy international maritime route, MS is highly exposed to seawater pollution, harmful algal blooms, and jellyfish blooms. To understand the physical properties of the water column in MS, scientific cruise data were used to examine mixed layer depth, stratification frequency, and water mass distribution during two monsoon seasons (March and August). Results showed that surface water in March is fresher and warmer than in August, whereas the bottom depth in March is more saline and cooler than in August. The mixed layer depth for both months did not exceed approximately 15 m for temperature and salinity, with thermocline and halocline layers observed below the mixed layer depth. The temperature and salinity diagram classified three water masses from the cruise dataset—surface warm water, mixed water, and subsurface water—with the potential density anomaly ranging between about 15.5 and 24 kg/m3. The highest density of water mass, subsurface water, was found only in March at a depth between 54 and 80 m. This cool, high-salinity water is the remaining NE monsoon water mass that settled near the bottom because March is the period of change from NE monsoon to SW monsoon. During this time, winds weaken and solar radiation increases, thus creating stable warm surface water. Strong stratification observed in March prevented mixing between warm surface water and cool bottom water. Meanwhile, August is characterized by a warm SW monsoon; thus, the whole MS is occupied by warmer water. This research presents the variation of physical properties in the water column and reveals the influences of monsoon season on shifts of stratification and water mass distribution. [ABSTRACT FROM AUTHOR]

This study examines the modifications of air-sea coupling processes by dust-radiation-cloud interactions over the North Atlantic Ocean using a high-resolution coupled atmosphere-wave-ocean-dust (AWOD) regional model. The dust-induced mechanisms that are responsible for changes of sea surface temperature (SST) and latent and sensible heat fluxes (LHF/SHF) are also examined. Two 3-month numerical experiments are conducted, and they differ only in the activation and deactivation of dust-radiation-cloud interactions. Model results show that the dust significantly reduces surface downward radiation fluxes (SDRF) over the ocean with the maximum change of 20-30 W m-2. Over the dust plume region, the dust effect creates a low-pressure anomaly and a cyclonic circulation anomaly, which drives a positive wind stress curl anomaly, thereby reducing sea surface height and mixed layer depth. However, the SST change by dust, ranging from -0.5 to 0.5 K, has a great spatial variation which differs from the dust plume shape. Dust cools SST around the West African coast, except under the maximum dust plume ridge, and extends westward asymmetrically along the northern and southern edges of the dust plume. Dust unexpectedly warms SST over a large area of the western tropical North Atlantic and north of the dust plume. These SST changes are controlled by different mechanisms. Unlike the SST change pattern, the LHF and SHF changes are mostly reduced underneath the dust plume region, though they are different in detail due to different dominant factors, and increased south of the dust plume over the tropic.

The ocean’s mixed layer depth (MLD) plays an important role in understanding climate dynamics, especially during extreme weather occurrences like hurricanes. This study investigates the effects of Hurricane Katrina (2005) on the MLD in the Gulf of Mexico, using the Delft3D modeling system. By integrating hydrodynamics and wave dynamics modules, we simulate the ocean’s response to extreme weather, focusing on temperature, salinity and MLD variations. Our analysis reveals significant cooling and mixing induced by Katrina, resulting in spatial and temporal fluctuations in temperature (~±4 °C) and salinity (~±1.5 ppt). The MLD is estimated using a simple threshold method, revealing a substantial deepening to ~120 m on 29–30 August during Hurricane Katrina in the middle of the northern Gulf of Mexico, compared to an average MLD of ~20–40 m during pre-storm conditions. It took about 18 days to recover to ~84% of the pre-storm level after Katrina. Compared to the stand-alone FLOW model, the coupled FLOW+WAVE model yields a deeper MLD of ~5%. The MLD recovery and wave effect on the MLD provide insights from various scientific, environmental and operational perspectives, offering a valuable basis for ocean management, planning and applications, particularly during extreme weather events.

Severe typhoon "In-Fa" passed through the northwestern region of East China Sea (ECS) in July 2021, affecting oceanic variables such as seawater temperature, salinity, and chlorophyll-a (Chl-a) concentration over the upwelling area. In this study, we analyzed the influence of the passage of typhoon "In-Fa" on the marine environment over the Upwelling Area off the Yangtze River Estuary (UAYRE) and the Upwelling Area of Zhoushan (UAZS). The results showed a significant decrease in sea surface temperature (SST) during the "In-Fa" typhoon, with maximum SST reductions of 2.98 °C in the UAYRE and 1.46 °C in the UAZS, which showed a "right bias" (indicating a greater cooling effect on the right side of the typhoon path compared to the left side). "In-Fa" influenced the temperature and salinity structure of the study areas and deepened the mixed layer depth (MLD). The MLD varied from the shallowest values of 2.02 m (18 July) to the deepest values of 19.4 m (26 July) in the UAYRE and from 2.43 m (18 July) to 16.79 m (25 July) in the UAZS. Furthermore, "In-Fa" led to an increase in sea surface Chl-a concentration, with a maximum Chl-a concentration enhancement of 285.58% (from 20 July to 28 July) in the UAYRE and 233.33% (from 20 July to 27 July) in the UAZS. The Ekman suction effect of "In-Fa" strengthened the upwelling, facilitating the transport of deep-sea nutrients to the upper ocean and providing favorable conditions for the growth of phytoplankton, thus benefiting the reproduction and survival of zooplankton, fish, and shrimp. This study contributes to understanding the mechanisms by which typhoons impact the ocean environment in upwelling area and provides valuable insights for the sustainable development of marine fisheries resources. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) anomalies in the Kuroshio Extension (KE) have been suggested to play a crucial role in decadal climate variability of the North Pacific affecting global climate and marine ecosystem variability. By analyzing the mixed layer heat budget, taking into account seasonality and mixed layer depth (MLD) variations, we here show for the first time that the KE SST front undergoes large decadal variations mainly owing to decadal modulations of the effective heat capacity affecting the SST sensitivity to surface heat fluxes during 1982–2015. More specifically, when the mixed layer becomes anomalously thick (shallow) to the south (north) of the front, it becomes less (more) sensitive to wintertime surface cooling. As a result, the SST front is strengthened, with positive (negative) SST anomalies to the south (north). A heat conservation model suggests that MLD anomalies are mainly due to thermocline depth anomalies. Plain Language Summary: Decadal climate variability of the North Pacific is known to affect global climate and marine ecosystem variability. The sea surface temperature (SST) front associated with the Kuroshio Extension (KE) in the northwestern Pacific is considered to play a key role in the turnabout of the North Pacific decadal climate variability. The fundamental question of how the strength of the SST front is modulated on the decadal timescale, however, has yet to be understood. Through quantitative analyses of oceanic and atmospheric data sets during 1982–2015, here we present for the first time that the KE SST front undergoes large decadal variations mainly owing to decadal modulation in thickness of the surface mixed layer that controls sensitivity of SST to surface heat exchange between the ocean and the atmosphere. Furthermore, a highly idealized model suggests that the anomalous mixed layer thickness is mainly due to anomalous oceanic stratification. Key Points: Generation mechanisms of decadal sea surface temperature (SST) anomalies associated with the Kuroshio Extension SST front is examinedA seasonally stratified mixed layer heat budget analysis with variable mixed layer depth (MLD) is conductedDecadal modulation in the MLD that controls sensitivity of SST to surface heat fluxes plays a key role during 1982–2015 [ABSTRACT FROM AUTHOR]

The frequent typhoons in the northwest Pacific drive oceanic responses, for example, changes in sea surface temperature (SST) and chlorophyll‐a (Chl‐a). A composite analysis shows that SST (Chl‐a) around the typhoon center decreases (increases). The SST begins to decrease 3 days before the typhoon's arrival and further decreases until 2 days following the typhoon's passage. The Chl‐a increases rapidly after the typhoon's arrival, and the maximum value is reached 3 days after the typhoon. Large oceanic responses are often associated with typhoons that have high wind speeds and slow translation speeds. From the perspective of the upper ocean structure, increasing changes in SST are identified with a shallower pre‐typhoon mixed layer depth (MLD). However, a significant dependence of the Chl‐a response on the pre‐typhoon MLD emerges only when the depth of typhoon‐induced mixing is greater than the pre‐typhoon MLD. This study helps to quantitatively describe typhoon‐induced changes with consideration of the determinant oceanic environment. Plain Language Summary: Typhoons in the northwestern Pacific induce strong oceanic responses. Using 17 years of satellite observations, the impacts of typhoons on sea surface temperature (SST) and chlorophyll‐a (Chl‐a) are investigated. The SST time series shows that the SST begins to decrease 2 days before the typhoon's arrival and continues to decrease until 2 days following the typhoon's passage. The Chl‐a has a weak peak 2 days prior to the typhoon's arrival, rapidly increases after the typhoon arrives, reaches the strongest response on the third day of the typhoon, and gradually decreases to a value slightly higher than the pre‐typhoon level. Prominent responses are associated with typhoons that have stronger intensity and slower translation speed. The pre‐typhoon upper ocean structure plays a dominant role in determining oceanic responses. Surface cooling is generally stronger where the pre‐typhoon mixed layer depth (MLD) is shallow. However, the change in Chl‐a shows a contrasting response in that the response prominently increases only when the depth of typhoon‐induced mixing exceeds the pre‐typhoon MLD. This study poses a quantitative approach to assess the influence of typhoons on the upper ocean from a statistical perspective with consideration of the upper ocean structure. Key Points: Sea surface temperature and chlorophyll‐a changes are composited to assess their responses to the passage of typhoons in the northwest PacificLarge responses are introduced by strong and slow‐moving typhoons, and surface cooling increases with shallow mixed layer depth (MLD)Responses in chlorophyll‐a increase only when the depth of typhoon‐induced mixing exceeds the pre‐typhoon MLD [ABSTRACT FROM AUTHOR]

Intraseasonal oscillation of the mixed layer and upper ocean temperature has been found to occur over the South China Sea (SCS) in the summer monsoon season based on the multiple reanalysis and observational data in this study. The method of composite analysis and an upper ocean temperature equation assisted the analysis of physical mechanisms. The results show that the mixed layer depth (MLD) in the SCS has a significant oscillation with a 30–60 d period over the SCS region, which is closely related to boreal summer intraseasonal oscillation (BSISO) activities. The MLD can increase (decrease) during the positive (negative) phase of the BSISO and usually lags behind by approximately one-eighth of the lifecycle (5 days) of the BSISO-related convection. The BSISO may cause periodic anomalies at the air-sea boundary, such as wind stress and heat flux, so it can play a dominant role in modulating the variation in MLD. There also are significant intraseasonal seawater temperature anomalies in both the surface and subsurface layers of the SCS. In addition, during the initial phase of the BSISO, the temperature anomaly signals of the thermocline are obviously opposite to the sea surface temperature (SST), especially in the southern SCS. According to the results from the analysis of the temperature equation, the vertical entrainment term caused by BSISO-related wind stress is stronger than the thermal forcing during the initial stage of convection, and it is more significant in the southern SCS. [ABSTRACT FROM AUTHOR]

The seasonality of the vertical mixing at coastal sites is not well characterized yet. Here, a time series of the dissipation rate of turbulent kinetic energy (ε) was obtained from weekly morning microstructure observations covering the destratification period (July 2015, February 2016) at a coastal site in the western Mediterranean Sea, influenced by freshwater runoffs. Estimated with bulk parameters from the public re-analyzed dataset ERA5, the Ekman layer, and the convective penetration depth scale with the mixed layer depth (MLD) with a good agreement. Below the MLD, peaks of ε are observed in the baroclinic layers that progressively overlap with the bottom layer, where repeated near-bottom turbidity peaks provide evidence of sediment resuspension, suggesting energetic processes within the bottom boundary layer. In the subsurface, moderate values ( 10 − 9 to 10 − 8   W   kg − 1 ) are observed, following a Burr type XII distribution. Significant correlation with ε at MLD is obtained with a model combining the effects of wind, wind–wave, and convection, highlighting a calm sea bias in our data, plus a sunrise bias when morning buoyancy fluxes are stabilizing. Another correlation, obtained from a pure-wind estimation 18 h before, suggests the role of wind in generating internal waves in the stratified layers, thus, impacting mixing intensity. [ABSTRACT FROM AUTHOR]

Green Noctiluca scintillans (NSG) is a mixotrophic dinoflagellate that frequently forms intense blooms in the north Indian Ocean, especially in the northeastern Arabian Sea during winter. This study investigates the conducive conditions and drivers associated with NSG blooms and proposes significant models for estimating NSG based on in situ (time-series) study during the bloom cycles. Two critical factors with regard to the blooms, i.e., phytoplankton abundance and sea surface temperature (SST), were examined. The first phase of heterotrophy dominance was when moderate blooms up to ~ 2.26 × 104 cells 1–1 occurred and, when NSG cells per unit chlorophyll-a (chl-a) increased, SST decreased up to ~ 24.5 ºC. The bloom intensity was proportional to the feed (diatoms/phytoplankton) availability and the degree of cooling (by the winter convection, i.e., nutrient enrichment). In the second phase of autotrophy dominance, intense blooms up to 1.9 × 105 cells l−1 occurred and NSG cells per unit chl-a fell, when the SST increased. During this period, bloom intensity was proportional to the degree of warming, i.e., nutrient and physiological stress. Phytoplankton are related to NSG by a single linear model through this SST cycle and is likely the NSG's essential biotic precursor. Attention is then focused on developing a remote sensing reflectance (Rrs) model for efficient synoptic monitoring of NSG using ocean color satellites. The Rrs band product ratio, a new metric, in combination with SST, notably modelled NSG abundance, which may be of potential routine application. [ABSTRACT FROM AUTHOR]

Abstract Sea surface temperature (SST) anomalies in the Kuroshio Extension (KE) have been suggested to play a crucial role in decadal climate variability of the North Pacific affecting global climate and marine ecosystem variability. By analyzing the mixed layer heat budget, taking into account seasonality and mixed layer depth (MLD) variations, we here show for the first time that the KE SST front undergoes large decadal variations mainly owing to decadal modulations of the effective heat capacity affecting the SST sensitivity to surface heat fluxes during 1982–2015. More specifically, when the mixed layer becomes anomalously thick (shallow) to the south (north) of the front, it becomes less (more) sensitive to wintertime surface cooling. As a result, the SST front is strengthened, with positive (negative) SST anomalies to the south (north). A heat conservation model suggests that MLD anomalies are mainly due to thermocline depth anomalies.

The Humboldt Upwelling System hosts a highly productive ecosystem with central importance for global fisheries, yet with strong seasonal and interannual variability in the planktonic base of the food chain ultimately affecting fish yield. Understanding the variability in energy transfer within the plankton community in the contemporary climate can provide valuable insights for future projections of planktonic dynamics. Therefore, we use a regional physical-biogeochemical ocean model simulation (CROCO-BioEBUS) from 1990 to 2010 to investigate the underlying mechanisms of seasonal and interannual variability of the trophic transfer. Our model simulations suggest that, on an interannual scale, variations in trophic transfer are governed by variations in the offshore surface flow that modulate the plankton cross-shore distribution. Weak offshore surface flow, as simulated during the El Niño period, allows the zooplankton to stay relatively close to the shore, leading to more efficient grazing and trophic transfer compared to years with strong offshore flow. This mechanism differs from the seasonal one, where the mixed layer depth is the primary driver of variations in plankton dynamics, including trophic transfer. Our results highlight that mechanisms controlling plankton trophic transfer differ across time scales, and thus stress that extrapolating solely from seasonal findings to understand long-term trophic transfer changes in the context of climate change may be insufficient.

The influence of mesoscale eddies on chlorophyll (Chl) has received significant attention due to Chl being a proxy for phytoplankton, which plays a crucial role in marine ecosystems. Solely relying on the analysis of satellite-observed Chl poses challenges in determining the phytoplankton response to mesoscale eddies. To address this, our study takes a collaborative approach, utilizing satellite-derived sea surface temperature anomalies (SSTA) and chlorophyll anomalies (CHLA) to comprehensively investigate the dynamical-biological processes associated with eddies in the subtropical and mid-latitude North Atlantic. In the subtropics, the patterns in CHLA and SSTA predominantly exhibit a dipole nature, with the dipole component providing more than 70% of the explained variance (EV). This suggests that eddy stirring is the dominant mechanism driving the observed anomaly patterns. Conversely, in the mid-latitudes, the monopole components ( T _M ) explain more than 60% of the EV, implying a more influential role for eddy trapping and vertical modulations. The signs of the T _M of eddy SSTA persist throughout their lifetime, being consistent with the lowering (raising) of isopycnals within AEs (CEs). However, the subtropical CHLA response is higher in AEs than CEs, indicating additional factors, such as eddy-induced Ekman pumping and/or mixing to a deeper level may be important. This finding is also corroborated by subsurface observations from Argo floats. At mid-latitudes, there is a clear inverse correspondence between the CHLA and mixed layer depth. In contrast, no significant correlation is observed in the subtropics, except during winter when a positive relationship emerges. These patterns suggest that phytoplankton exhibit highly diverse responses to the physical dynamics associated with eddies. Our work offers a method to estimate eddy dynamical-biological impacts on phytoplankton using satellite products, compensating for the limitations of in-situ observations. It also reveals potential contributions to marine primary production, global carbon cycles, and the development of biogeochemical models.

Ocean dynamic sea level (ODSL) is expected to be one of the major contributors to sea level rise in the North Sea during the 21st century. This component is defined as the spatial sea level anomaly due to ocean currents, wind stresses and local thermosteric and halosteric effects. Climate models from CMIP5 and CMIP6 show a large spread, as well as an increase between CMIP5 and CMIP6 North Sea ODSL projections. In this study, we apply linear regression models on CMIP5 and CMIP6 data to get a better understanding of the processes that influence ODSL change in the North Sea. We find that neither global surface air temperature nor global mean thermosteric sea level can reproduce ODSL projections based on a linear relation in CMIP6, whereas this was the case for CMIP5. Including the strength of the Atlantic meridional overturning circulation (AMOC) as an additional predictor enables us to reproduce long-term changes in ODSL for both ensembles. The sensitivity to the AMOC increased in CMIP6, which points to a difference in model dynamics between CMIP5 and CMIP6, and a more important role of the deep ocean. To investigate this further, we analyse mixed layer depth data in the North Atlantic. We find that models with a relatively deep mixed layer in the Greenland Sea over the period 1985–2004, project larger rise in ODSL in the North Sea for both CMIP5 and CMIP6. This implies that the location of deep water formation in the North Atlantic potentially influences ODSL in the North Sea. The number of these models increased from CMIP5 to CMIP6, again pointing to a different sensitivity to larger scale processes, potentially explaining the difference between the two ensembles.

Abstract The frequent typhoons in the northwest Pacific drive oceanic responses, for example, changes in sea surface temperature (SST) and chlorophyll‐a (Chl‐a). A composite analysis shows that SST (Chl‐a) around the typhoon center decreases (increases). The SST begins to decrease 3 days before the typhoon’s arrival and further decreases until 2 days following the typhoon’s passage. The Chl‐a increases rapidly after the typhoon’s arrival, and the maximum value is reached 3 days after the typhoon. Large oceanic responses are often associated with typhoons that have high wind speeds and slow translation speeds. From the perspective of the upper ocean structure, increasing changes in SST are identified with a shallower pre‐typhoon mixed layer depth (MLD). However, a significant dependence of the Chl‐a response on the pre‐typhoon MLD emerges only when the depth of typhoon‐induced mixing is greater than the pre‐typhoon MLD. This study helps to quantitatively describe typhoon‐induced changes with consideration of the determinant oceanic environment.

The spatial distribution and seasonal variation of near-inertial kinetic energy (NIKE) in the upper Ross Sea (RS) are examined using the 1/4° NEMO3.6-LIM3 sea ice physical–biological coupled model. The annual-mean surface and mixed layer-integrated NIKE have large values at the shelf break around Iselin Bank and the northeast area outside the continental shelf. The spatial distribution of the surface and mixed layer-integrated NIKE in the austral summer RS is mainly controlled by the near-inertial wind stress magnitude (NIWSM), which acts as the energy source. The northeast high NIKE agrees with the large NIWSM there. Semidiurnal tides only contribute to the NIKE peak around the Iselin Bank and shelf break. The change of mixed layer depth (MLD) can induce spatial discrepancies between surface and mixed layer-integrated NIKE. The shallower MLD in the western RS limits the near-inertial energy in a thin layer, which has induced the large surface NIKE there. The NIKE in the upper RS has a pronounced seasonal variation with the largest surface and mixed layer-integrated NIKE both observed during austral summer. The seasonal variation of NIWSM and sea ice concentration play an important role in inducing the seasonal variation of NIKE. While the NIWSM acts as the main energy source of NIKE, the presence of sea ice can prevent the wind from directly acting on the surface ocean, inducing the low NIKE during austral winter.

Climatologies of the mixed layer depth (MLD) have been provided using several definitions based on temperature/density thresholds or hybrid approaches. The upper ocean pycnocline (UOP) that sits below the mixed layer base remains poorly characterized, though this transition layer is an ubiquitous feature of the ocean surface layer. Available hydrographic profiles provide near-global coverage of the world’s ocean and are used to build a seasonal climatology of UOP properties – intensity, depth, thickness – to characterize the spatial and seasonal variations of upper ocean stratification. The largest stratification values O(10−3s−2) are found in the intertropical band, where seasonal variations of the UOP are also very small. The deepest (> 200 m) and least stratified O(10−6s−2) UOPs are found in winter along the Antarctic Circumpolar Current (ACC) and at high latitudes of the North Atlantic. The UOP thickness has a median value of 23 m with limited seasonal and spatial variations; only a few regions have UOP thicknesses exceeding 35 m. The UOP properties allow the characterization of the upper ocean restratification that generally occurs in early spring and is generally associated with large variability. Depending on the region, this restratification may happen gradually as around the Rockall plateau or abruptly as in the Kuroshio Extension. The UOP is also likely to merge intermittently with the permanent pycnocline in winter. The upper edge of the UOP is eventually close to MLD estimates, except in a few notable regions such as in the Pacific Warm Pool where barrier layers are important, and during wintertime at high latitudes of the North Pacific.

In this study, we mainly used in-situ observations from underwater gliders to analyze the ocean response in the northern South China Sea affected by Son-tinh (2018), Mandal et al. (2018) Mangkhut (2018)and Noul (2020). The results showed that these TCs caused 0.6 °C, 1.1 °C and 1.7 °C maximum sea surface temperature cooling respectively, which were weaker than general conditions because of long distance, weak intensity and fast movement speed. Net solar radiation, precipitation, 10-m wind and sea surface heat flux also made contribution in changes of SST. The mixed layer depth (MLD) became shallower after Son-Tinh and Noul passed through, while during Mangkhut it did not change significantly. After TCs passed through, the stratification around MLD became more obvious, with a banded distribution and stronger high-value areas of buoyancy frequency. Within 1 week after the shortest distance, the temperature and salinity responses in the upper ocean were stronger than those at the sea surface, and the gradients of temperature and salinity and their anomalies were more evident in the subsurface layer. The results of this study show that underwater glider observations are important for understanding oceanic responses to tropical cyclones and are useful for studying tropical cyclone activities. [ABSTRACT FROM AUTHOR]

Deviations of surface ocean dissolved oxygen (O 2) from equilibrium with the atmosphere should be rectified about twenty times more quickly than deviations of dissolved carbon dioxide (CO 2). Therefore, persistent O 2 disequilibria in the Labrador Sea, while CO 2 is close to equilibrium, has been a matter of interest to many previous works. Here we investigate this phenomenon by using a novel analytical technique, the 'CORS (Carbon Dioxide and Oxygen Relative to Saturation) method', and also by using more data than was available previously. We compare observations to results from a model we developed for the Labrador Sea which combines plankton ecology with biogeochemical cycling of oxygen, carbon and nitrogen. In contrast to earlier works which mostly considered individual factors in isolation, here we used the model, together with data, to distinguish between the varying influences of several processes potentially contributing to the long-lasting O 2 undersaturation: mixed layer depth, duration of mixed layer deepening, convection, entrainment and bottom water O 2 content. Our model experiments confirm that, for the same gas exchange rate, the effects on surface O 2 concentration differ significantly among the identified drivers. Our results suggest that prolonged surface O 2 undersaturation is not always dependent on the extreme winter mixed layer depths, but rather that even moderately deep mixed layers (e.g. 300 m), when prolonged and in conjunction with continuous entrainment of oxygen-depleted deep water, can also drive persistent surface O 2 anomalies. An implication of our results is that regions in the North Atlantic with maximum winter mixed layer depths of only a few hundred metres should also show persistent surface O 2 undersaturation. We further reveal that convection in deep water formation regions produces trendlines that do not pass through the origin of a plot of CO 2 vs. O 2 deviations which have previously been thought to indicate erroneous data. • Extreme winter mixed layer depths in the Labrador Sea drives its surface O 2 anomaly because of continuous entrainment of O 2 -poor deep waters. • Moderately shallow maximum winter mixing depths (e.g. 300 m) can also drive long-duration surface O 2 undersaturation. • Questionable autonomous data previously assumed to be erroneous may in fact be real. [ABSTRACT FROM AUTHOR]

Recent studies have suggested that there is a dynamic connection between the Atlantic Meridional Overturning Circulation (AMOC) and the North Atlantic subpolar gyre (SPG). This modeling study uses a fully coupled atmosphere–ocean–sea ice Earth system model to investigate the SPG dynamics throughout the past twenty-two thousand years. We found that the variations in the SPG and AMOC strength are synchronized. Consequently, during cold events in the Northern Hemisphere, the SPG strength declined simultaneously with the AMOC strength and with shallower mixed layer depths, which reduced the northward meridional heat transport and increased Atlantic sea ice coverage.

Oceanic general circulation models (OGCMs) are important tools used to investigate mechanisms for ocean climate variability and predict the ocean change in the future. However, in most current ocean models, the impact of sea surface waves as one of the most significant dynamic processes in the upper ocean is absent. In this study, the Stokes drift and the vertical mixing induced by nonbreaking surface waves derived from the wave model (WAVEWATCH III) are incorporated into a Climate System Ocean Model, and their effects on an ocean climate simulation are analyzed. Numerical experiments show that both physical processes can improve the simulation of sea surface temperature (SST) and mixed layer depth (MLD) in the Southern Hemisphere. The introduction of Stokes drift effectively reduces the subsurface warm bias in the equatorial tropics, which is caused by the weakening of vertical mixing in the equatorial region. The nonbreaking surface wave mainly reduces the temperature bias in the Southern Ocean by enhancing mixing in the upper ocean. For the MLD, the Stokes drift mainly improves the simulation of the winter MLD, and the nonbreaking surface wave improves the summer MLD. For MLD south of 40° S in summer, the introduction of nonbreaking surface waves resulted in a reduction of 11.86 m in MLD bias and 7.8 m in root mean square errors (RMSEs), respectively. For winter subtropical MLD in the Southern Hemisphere, considering the Stokes drift, the MLD bias and RMSEs were reduced by 2.49 and 5.39 m, respectively. Adding these two physical processes simultaneously provides the best simulation performance for the structure of the upper layer. The introduction of sea surface waves effectively modulates the vertical mixing of the upper ocean and then improves the simulation of the MLD. Thus, sea surface waves are very important for ocean simulation, so we will further couple a sea waves model in the Chinese Academy of Sciences Earth System Model (CAS-ESM) as part of their default model component.

The substantial productivity of the northern Norwegian Sea is closely related to its strong mesoscale eddy activity, but how eddies affect phytoplankton biomass levels in the upper ocean through horizontal and vertical transport-mixing has not been well quantified. To assess mesoscale eddy induced ocean surface chlorophyll-a concentration (CHL) anomalies and modulation of eddy-wind interactions in the region, we constructed composite averaged CHL and wind anomalies from 3,841 snapshots of anticyclonic eddies (ACEs) and 2,727 snapshots of cyclonic eddies (CEs) over the period 2000-2020 using satellite altimetry, scatterometry, and ocean color products. Results indicate that eddy pumping induces negative (positive) CHL anomalies within ACEs (CEs), while Ekman pumping caused by wind-eddy interactions induces positive (negative) CHL anomalies within ACEs (CEs). Eddy-induced Ekman upwelling plays a key role in the unusual positive CHL anomalies within the ACEs and results in the vertical transport of nutrients that stimulates phytoplankton growth and elevated productivity of the region. Seasonal shoaling of the mixed layer depth (MLD) results in greater irradiance levels available for phytoplankton growth, thereby promoting spring blooms, which in combination with strong eddy activity leads to large CHL anomalies in May and June. The combined processes of wind-eddy interactions and seasonal shallowing of MLD play a key role in generating surface CHL anomalies and is a major factor in the regulation of phytoplankton biomass in the northern Norwegian Sea.

Severe typhoon “In-Fa” passed through the northwestern region of East China Sea (ECS) in July 2021, affecting oceanic variables such as seawater temperature, salinity, and chlorophyll-a (Chl-a) concentration over the upwelling area. In this study, we analyzed the influence of the passage of typhoon “In-Fa” on the marine environment over the Upwelling Area off the Yangtze River Estuary (UAYRE) and the Upwelling Area of Zhoushan (UAZS). The results showed a significant decrease in sea surface temperature (SST) during the “In-Fa” typhoon, with maximum SST reductions of 2.98 °C in the UAYRE and 1.46 °C in the UAZS, which showed a “right bias” (indicating a greater cooling effect on the right side of the typhoon path compared to the left side). “In-Fa” influenced the temperature and salinity structure of the study areas and deepened the mixed layer depth (MLD). The MLD varied from the shallowest values of 2.02 m (18 July) to the deepest values of 19.4 m (26 July) in the UAYRE and from 2.43 m (18 July) to 16.79 m (25 July) in the UAZS. Furthermore, “In-Fa” led to an increase in sea surface Chl-a concentration, with a maximum Chl-a concentration enhancement of 285.58% (from 20 July to 28 July) in the UAYRE and 233.33% (from 20 July to 27 July) in the UAZS. The Ekman suction effect of “In-Fa” strengthened the upwelling, facilitating the transport of deep-sea nutrients to the upper ocean and providing favorable conditions for the growth of phytoplankton, thus benefiting the reproduction and survival of zooplankton, fish, and shrimp. This study contributes to understanding the mechanisms by which typhoons impact the ocean environment in upwelling area and provides valuable insights for the sustainable development of marine fisheries resources.

Warm blobs are persistent warm anomalies in the Northeast Pacific (NEP) upper ocean. Here, we assess the role of mixed layer depth (MLD) in their location and development based on ocean‐atmosphere reanalysis data. We find that warm blobs occur more frequently over 165°–130°W and 35°–50°N with shallow MLD. They are largely confined to the mixed layer, although substantial portions exist beneath it in summer when the MLD shoals. Based on a mixed‐layer heat budget analysis, we reveal that anomalous MLD and heat flux contribute dominantly to the surface heat flux term over the study area from May to July and from September to March, respectively, and have a large effect on warm blob development. Therefore, the MLD has important implications for the location and seasonal evolution of warm blobs and temperature diagnosis over the NEP. Plain Language Summary: Warm blobs are anomalous warm waters in the upper ocean of the Northeast Pacific (NEP). Oceanic mixed layer is the warm surface layer that maintains homogeneous temperature or density. This study aims to clarify the role of mixed layer depth (MLD) in the location and development of warm blobs. First, we determine a region of 165°–130°W and 35°–50°N in which warm blobs are most likely to occur due to strong sea surface temperature variability and shallow MLD. In this region, temperature anomalies of the warm blobs are largely confined to the mixed layer, although parts of the anomalies are beneath it when MLD shoals in summer. Then, we separate and quantify the relative contributions of MLD and surface heat flux anomaly to the development of the warm blobs from the surface heat flux term in a mixed‐layer heat budget. We find that the contribution of MLD anomaly is dominant from May to July, while that of surface heat flux anomaly (i.e., less ocean heat loss) is pronounced from September to the following March. Therefore, the effect of MLD on the location and seasonal evolution of the warm blobs gives implications for the temperature diagnosis over the NEP. Key Points: Northeast Pacific warm blobs appear more frequently in areas with a shallow mixed layerTemperature anomalies of warm blobs are not only largely confined to the mixed layer but also stored beneath the mixed layer in summerMixed layer depth anomalies are the dominant contributor to the temperature anomalies over the warm blob area from May to July [ABSTRACT FROM AUTHOR]