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The study of upper ocean mixing processes, including their dynamics and thermodynamics, has been a primary focus for oceanographers and meteorologists. Wave breaking in deep water is believed to play a significant role in these processes, affecting air–sea interactions and contributing to the energy dissipation of surface waves. This, in turn, enhances the transfer of gas, heat, and mass at the ocean surface. In this paper, we use the FVCOM-SWAVE coupled wave and current model, which is based on the MY-2.5 turbulent closure model, to examine the response of upper ocean turbulent kinetic energy (TKE) and temperature to various wave breaking parametric schemes. We propose a new parametric scheme for wave breaking energy at the sea surface, which is based on the correlation between breaking wave parameter R B and whitecap coverage. The impact of this new wave breaking parametric scheme on the upper ocean under typhoon conditions is analyzed by comparing it with the original parametric scheme that is primarily influenced by wave age. The wave field simulated by SWAVE was verified using Jason-3 satellite altimeter data, confirming the effectiveness of the simulation. The simulation results for upper ocean temperature were also validated using OISST data and Argo float observational data. Our findings indicate that, under the influence of Typhoon Nanmadol, both parametric schemes can transfer the energy of sea surface wave breaking into the seawater. The new wave breaking parameter R B scheme effectively enhances turbulent mixing at the ocean surface, leading to a decrease in sea surface temperature (SST) and an increase in mixed layer depth (MLD). This further improves upon the issue of uneven mixing of seawater at the air–sea interface in the MY-2.5 turbulent closure model. However, it is important to note that wave breaking under typhoon conditions is only one aspect of wave impact on ocean disturbances. Therefore, further research is needed to fully understand the impact of waves on upper ocean mixing, including the consideration of other wave mechanisms. [ABSTRACT FROM AUTHOR]

Ocean stratification on Arctic shelves critically influences nutrient availability, essential for primary production. However, discerning the changes in stratification and their drivers is challenging. Through the use of a highresolution ocean-sea-ice model, this study investigates the variability in stratification within the northeastern Chukchi Sea over the period from 1987 to 2016. Our analysis, validated against available observations, reveals that summers with weak stratification are marked by a warmer water column that features a saltier upper layer and a fresher lower layer, thereby diminishing the vertical density gradient. In contrast, summers with strong stratification are characterized by a cooler column with a fresher upper layer and a saltier lower layer, resulting in an increased density gradient. This variability is primarily driven by the timing of sea-ice retreat and the consequent variations in meltwater flux, with early retreat leading to less meltwater and saltier surface conditions. This factor significantly outweighs the influence of changes in circulation and associated lateral freshwater transport driven by the Bering Strait inflow. We also find that the synchronization of sea-ice retreat and Bering Strait inflow intensity is linked to the timing and strength of the Aleutian Low's westward shift from the Gulf of Alaska to the Aleutian Basin in the early winter. These insights are crucial for understanding nutrient dynamics and primary production in the region. Furthermore, monitoring sea-ice retreat timing could serve as a useful proxy for predicting subsequent summer stratification changes. [ABSTRACT FROM AUTHOR]

Ocean waves and Stokes drift are generated by typhoons. This study investigated the characteristics of ocean waves and wave-induced Stokes drift and their effects during Typhoon Mangkhut using European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 datasets and observational data. The results revealed that the typhoon generated intense cyclones and huge typhoon waves with a maximum wind speed of 45 m/s, a minimum pressure of 955 hPa, and a maximum significant wave height of 12 m. The Stokes drift caused by typhoon waves exceeded 0.6 m/s, the Stokes depth scale exceeded 18 m, and the maximum Stokes transport reached 6 m2/s. The spatial distribution of 10-m wind speed, typhoon wave height, Stokes drift, Stokes depth, and Stokes transport during the typhoon was highly correlated with the typhoon track. The distribution along the typhoon track showed significant zonal asymmetry, with greater intensity on the right side of the typhoon track than on the left side. These findings provide important insights into the impact of typhoons on ocean waves and Stokes drift, thus improving our understanding of the interactions between typhoons and the ocean environment. This study also investigated the contribution of Stokes transport to the total net transport during typhoons using Ekman-Stokes Numbers as a comparative measure. The results indicated that the ratio of Stokes transport to the total net transport reached up to 50% within the typhoon radius, while it was approximately 30% outside the radius. Strong Stokes transport induced by typhoon waves led to divergence in the transport direction, which resulted in upwelling of the lower ocean as a compensation current. Thus, Stokes transport played a crucial role in the vertical mixing of the ocean during typhoons. The findings suggested that Stokes transport should be paid more attention to, particularly in high latitude ocean regions, where strong winds can amplify its effects. [ABSTRACT FROM AUTHOR]

The non‐local model of mixing based on internal wave breaking, IDEMIX, is implemented as an enhancement of a turbulent kinetic energy closure model in three non‐eddy resolving general circulation ocean models that differ in the discretization and choice of computational grids. In IDEMIX internal wave energy is generated by an energy flux resulting from near‐inertial waves induced by wind forcing at the surface, and at the bottom, by an energy flux that parameterizes the transfer of energy between baroclinic and barotropic tides. In all model simulations with IDEMIX, the mixing work is increased compared to the reference solutions without IDEMIX, reaching values in better agreement with finestructure observations. Furthermore, the horizontal structure of the mixing work is more realistic as a consequence of the heterogeneous forcing functions. All models with IDEMIX simulate deeper thermocline depths related to stronger shallow overturning cells in the Indo‐Pacific. In the North Atlantic, deeper mixed layers in simulations with IDEMIX are associated with an increased Atlantic overturning circulation and an increase of northward heat transports toward more realistic values. The response of the deep Indo‐Pacific overturning circulation and the weak bottom cell of the Atlantic to the inclusion of IDEMIX is incoherent between the models, suggesting that additional unidentified processes and numerical mixing may confound the analysis. Applying different tidal forcing functions leads to simulation differences that are small compared to differences between the different models or between simulations with IDEMIX and without IDEMIX. Plain Language Summary: Waves in the ocean interior play a fundamental role for ocean dynamics since they can carry energy over long distances and, once they break, lead to turbulent mixing. This turbulent mixing can cause dense water masses to rise from the deep ocean with a direct impact on large‐scale currents. The wave dynamics occur on spatial scales that cannot be resolved in global ocean or climate models. To account for these processes, we apply the new parameterization IDEMIX that describes internal wave generation, propagation, and mixing. Using three different ocean models with and without IDEMIX ensures that we can identify model‐specific effects of the parameterization and discriminate them from those independent of the model. We find that the simulated mixing patterns agree better with observations once IDEMIX is applied. Large‐scale currents and the vertical temperature distribution are substantially affected by the internal wave parameterization. Whether this leads to an improved agreement with observed currents and water mass properties depends on the specific model and on numerical effects. In most cases, simulations with IDEMIX are not very sensitive to details of how the internal wave model is driven by tidal energy input. Key Points: the IDEMIX closure for a consistent representation of internal wave‐induced mixing is evaluated in three state‐of‐the‐art ocean modelsonly in simulations with IDEMIX can the models reproduce the magnitude and spatial variability of the observed mixing workmost changes with IDEMIX can be attributed to stronger mixing, but some effects are confounded by other processes and numerical mixing [ABSTRACT FROM AUTHOR]

Seasonal evolution of both surface signature and subsurface structure of a Mediterranean mesoscale anticyclones is assessed using the Coastal and Regional Ocean Community high‐resolution numerical model with realistic background stratification and fluxes. In good agreement with remote‐sensing and in‐situ observations, our numerical simulations capture the seasonal cycle of the anomalies induced by the anticyclone, both in the sea surface temperature (SST) and in the mixed layer depth (MLD). The eddy signature on the SST shifts from warm‐core in winter to cold‐core in summer, while the MLD deepens significantly in the core of the anticyclone in late winter. Our sensitivity analysis shows that the eddy SST anomaly can be accurately reproduced only if the vertical resolution is high enough (∼4 m in near surface) and if the atmospheric forcing contains high‐frequency. In summer with this configuration, the vertical mixing parameterized by the k − ϵ closure scheme is three times higher inside the eddy than outside the eddy, and leads to an anticyclonic cold core SST anomaly. This differential mixing is explained by near‐inertial waves, triggered by the high‐frequency atmospheric forcing. Near‐inertial waves propagate more energy inside the eddy because of the lower effective Coriolis parameter in the anticyclone core. On the other hand, eddy MLD anomaly appears more sensitive to horizontal resolution, and requires SST retroaction on air‐sea fluxes. These results detail the need of high frequency forcing, high vertical and horizontal resolutions to accurately reproduce the evolution of a mesoscale eddy. Plain Language Summary: Mesoscale eddies are turbulent structures present in every regions of the world ocean, and accounting for a significant part of its kinetic energy budget. These structures can be tracked in time and recently revealed a seasonal cycle from in situ data. An anticyclone (clockwise rotating eddy in the northern hemisphere) is observed in the Mediterranean to be predominantly warm at the surface and to deepen the mixed layer in winter, but shifts to a cold‐core summer signature. This seasonal signal is not yet understood and studied in ocean models. In this study we assess the realism of an anticyclone seasonal evolution in high resolution numerical simulations. Eddy surface temperature seasonal shift is retrieved and is linked to an increased mixing at the eddy core spontaneously appearing at high vertical resolution (vertical grid size smaller than 4 m) in the presence of high frequency atmospheric forcing. This increased mixed is due to the preferred propagation of near‐inertial waves in the anticyclone due to its negative relative vorticity. Eddy‐induced mixed layer depth anomalies also appear to be triggered by sea surface temperature retroaction on air‐sea fluxes. These results suggest that present‐day operational ocean forecast models are too coarse to accurately retrieve mesoscale evolution. Key Points: Enhanced mixing in anticyclones explain inverse eddy sea surface temperature (SST) signatureVertical resolution is crucial to model eddy core mixing triggered by near‐inertial wavesMixed layer anomaly is mainly driven by SST retroaction on air‐sea fluxes [ABSTRACT FROM AUTHOR]

Winter conditions in the NW Mediterranean cause instability of the water column and non‐geostrophic dynamics, such as vertical mixing and convection are significant. These events involve nutrient supply to the photic zone that can sustain high productivity. In this study, we aim to investigate the role of winter hydrodynamics on the spawning strategies of Sardina pilchardus and Micromesistius poutassou. Data were obtained on two oceanographic cruises (February 2017 and 2018) off the Catalan coast. The occurrence of S. pilchardus eggs very close to the coast indicated a clear preference of the species for spawning in coastal areas. Preflexion and postflexion larvae exhibited a slightly wider distribution showing a clear association with the cold, less saline and more productive coastal waters. Preflexion larvae of M. poutassou were found on the upper slope and over the shelf, being offshore limited by the shelf/slope front present all along the slope. The front would act as a barrier preventing their dispersion towards the open sea. M. poutassou larvae in advanced developmental stages were located close to the coast in the productive shelf waters, with instabilities of the front contributing to larval transport from offshore waters to the coast. The vertical distribution of both species showed high variability, not only related with the daily cycle or developmental stage, but also with the vertical structure of the water column. Overall, the results provide some clues on how the spawning strategies of both species may evolve under future scenarios of higher winter‐stratification, because of the global warming. [ABSTRACT FROM AUTHOR]

An indigenously designed High Energy Pulse LIDAR (HEPL) system was re-integrated and installed at ARIES, Nainital, for probing the atmosphere and to analyse the Mie scatterers. After the characterization of the Photomultiplier tube and Discriminator, about 50 quality checked valid measurements were investigated. Reasonably good signals were obtained upto ~2.5 km for 30 m altitude resolution, ~5 km for 150 m, and ~8 km for 300 m resolutions in the vertical, which were utilized to investigate the aerosol and cloud profiles. The system was able to capture the clouds at various altitudes under different atmospheric conditions. Elevated aerosol layers were also detected at about 3 and 3.5 km over the observation site. Such unique measurements from the data void complex Himalayan terrain highlight the need for continuous monitoring of the aerosol loading caused by transport of pollutants from continental locations and far-off distances that affect the air quality and Himalayan glaciers as well. [ABSTRACT FROM AUTHOR]

Abstract Seasonal evolution of both surface signature and subsurface structure of a Mediterranean mesoscale anticyclones is assessed using the Coastal and Regional Ocean Community high‐resolution numerical model with realistic background stratification and fluxes. In good agreement with remote‐sensing and in‐situ observations, our numerical simulations capture the seasonal cycle of the anomalies induced by the anticyclone, both in the sea surface temperature (SST) and in the mixed layer depth (MLD). The eddy signature on the SST shifts from warm‐core in winter to cold‐core in summer, while the MLD deepens significantly in the core of the anticyclone in late winter. Our sensitivity analysis shows that the eddy SST anomaly can be accurately reproduced only if the vertical resolution is high enough (∼4 m in near surface) and if the atmospheric forcing contains high‐frequency. In summer with this configuration, the vertical mixing parameterized by the k − ϵ closure scheme is three times higher inside the eddy than outside the eddy, and leads to an anticyclonic cold core SST anomaly. This differential mixing is explained by near‐inertial waves, triggered by the high‐frequency atmospheric forcing. Near‐inertial waves propagate more energy inside the eddy because of the lower effective Coriolis parameter in the anticyclone core. On the other hand, eddy MLD anomaly appears more sensitive to horizontal resolution, and requires SST retroaction on air‐sea fluxes. These results detail the need of high frequency forcing, high vertical and horizontal resolutions to accurately reproduce the evolution of a mesoscale eddy.

Abstract The non‐local model of mixing based on internal wave breaking, IDEMIX, is implemented as an enhancement of a turbulent kinetic energy closure model in three non‐eddy resolving general circulation ocean models that differ in the discretization and choice of computational grids. In IDEMIX internal wave energy is generated by an energy flux resulting from near‐inertial waves induced by wind forcing at the surface, and at the bottom, by an energy flux that parameterizes the transfer of energy between baroclinic and barotropic tides. In all model simulations with IDEMIX, the mixing work is increased compared to the reference solutions without IDEMIX, reaching values in better agreement with finestructure observations. Furthermore, the horizontal structure of the mixing work is more realistic as a consequence of the heterogeneous forcing functions. All models with IDEMIX simulate deeper thermocline depths related to stronger shallow overturning cells in the Indo‐Pacific. In the North Atlantic, deeper mixed layers in simulations with IDEMIX are associated with an increased Atlantic overturning circulation and an increase of northward heat transports toward more realistic values. The response of the deep Indo‐Pacific overturning circulation and the weak bottom cell of the Atlantic to the inclusion of IDEMIX is incoherent between the models, suggesting that additional unidentified processes and numerical mixing may confound the analysis. Applying different tidal forcing functions leads to simulation differences that are small compared to differences between the different models or between simulations with IDEMIX and without IDEMIX.

The study of upper ocean mixing processes, including their dynamics and thermodynamics, has been a primary focus for oceanographers and meteorologists. Wave breaking in deep water is believed to play a significant role in these processes, affecting air–sea interactions and contributing to the energy dissipation of surface waves. This, in turn, enhances the transfer of gas, heat, and mass at the ocean surface. In this paper, we use the FVCOM-SWAVE coupled wave and current model, which is based on the MY-2.5 turbulent closure model, to examine the response of upper ocean turbulent kinetic energy (TKE) and temperature to various wave breaking parametric schemes. We propose a new parametric scheme for wave breaking energy at the sea surface, which is based on the correlation between breaking wave parameter RB and whitecap coverage. The impact of this new wave breaking parametric scheme on the upper ocean under typhoon conditions is analyzed by comparing it with the original parametric scheme that is primarily influenced by wave age. The wave field simulated by SWAVE was verified using Jason-3 satellite altimeter data, confirming the effectiveness of the simulation. The simulation results for upper ocean temperature were also validated using OISST data and Argo float observational data. Our findings indicate that, under the influence of Typhoon Nanmadol, both parametric schemes can transfer the energy of sea surface wave breaking into the seawater. The new wave breaking parameter RB scheme effectively enhances turbulent mixing at the ocean surface, leading to a decrease in sea surface temperature (SST) and an increase in mixed layer depth (MLD). This further improves upon the issue of uneven mixing of seawater at the air–sea interface in the MY-2.5 turbulent closure model. However, it is important to note that wave breaking under typhoon conditions is only one aspect of wave impact on ocean disturbances. Therefore, further research is needed to fully understand the impact of waves on upper ocean mixing, including the consideration of other wave mechanisms.

Under‐ice phytoplankton "blooms" have been observed in the Southern Ocean, although irradiance is extremely low and vertical mixing is assumed to be deep. Most under‐ice data have been collected using Argo floats, as research expeditions during austral fall and winter are limited. Hydrographic measurements under dense ice cover indicate that vertical mixing in weakly stratified systems may be less than previously suggested, and that the accepted determinations of mixed layer depths are inappropriate in regions with extremely weak stratification, such as those under ice. Vertical gradients in density suggest that mixed layers in the Ross Sea in early October are not extremely deep; furthermore, while phytoplankton biomass is low, it has begun to accumulate under ice. Growth rates indicate that phytoplankton growth in the Ross Sea begins in early September. Extending the period of growth may have substantial impacts on carbon biogeochemistry and food web energetics in ice‐covered waters. Plain Language Summary: Data from profiling floats under Southern Ocean ice indicate that some growth of plankton occurs there, even though available irradiance is extremely low and vertical mixing is deep. Few research expeditions during austral fall and winter occur due to the difficulty of sampling and penetrating deep into ice. Estimates of vertical mixing in October from the Ross Sea suggest that the accepted criterion of mixed layer depths are inappropriate in regions with extremely weak stratification, such as those under ice. Mixed layers in the Ross Sea in early October are not extremely deep; furthermore, while phytoplankton biomass is low, it has begun to accumulate under ice. Growth rates indicate that the onset of phytoplankton growth in the Ross Sea likely occurs by early September, soon after positive irradiance occurs. Extending the period of growth may have substantial impacts on carbon biogeochemistry and food web energetics in ice‐covered waters. Key Points: Mixed layer depths under extremely weak vertical stratification (like under ice) are overestimated using conventional criteriaPhytoplankton growth in the Ross Sea is initiated soon after the start of solar day, far earlier than has been previously suggestedEarly spring growth can potentially alter our views of Southern Ocean carbon biogeochemistry and food web phenology [ABSTRACT FROM AUTHOR]

The Luzon Strait (LS) is a key region for estimating carbon sources and sinks in the South China Sea (SCS) and is highly influenced by the Kuroshio Current (KC) and typhoons. Understanding the variations in the sea surface partial pressure of carbon dioxide (pCO2-sw) under the combined effects of typhoons and KC in this region is crucial for estimating local and regional changes in ocean carbon flux. Based on valuable in situ pCO2-sw and remote sensing data, this study aimed to reveal the temporal variations and the physical mechanisms of pCO2-sw variations under the comprehensive effects of both typhoons and Kuroshio Intrusion (KI) in the LS. One week after the passage of the tropical cyclone (TC) Nanmadol, the concentration in the pCO2-sw and the influencing mechanisms varied in three different regions (W1–W3) on Transect A (120°E). In the region dominated by SCS waters (W1), the average pCO2-sw increased by 5.1 μatm after TC, which was mainly due to the TC "Wind Pump" inducing strong vertical mixing, which brought dissolved inorganic carbon (DIC)-rich deeper water up to the surface. In the region affected by KC (W2 and W3), pCO2-sw decreased after the TC (−8.2 μatm and −1.8 μatm, respectively) with TC-enhanced KI because the invasion of lower pCO2-sw of Kuroshio waters inhibited the TC-induced upwelling. More significant TC-induced upwelling (W3) would alleviate the decrease in pCO2-sw caused by the TC-enhanced KI. This study is a rare case providing a better understanding of the variations in pCO2-sw under TC-enhanced KI, which provides support for regional climate change prediction and carbon flux estimation in the western boundary current regions. [ABSTRACT FROM AUTHOR]

The lack of adequate vertical mixing is one of the factors limiting the productivity of open raceway microalgae reactors. The existence of large gradients of light involves the cells being mainly adapted to local irradiance instead of average irradiance, which would allow for maximizing the light utilization efficiency, thus maximizing the biomass productivity of microalgae cultures. To overcome this problem, different alternatives have been proposed, one of the more suitable being the utilization of airfoils to improve vertical mixing. In this work, numerical and experimental studies were performed to analyse the effect of the aerodynamic airfoils patented by the University of Seville (WO2020120818A1). The goal is to improve the photosynthetic efficiency, but also a better understanding of the light regime to which the microalgae cells are exposed in these systems and how to improve it. Computational Fluid Dynamics (CFD) was used to optimize the flow generated by the airfoils. A dynamic photosynthesis model of Rubio Camacho et al. (Biotechnol Bioeng 81:459–473, 2003) was used to estimate the photosynthesis rate as a function of the light regime to which the cells are exposed, including photo-adaptation and photo-inhibition phenomena, the results confirm that the use of airfoils improves the vertical mixing and the photosynthesis rate. The photosynthetic benefits were observed 10 m downstream of the airfoils, resulting in an increase in photosynthesis rate and productivity by up to 30%. These results confirm the benefits of an increase in mixing in microalgae cultures, especially when focusing on the movement of the cells between the different illuminated zones while maintaining low energy consumption and capital expenses. [ABSTRACT FROM AUTHOR]

ABSTRACTViruses infect and kill prokaryotic populations in a density- or frequency-dependent manner and affect carbon cycling. However, the effects of the stratification transition, including the stratified and de-stratified periods, on the changes in prokaryotic and viral communities and their interactions remain unclear. We conducted a monthly survey of the surface and deep layers of a large and deep freshwater lake (Lake Biwa, Japan) for a year and analyzed the prokaryotic production and prokaryotic and viral community composition. Our analysis revealed that, in the surface layer, 19 prokaryotic species, accounting for approximately 40% of the total prokaryotic abundance, could potentially contribute to the majority of prokaryotic production, which is the highest during the summer and is suppressed by viruses. This suggests that a small fraction of prokaryotes and phages were the key infection pairs during the peak period of prokaryotic activity in the freshwater lake. We also found that approximately 50% of the dominant prokaryotic and viral species in the deep layer were present throughout the study period. This suggests that the “kill the winner” model could explain the viral impact on prokaryotes in the surface layer, but other dynamics may be at play in the deep layer. Furthermore, we found that annual vertical mixing could result in a similar rate of community change between the surface and deep layers. These findings may be valuable in understanding how communities and the interaction among them change when freshwater lake stratification is affected by global warming in the future.IMPORTANCEViral infection associated with prokaryotic production occurs in a density- or frequency-dependent manner and regulates the prokaryotic community. Stratification transition and annual vertical mixing in freshwater lakes are known to affect the prokaryotic community and the interaction between prokaryotes and viruses. By pairing measurements of virome analysis and prokaryotic production of a 1-year survey of the depths of surface and deep layers, we revealed (i) the prokaryotic infection pairs associated with prokaryotic production and (ii) the reset in prokaryotic and viral communities through annual vertical mixing in a freshwater lake. Our results provide a basis for future work into changes in stratification that may impact the biogeochemical cycling in freshwater lakes.

Abstract Under‐ice phytoplankton “blooms” have been observed in the Southern Ocean, although irradiance is extremely low and vertical mixing is assumed to be deep. Most under‐ice data have been collected using Argo floats, as research expeditions during austral fall and winter are limited. Hydrographic measurements under dense ice cover indicate that vertical mixing in weakly stratified systems may be less than previously suggested, and that the accepted determinations of mixed layer depths are inappropriate in regions with extremely weak stratification, such as those under ice. Vertical gradients in density suggest that mixed layers in the Ross Sea in early October are not extremely deep; furthermore, while phytoplankton biomass is low, it has begun to accumulate under ice. Growth rates indicate that phytoplankton growth in the Ross Sea begins in early September. Extending the period of growth may have substantial impacts on carbon biogeochemistry and food web energetics in ice‐covered waters.

Selective withdrawal from reservoirs is an effective strategy for balancing economic goals and environmental demands. Multilevel intake operation (MIO) is one of the main options for improving the thermal regime of deep reservoirs. In this study, a combination of measured data and a two-dimensional hydrodynamic model (CE-QUAL-W2) was utilized to analyze the impact of MIO processes on the thermal stability of a reservoir and the improvement effect of the withdrawal water temperature (WWT). The purpose of this study was to identify an optimal selective withdrawal strategy to meet the water temperature needs of fish downstream during their spawning and breeding activities. The results showed that the MIOs raised the withdrawal water intake position (8.3–11.2 m) for the WWT. Meanwhile, MIOs resulted in statistically significant changes in the water temperature structure of the reservoir (p

Vertical mixing parameterizations in ocean models are formulated on the basis of the physical principles that govern turbulent mixing. However, many parameterizations include ad hoc components that are not well constrained by theory or data. One such component is the eddy diffusivity model, where vertical turbulent fluxes of a quantity are parameterized from a variable eddy diffusion coefficient and the mean vertical gradient of the quantity. In this work, we improve a parameterization of vertical mixing in the ocean surface boundary layer by enhancing its eddy diffusivity model using data‐driven methods, specifically neural networks. The neural networks are designed to take extrinsic and intrinsic forcing parameters as input to predict the eddy diffusivity profile and are trained using output data from a second moment closure turbulent mixing scheme. The modified vertical mixing scheme predicts the eddy diffusivity profile through online inference of neural networks and maintains the conservation principles of the standard ocean model equations, which is particularly important for its targeted use in climate simulations. We describe the development and stable implementation of neural networks in an ocean general circulation model and demonstrate that the enhanced scheme outperforms its predecessor by reducing biases in the mixed‐layer depth and upper ocean stratification. Our results demonstrate the potential for data‐driven physics‐aware parameterizations to improve global climate models. Plain Language Summary: The upper region of the ocean is highly energetic and is responsible for transferring mass, energy and biogeochemical tracers between the atmosphere and the deeper regions of the ocean. This transport takes place because of turbulent swirling motions, which are found to be of varying sizes. Climate models cannot represent all of these motions because smaller‐scale swirls are complex and require additional computational resources. As we cannot neglect those small swirls, we try to approximate their effects on larger‐scale motions using mathematical models. These models have a few ad hoc or empirical assumptions that lead to uncertainty when these climate models are used to project the future climate. To reduce this uncertainty, we augment an existing model of turbulent swirling process with machine learning, which replaces some ad hoc approximations with data‐driven neural networks. Neural networks can learn those missing processes more accurately than a traditional physics‐based model. The neural networks are shown to improve physics in climate simulations. Although we only touch on one component in an ocean climate model, this approach can be replicated to improve any other component that was using ad hoc assumptions and replace them with data‐driven models using techniques from machine learning. Key Points: We improve a parameterization of vertical mixing in the ocean surface boundary layer using neural networksNeural networks are trained to predict the diffusivity of second moment closure and maintain energetic constraints of the original parameterizationThe improved scheme reduces biases of mixed layer depth and thermocline in an atmospherically forced ocean model [ABSTRACT FROM AUTHOR]

In the western subarctic Pacific (WSP), iron (Fe) can enhance the biological carbon pump by stimulating phytoplankton photosynthesis. However, little is known about how Fe and light availability controls the phytoplankton photophysiology in the WSP near the Kuril Islands, where water mixing is sometimes enhanced, even in summer. Here, we conducted on-deck Fe-manipulated incubation experiments at two stations where surface mixed layer depths were distinct but showed similar initial macronutrient and dissolved Fe concentrations and phytoplankton community composition even experienced light availability for phytoplankton in the water column could be different. An Fe addition to water samples from the deeper surface-mixed layer enhanced the phytoplankton biomass and the photosynthetic competence in photosystem II (Fv/Fm), suggesting an Fe-light co-limitation, which was supported by the results from high functional absorption cross section of photosystem II (σPSII) and low light saturation index (Ek). At the shallower mixed layer station, Fe amendment did not stimulate phytoplankton biomass and photosynthesis, indicating that Fe was sufficient for the phytoplankton. Although the centric diatom Chaetoceros species were predominant at both stations throughout incubation, the pennate diatoms Pseudo-nitzschia spp. and Cylindrotheca closterium significantly increased at the deeper and shallower-mixed layer stations, respectively, after Fe enrichment. The elongation of the pennate diatom Neodenticula seminae chains was observed under low Fe availability caused by a chelator. This could be related to the fact that N. seminae is abundant in sediment trap samples from the WSP under Fe-limited conditions and contributed to the high biological pump efficiency. [ABSTRACT FROM AUTHOR]

Regional transport and vertical mixing are important for aerosol pollution of megacities, but their roles are often challenging to assess via ground observations. In this study, we measured aerosol chemical composition simultaneously above urban canopy (Shanghai Tower, 609 m), a site representative of aerosols from regional scale, and a nearby ground site and investigated the roles of regional transport and vertical mixing over 2020 COVID‐19 lockdown period (1 January to 18 April), when local emissions were first drastically reduced and then recovered. During the lockdown period, regional transport was the major source of most aerosol species (organics, NO3−, SO42−, and NH4+) at both heights according to the high correlation coefficients (R = 0.81–0.87) between both heights. Correlation coefficients and vertical ratios (609 m/ground) of most aerosol components showed similar diurnal variations with the evolution of planetary boundary layer height, indicating the role of vertical mixing in aerosol pollution. Moreover, the concentrations of aerosol components at 609 m generally preceded those at ground level by 1–2 hr, indicating that aerosols were first transported at upper boundary layer, and then were mixed downwards to ground level. At 609 m, highly oxidized oxygenated organic aerosol (OOA; a surrogate of secondary organic aerosol (SOA)) dominated in organic aerosol (≥75%). The high correlations (R = 0.96) between OOA and hydrocarbon‐like organic aerosol (HOA; a surrogate of primary organic aerosol (POA)) at 609 m indicated that they originated similarly from regional transport. This study highlights the importance of regional joint prevention and control of pollutant emissions and observation above urban canopy. Plain Language Summary: Aerosol pollution in megacities is mainly due to local anthropogenic emissions and regional transport from surrounding areas. Based on ground observations and model simulations, previous studies have found that regional transport was the main source of aerosols in Shanghai when local emissions were limited during the COVID‐19 lockdown in 2020, but lacking direct observational evidence at height above urban canopy and the role of planetary boundary layer dynamics. By combining the observations of aerosol chemical composition at a site of Shanghai Tower (609 m above ground level) and a nearby ground site, this study points out the roles of regional transport and vertical mixing within the boundary layer in the occurrence of aerosol pollution in Shanghai during COVID‐19 in 2020. Specifically, most aerosol species are transported mainly from northern surrounding areas and East China Sea to Shanghai urban canopy, and the subsequent vertical mixing process along with the development of boundary layer contributes to aerosols at ground level. Moreover, both secondary and primary organic aerosols are transported from regional scale which could age during transport. We highlight that combination of observations at different heights will facilitate analyzing the roles of regional transport and boundary layer mixing in aerosol pollution. Key Points: Regional transport is the major source of aerosols in Shanghai during the COVID‐19 lockdown in 2020Downward vertical mixing within planetary boundary layer coupled with regional transport leads to aerosol pollution observed on the groundOrganic aerosol at 609 m is mostly constituted by SOA (≥75%) and both the SOA and POA are mainly from regional source [ABSTRACT FROM AUTHOR]

Large herbivores have been described as agents of change in terrestrial habitats. Their effect on aquatic ecosystems are, however, underexplored. We raise the question of whether elephants and hippopotamus have the potential to significantly alter limnological properties and, indirectly, primary and secondary productivity within small and shallow freshwaters in arid and semi‐arid African landscapes. In this note we discuss hypothetical means by which elephants and hippopotamus alter shallow freshwater bodies. We further assimilate this with known ways by which these mega‐herbivores alter aquatic environments giving an overview of their potential functional role in structuring aquatic habitats. [ABSTRACT FROM AUTHOR]

The North Atlantic gyre experiences both a significant temperature rise at high latitudes and a considerable weakening of the geostrophic component of the Gulf Stream, which is reflected by the 64-year fundamental gyral Rossby wave (GRW). This singular behavior compared to the South Atlantic and South Indian Ocean gyres highlights a feedback loop of Arctic ice sheet melting on mid-latitude Atlantic Ocean temperature. The warming of the northern oceanic gyre at high latitudes due to the retreat of Arctic ice sheet via the Labrador Current decreases the thermal gradient between the high and low latitudes of the north Atlantic gyre. This results in a weakening of the geostrophic forces at the basin scale and a reduction in the amplitude of the GRWs. Reducing the amplitude of the variation of the upward and downward movement of the pycnocline modifies air–sea interactions, weakening vertical mixing as well as the evaporation processes and the departure of latent heat when the pycnocline rises. The resulting thermal anomaly stretching along the Gulf Stream from where it leaves the American continent is partly transferred to the Arctic sea ice via the drift current and thermohaline circulation, which contributes to the retreat of the ice sheet, and the closing of the feedback loop. The 64-year-period GRW should disappear around 2050 if its damping continues linearly, favoring an increasingly rapid warming of the ocean at mid-latitudes. These interactions are less acute in the southern hemisphere due to the circumpolar current. [ABSTRACT FROM AUTHOR]

Abstract Vertical mixing parameterizations in ocean models are formulated on the basis of the physical principles that govern turbulent mixing. However, many parameterizations include ad hoc components that are not well constrained by theory or data. One such component is the eddy diffusivity model, where vertical turbulent fluxes of a quantity are parameterized from a variable eddy diffusion coefficient and the mean vertical gradient of the quantity. In this work, we improve a parameterization of vertical mixing in the ocean surface boundary layer by enhancing its eddy diffusivity model using data‐driven methods, specifically neural networks. The neural networks are designed to take extrinsic and intrinsic forcing parameters as input to predict the eddy diffusivity profile and are trained using output data from a second moment closure turbulent mixing scheme. The modified vertical mixing scheme predicts the eddy diffusivity profile through online inference of neural networks and maintains the conservation principles of the standard ocean model equations, which is particularly important for its targeted use in climate simulations. We describe the development and stable implementation of neural networks in an ocean general circulation model and demonstrate that the enhanced scheme outperforms its predecessor by reducing biases in the mixed‐layer depth and upper ocean stratification. Our results demonstrate the potential for data‐driven physics‐aware parameterizations to improve global climate models.

Sea surface temperature (SST) cooling is a typical ocean response to tropical cyclones (TCs). Previous studies have shown that TC-induced SST cooling is influenced by the TC characteristics, ocean stratification, and background vorticity. However, whether internal tides (ITs) affect TC-induced SST cooling remains unclear. Based on a series of numerical simulations in the South China Sea, the influence of ITs on TC-induced SST cooling is explored in this study. Results indicate that ITs can enhance TC-induced SST cooling by 0.2 − 0.4 °C in the Luzon Strait. Temperature budget analysis shows that ITs modulate the SST cooling mainly through changing the vertical mixing and advection. In the Luzon Strait, ITs enhance the vertical mixing but suppress the vertical advection. Because the contribution of vertical mixing to SST cooling is one order of magnitude higher than that of vertical advection, ITs enhance TC-induced SST cooling in the Luzon Strait mainly through enhancing the vertical mixing. Moreover, the enhancements of vertical mixing and SST cooling induced by ITs are found to be related to the TC characteristics. One reason is that in addition to ITs, the high-frequency waves generated by the interaction between TC-induced near-inertial waves and ITs also contribute to the shear and hence the turbulent mixing. [ABSTRACT FROM AUTHOR]

In this study, a regional modelling approach is adopted using the Modular Ocean Model version 5 (MOM5) with implementation of open boundary condition (OBC) in the Bay of Bengal (BoB) region. The domain is given an initial stratification profile of climatological temperature and salinity and is forced using seasonally varying wind and air-sea fluxes at the surface. A sensitivity analysis is performed by changing the vertical mixing schemes. Changes in vertical mixing schemes have significant impact on the ocean system, including the surface properties. Two different vertical mixing models are used: K-profile parameterization (KPP), and k- ∈ turbulence model. Inter-comparison between these two experiments suggests that k- ∈ model performs seemingly better than the KPP model. With k- ∈ mixing scheme, the simulated sea surface temperature (SST) and mixed layer depth (MLD) are closer to observations. Model generated turbulent kinetic energy (k) and dissipation ( ∈) are different in the two experiments because of different representations of k and ∈ in the two models. The results show the importance of regional modelling and the use of different mixing schemes. Employing a physics-based vertical mixing scheme, such as the k- ∈ turbulence model, shows improvement in the model physics and the model simulated ocean parameters. [ABSTRACT FROM AUTHOR]

Seasonal nearshore-offshore exchanges by coastal upwelling events in large lakes can play a significant role in nearshore nutrient dynamics, affecting lake productivity and water quality. We analyzed field observations along the north shore of Lake Ontario, collected in the summer of 2018, focusing on the littoral zone and specifically the Cladophora habitat zone (<15 m), to investigate episodic enhanced vertical mixing by coastal up/downwelling events. Vertical turbulent diffusivity (K z) based on the buoyancy Reynolds number above the metalimnion layer during downwelling events was generally higher than those below the metalimnion layer during upwelling events; while K z at the metalimnion layer can increase by ∼ two orders of magnitude during upwelling events. Our results suggest that K z based on the Richardson number parameterization, which only accounts for large-scale current shear and stability, and incorporates an adjustable parameter is ∼ ten times higher than the K z based on the buoyancy Reynolds number. Analysis of historical wind records indicates that the frequency of coastal upwelling favorable winds on the north shore of Lake Ontario has increased by > 45% over the last thirty years - suggesting an increasing trend of nearshore-offshore nutrient exchanges as a contributing factor for the nearshore water quality management. [ABSTRACT FROM AUTHOR]

The Lofoten Vortex (LV) is a quasi-permanent anticyclonic eddy with the characteristic of periodic regeneration in the Lofoten Basin (LB), which is one of the major areas of deep vertical mixing in the Nordic Sea. Our analysis of the LV contributes to our understanding of the variations in convective mixing in the LB. Based on drifter data and satellite altimeter data, the climatological results show that the LV has the sea surface characteristics of relative stability in terms of its spatial position and significant seasonal variations in its physical characteristics. Combined with the temperature and salinity data of Argo profiles, the vertical structures of the LV are presented here in terms of their spatial distribution and monthly variations. The wavelet analysis of the satellite sea surface temperature (SST) data shows that the period of SST anomaly (SSTA) in the LV sea area is 8–16 years. In the stage marked by a decreasing (increasing) trend of SSTA, the vertical mixing is strengthened (weakened). Current vertical mixing is clearly revealed by the Argo profiles, and the SSTA shows a significant impact of cooling. However, against a background of warming and freshening, this vertical mixing will be greatly weakened in the next increasing trending stage of the SSTA. [ABSTRACT FROM AUTHOR]

Vertical transport caused by mixing is essential for understanding physical processes in lakes. However, mixing processes in shallow lakes are not well understood because of the lack of turbulence measurements. This study presents observations of vertical mixing and oxygen flux in a shallow lake, Lake Kitaura of the Lake Kasumigaura continuous lake system, which is located along the central eastern coast of the Japanese mainland. Mooring and microstructure surveys were conducted in August 2020. The vertical eddy diffusivity was estimated from the Ellison scale using high-frequency sampled temperature data from the mooring location, and the estimations were consistent with those observed from the microstructure profiler. The estimated eddy diffusivity revealed a mixing structure and oxygen flux in the lake during the study period. The daily cycle of stratification and mixing was caused by daily heating due to solar radiation and by winds from daily sea breezes, respectively. The daily stratification maximum occurred around noon, which suppressed vertical mixing. Vertical mixing was intensified due to sea breezes in the afternoon, which led to movement of oxygen from the surface layer to the bottom layer. The maximum vertical mixing was observed at 18:00. The oxygen concentration did not increase during nighttime when the negative surface heat flux was observed; thus, nighttime cooling may not primarily contribute to the vertical oxygen supply. [ABSTRACT FROM AUTHOR]

In recent years, a decrease in dissolved oxygen in the lake bottom has been observed in the northern part of Lake Biwa, the largest freshwater lake in Japan. The main physical controlling factor is considered to be changes in stratification, which are influenced by weather and climatic conditions. As atmospheric temperature increases, lake surface temperature rises, and stratification is formed. The strength of stratification has a significant impact on the transport of various material cycles, such as dissolved oxygen and nutrients, from the lake surface to the bottom layer. Therefore, in this study, the analysis of the effect of strong winds during the stratification collapse period (autumn) on the stratification structure of Lake Biwa was performed. Baseline simulations using actual meteorological data and experimental simulations using meteorological data with modified wind speed and direction were conducted. The numerical experiments showed that collapse of stratification was more likely to occur in autumn than in summer, and vertical mixing was enhanced. When strong winds blew more in one direction, vertical mixing was further enhanced when the duration of strong winds was longer during periods of weak stratification, as well as the results during the stratification period. When winds of the same magnitude as during the stratification period blew during periods of decreasing water temperature, a decrease in bottom water temperature was observed prior to the full-layer circulation in the following year. This suggested that strong winds during the decreasing temperature period had a strong influence on the water temperature environment next year. [ABSTRACT FROM AUTHOR]

Convective clouds are important for both the vertical redistribution of tropospheric trace gases and the removal, via microphysical scavenging, of soluble trace gas precursors of ozone. We investigate wet scavenging of formaldehyde (CH2O), hydrogen peroxide (H2O2), and methyl hydrogen peroxide (CH3OOH) in quasi‐marine and land convective storms over Texas, USA observed on 18 September 2013 during the 2013 SEAC4RS campaign. Cloud‐resolving simulations using the Weather Research and Forecasting model with Chemistry (WRF‐Chem) were performed to understand the effect of entrainment, scavenging efficiency (SE), and ice physics processes on these trace gases with varying solubility. While marine and land convection can have distinctly different microphysical properties, we did not find significant differences in the SEs of CH2O, H2O2, or CH3OOH. The SEs of 44%–53% for CH2O and 85%–90% for H2O2 are consistent with our previous studies from SEAC4RS and the 2012 DC3 field experiment storms. Using WRF‐Chem simulations, the ice retention factor (rf) for CH2O was determined to be 0.5–0.9, which is higher than found in previous studies. We show that the CH2O rf is higher in airmass and multicell storms than in severe storms and hypothesize that ice shattering may affect CH2O rf values. The CH3OOH SEs (39%–73%) were higher than expected from Henry's Law equilibrium. While recent studies suggest that CH3OOH measurements have interference due to methane diol, we find that this interference cannot fully explain the higher‐than‐expected CH3OOH SEs determined here and during DC3, suggesting further research is needed to understand CH3OOH vertical redistribution. Plain Language Summary: Thunderstorms are important mechanisms for the transport and removal of trace gases in the atmosphere. This study investigates the transport and removal processes of tropospheric ozone trace gas precursors in marine and land convective storms over Texas, USA. Numerical simulations were performed to understand storm processes affecting atmospheric gases with different solubilities. While marine and land storms can have distinctly different storm structure and updraft winds, we did not find significant differences in the amount of gases removed by these storms. The results show that the efficiency of the storm in removing gas from the atmosphere is consistent with our previous findings for moderate to severe storms over the US. In addition, the analysis showed when cloud drops freeze onto ice and snow particles that formaldehyde had more retention in the frozen particle in moderate storms compared to severe storms suggesting that there are many processes affecting this key ozone precursor in deep convection. The more soluble hydrogen peroxide did not show the same ice retention behavior because it is removed by precipitation in the warmer, liquid‐only region of the storm. Key Points: Although marine and land convection have different microphysical properties, there are no relevant differences in the scavenging efficiencyWRF‐Chem simulations analysis shows ice retention factor for formaldehyde is higher than found in previous studiesMethyl hydrogen peroxide scavenging efficiencies are higher than expected from Henry's Law, even with potential measurement interference [ABSTRACT FROM AUTHOR]

Transport processes in coastal oceans are important for understanding mass transport between land and ocean. Investigations of mixing processes are essential to reveal the transport processes. This paper presents mixing processes due to internal tides and the associated mass transport, on the basis of field observations and numerical simulations conducted by the author who is a recipient of Okada Prize from the Oceanographic Society of Japan, 2022. The Yoing Ocean Data Acquisition (YODA) Profiler, has been developed to observe small scale dynamics and mixing processes in coastal oceans. Results from the profiler revealed mixing in river plumes and breaking of nonlinear internal tides on a shallow slope. Breaking of internal tides results in strong turbulent mixing, sediment resuspension and generation of intermediate nepheloid layers in Otsuchi Bay, Japan. It was found that vertical mixing was enhanced by a collision of a receding internal tide and a subsequent run-up nonlinear internal bore on a gentle slope. In addition, numerical models were developed to investigate internal tides breaking. Large scale oceanic numerical models showed enhanced diurnal internal tides caused by a resonance of Kelvin waves around islands over the Izu-Ogasawara Ridge and strongly enhanced the internal tide energy flux toward the upstream of the Kuroshio owing to an interaction of the Kuroshio and tides. This study also applies methods established in oceanic studies to analysis of mixing processes in a lake. [ABSTRACT FROM AUTHOR]

This study aimed to evaluate the seasonal variability in surface heat content in the Arabian Gulf (AG) based on hydrographic data. The ocean heat content (OHC) was initially estimated from surface to maximum depth (75 m) to show the seasonal variability, where the seasonal temperature reaches to that depth. Then OHC was re-estimated from the surface to a depth of 35 m, which represents the average depth of AG, to obtain accurate horizontal distributions. Results showed that during winter, the northern part of AG experiences the lowest OHC compared to the southeastern part. The monthly spatial average implies that the highest OHC of AG water was in September and October, while the lowest heat content was found in February and March. However, the OHC horizontal distributions were almost the same for the entire gulf during summer. In general, there was increasing in the OHC in the southeast region of the gulf. OHC anomalies are concentrated in the northern region of the AG, while the southeastern part near the Strait of Hormuz has the lowest values. Regarding heat flux, the highest heat gains were during spring, while the highest loss was in autumn. The water exchange between the AG and the Indian Ocean through the Strait of Hormuz may play a major role in the seasonal variability in OHC. [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]

High biological productivity and the efficient export of carbon-enriched subsurface waters to the open ocean via the continental shelf pump mechanism make mid-latitude continental shelves like the northwest European shelf (NWES) significant sinks for atmospheric CO2. Tidal forcing, as one of the regionally dominant physical forcing mechanisms, regulates the mixing-stratification status of the water column that acts as a major control for biological productivity on the NWES. Because of the complexity of the shelf system and the spatial heterogeneity of tidal impacts, there still are large knowledge gaps on the role of tides for the magnitude and variability of biological carbon fixation on the NWES. In our study, we utilize the flexible cross-scale modeling capabilities of the novel coupled hydrodynamic–biogeochemical modeling system SCHISM–ECOSMO to quantify the tidal impacts on primary production on the NWES. We assess the impact of both the barotropic tide and the kilometrical-scale internal tide field explicitly resolved in this study by comparing simulated hindcasts with and without tidal forcing. Our results suggest that tidal forcing increases biological productivity on the NWES and that around 16% (14.47 Mt C) of annual mean primary production on the shelf is related to tidal forcing. Vertical mixing of nutrients by the barotropic tide particularly invigorates primary production in tidal frontal regions, whereas resuspension and mixing of particulate organic matter by tides locally hinders primary production in shallow permanently mixed regions. The tidal impact on primary production is generally low in deep central and outer shelf areas except for the southwestern Celtic Sea, where tidal forcing substantially increases annual mean primary production by 25% (1.53 Mt C). Tide-generated vertical mixing of nutrients across the pycnocline, largely attributed to the internal tide field, explains one-fifth of the tidal response of summer NPP in the southwestern Celtic Sea. Our results therefore suggest that the tidal NPP response in the southwestern Celtic Sea is caused by a combination of processes likely including tide-induced lateral on-shelf transport of nutrients. The tidally enhanced turbulent mixing of nutrients fuels new production in the seasonally stratified parts of the NWES, which may impact the air–sea CO2 exchange on the shelf.

The physical and biogeochemical functioning of eastern boundary upwelling systems is generally understood within the context of the upwelling - relaxation cycle, driven by sub-diurnal wind variability (i.e. with a time-scale of greater than a day). Here, we employ a realistically configured and validated 3D model of the southern Benguela upwelling system to quantify the impact of super-diurnal winds associated with the land-sea breeze (LSB). The ocean response to the LSB is found to be particularly enhanced within St Helena Bay (SHB), a hotspot for productivity which is also prone to Harmful Algal Bloom (HAB) development. We attribute the enhanced response to a combination of near-critical latitude for diurnal-inertial resonance (~32.5°S), the local enhancement of the LSB, and the local development of a shallow stratified surface layer through bay retention. Pronounced advection of the surface layer by diurnal-inertial oscillations contributes to large differences in day- and night-time sea surface temperatures (SST’s) (more than 2°C on average in SHB). Event-scale diapycnal mixing is particularly enhanced within SHB, as highlighted by a numerical experiment initialised with a subsurface passive tracer. These super-diurnal processes are shown to influence sub-diurnal dynamics within SHB through their modulation of the vertical water column structure. A deeper thermocline retains the upwelling front closer to land during active upwelling, while geostrophically-driven alongshore flow is impacted through the modulation of cross-shore pressure gradients. The results suggest that the LSB is likely to play an important role in the productivity and therefore HAB development within SHB, and highlight potential challenges for observational systems and models aiming to improve our understanding of the physical and biological functioning of the system.

The oceanography of the northern Canadian Arctic Archipelago (CAA) remains poorly studied. Here we present a unique set of conductivity–temperature–depth (CTD) and nitrate profiles collected in a fjord system around Axel Heiberg Island in the northern CAA during April–May 2022. The profiles are examined within the context of upstream observations in the Arctic Ocean and downstream observations in the central CAA, and reveal the origin of water masses and their interactions with ambient water from the continental slope and the nearby tidewater glacier outlet. The subsurface water (~25–180 m depth) is associated with the Pacific water outflow from the Arctic Ocean. The underlying halocline separates Pacific water from a deeper layer of polar water that has interacted with the warm (>0°C) Atlantic water observed below 240 m depth. Pacific water is significantly modified compared to the adjoining Arctic Ocean, as evidenced by the following details. Cold water intrusions from the tidewater glacier create deviations of ~0.25°C in the temperature profile through the subsurface water down to a depth of 140 m. Profiles show no thermal signature of Pacific summer water. Compared to the adjacent Arctic Ocean, the deeper fraction of Pacific-derived water and the Atlantic-modified polar water are warmer, while the underlying Atlantic water is colder. Overall, our results suggest that Pacific and Atlantic water in this area of the northern CAA are modified due to enhanced vertical mixing in a narrow band over the continental slope and shelf off the CAA, and are further modified by interactions with outlet glaciers in the area. This implies that tracing the initial thermohaline signature of the Pacific and Atlantic water flow through the CAA seems to be hardly possible without the use of additional tracers. We also find evidence of geothermal heating near the seafloor, which is not surprising given the observed presence of terrestrial geothermal vents around Axel Heiberg Island, and speculate this heat flux limits ice growth near the glacier terminus.

The South China Sea is a typhoon-prone region, and previous studies have shown that typhoons have significant impacts on air-sea CO _2 fluxes. However, the effect of typhoons on the northern coastal area of the South China Sea is not well understood owing to limited observational data. In this study, we used a coupled model to simulate the impact of four typhoons (Hato, Mangkhut, Nida, and Merbok) on the partial pressure of CO _2 in seawater (pCO _2sea ) and the CO _2 fluxes in this area. Our results show that the coupled model effectively reproduces the spatial pattern of pCO _2sea in this region. The response of pCO _2sea to typhoons was determined by typhoon-induced vertical mixing and coastal upwelling, along with initial oceanic conditions. Typhoon Nida caused a decrease in pCO _2sea with Total Alkalinity and Sea Surface Temperature being the primary factors. However, typhoons Hato, Mangkhut, and Merbok caused an increase in pCO _2sea with Dissolved Inorganic Carbon playing a more prominent role. The average CO _2 fluxes during the passage were approximately 6–14 times higher than those before typhoon passage. These results enhance our understanding of the effect of typhoons on air-sea CO _2 fluxes over the northern coastal area of the South China Sea.

The Luzon Strait (LS) is a key region for estimating carbon sources and sinks in the South China Sea (SCS) and is highly influenced by the Kuroshio Current (KC) and typhoons. Understanding the variations in the sea surface partial pressure of carbon dioxide (pCO2-sw) under the combined effects of typhoons and KC in this region is crucial for estimating local and regional changes in ocean carbon flux. Based on valuable in situ pCO2-sw and remote sensing data, this study aimed to reveal the temporal variations and the physical mechanisms of pCO2-sw variations under the comprehensive effects of both typhoons and Kuroshio Intrusion (KI) in the LS. One week after the passage of the tropical cyclone (TC) Nanmadol, the concentration in the pCO2-sw and the influencing mechanisms varied in three different regions (W1–W3) on Transect A (120°E). In the region dominated by SCS waters (W1), the average pCO2-sw increased by 5.1 μatm after TC, which was mainly due to the TC “Wind Pump” inducing strong vertical mixing, which brought dissolved inorganic carbon (DIC)-rich deeper water up to the surface. In the region affected by KC (W2 and W3), pCO2-sw decreased after the TC (−8.2 μatm and −1.8 μatm, respectively) with TC-enhanced KI because the invasion of lower pCO2-sw of Kuroshio waters inhibited the TC-induced upwelling. More significant TC-induced upwelling (W3) would alleviate the decrease in pCO2-sw caused by the TC-enhanced KI. This study is a rare case providing a better understanding of the variations in pCO2-sw under TC-enhanced KI, which provides support for regional climate change prediction and carbon flux estimation in the western boundary current regions.

The cold wake caused by a tropical cyclone (TC) extends for hundreds of kilometers and persists for several weeks, thus influencing the surface response for any subsequent TCs that might pass over it. It is commonly accepted that sea-surface temperature (SST) cooling, as produced by a single TC, occurs primarily through vertical mixing. However, when there are sequential TCs, the earlier TC can dramatically change the thermal structure of the upper ocean, which may influence the subsequent development of a latter-occurring TC (LTC). Therefore, the contribution of horizontal advection and vertical mixing to SST-cooling during the passage of LTCs is of great interest. Using a 19-year-long observational dataset and the heat budget analysis of an idealized numerical simulation, the SST change during the passage of sequential TCs is investigated. The results demonstrate that, on average, the SST cooling caused by the LTC shows an overall decreasing trend with enhanced lingering wakes. Budget analysis of the model simulations suggests that an earlier TC can suppress the vertical mixing induced by an LTC mainly through an alteration of dynamics within the deepened mixed layer and that the contribution of vertical mixing to the SST cooling is weaker due to the intensification of the earlier TC. The weakened vertical mixing dominates the decreased SST cooling induced by an LTC. In contrast, the cold wake generated by an earlier TC can produce more cold water on the right side of the TC’s track, which contributes to stronger horizontal advection upon the arrival of the LTC. In general, the effects of the earlier TC can suppress the sea-surface thermal response to an LTC. If the contribution of the horizontal advection to SST cooling is neglected, the SST cooling induced by an LTC could be reduced by about 40%. As for the response of the sub-surface water to the passage of an LTC, the weakened warm anomaly induced by vertical mixing and the enhanced cooling anomaly caused by the vertical advection explain the reduced tendency for the mixed layer to deepen. As a result, the tendency for the mixed layer depth (MLD) to increase is suppressed during the passage of an LTC. These results highlight the importance of optimally depicting cold wakes in numerical simulations to improve the prediction of the upper ocean’s response to sequential TCs.

Despite the rapid increase of greenhouse gases (GHGs) in the atmosphere during the past 50 years, observed global mean surface temperature (GMST) showed a pause in the warming trend during the first decade of the twenty-first century. This is referred to as the global warming "hiatus". A dominant hypothesis emphasizes that the superimposition of the cold phase of the Pacific decadal variability and the global warming trend can lead to the hiatus. Using simply energy balance models, we explore two potential mechanisms that may supress the GMST warming trend: enhanced negative climate feedback and downward heat mixing. Forced by linearly increasing heating, a stronger negative climate feedback can reduce the GMST warming rate, but cannot result in a warming hiatus. Downward mixing of heat can cause a short-lived hiatus of surface warming rate due to enhanced nonlinear ocean heat uptake by the lower ocean, but the surface warming would be accelerated in the long run due to the decline of downward heat mixing rate. This study provides further evidence, both theoretically and numerically, that in the long run, the only route to contain the global warming effectively is to reduce GHG emissions. [ABSTRACT FROM AUTHOR]

Limited light availability due to insufficient vertical mixing is one of the main drawbacks of raceway ponds (RWPs), the most common type of microalgae cultivation system. In this study, we have investigated the application of vortex induced vibration (VIV) systems to improve vertical mixing in order to enhance algal biomass productivity. The system consists of a cylinder submerged parallel to the bottom in the pond with two springs attached at its ends. The cylinder oscillates perpendicularly to the flow direction at the pond to increase vertical mixing. A VIV system, which requires no additional energy input, was installed in a 0.3 m deep raceway pond and continuous cylinder oscillation was successfully achieved. Cylinder oscillation frequency of 1.24 s−1 and amplitude of 6.5 cm have been obtained experimentally for 0.3 m s−1 flow velocity. Numerical simulations were carried out with experimental parameters using CFD code and were in good accordance with experimental results. Numerical analysis revealed that it is possible to create high frequency light/dark cycles; mean light/dark cycle frequencies were found to be 2.33 s−1, 5.28 s−1 and 21.17 s−1, at lowermost, middle and uppermost cylinder positions, respectively. Enhanced velocity magnitude of 0.3 m s−1 was achieved in the vertical direction; vertical motion of flow resulting from cylinder oscillation covers about two thirds of pond depth. Effectiveness of the VIV system on biomass growth was also verified by comparative Chlorella vulgaris cultivation under outdoor conditions. It has been observed that the VIV system installed reactor enhanced biomass production capacity by over 20% compared to the control pond. These results indicate that the presented method possesses a potential for enhanced algal biomass production without significant increase in installation and operating costs. [ABSTRACT FROM AUTHOR]

Ocean turbulent mixing is a key process affecting the uptake and redistribution of heat, carbon, nutrients, oxygen and other dissolved gasses. Vertical turbulent diffusivity sets the rates of water mass transformations and ocean mixing, and is intrinsically an average quantity over process time scales. Estimates based on microstructure profiling, however, are typically obtained as averages over individual profiles. How representative such averaged diffusivities are, remains unexplored in the quiescent Arctic Ocean. Here, we compare upper ocean vertical diffusivities in winter, derived from the 7Be tracer‐based approach to those estimated from direct turbulence measurements during the year‐long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, 2019–2020. We found that diffusivity estimates from both methods agree within their respective measurement uncertainties. Diffusivity estimates obtained from dissipation rate profiles are sensitive to the averaging method applied, and the processing and analysis of similar data sets must take this sensitivity into account. Our findings indicate low characteristic diffusivities around 10−6 m2 s−1 and correspondingly low vertical heat fluxes. Plain Language Summary: Ocean turbulent mixing plays an important role in the uptake and redistribution of heat, carbon, nutrients, oxygen and other properties. For example, this process delivers nutrients to the sunlit surface ocean where they are utilized to produce plants (phytoplankton) for the ecosystem food web. However, strong changes in density within the upper Arctic Ocean hinder vertical transport of nutrients, such that nutrient fluxes are generally smaller than those observed elsewhere in the world ocean. Furthermore, low vertical transport rates isolate the surface ocean from heat input from below which helps protect the ice from melting. Here, we compare the strength of upper ocean mixing, an important parameter for the calculation of vertical transport, derived from two independent methods during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) ice drift experiment, 2019–2020. This comparison allows us to better quantify the vertical diffusivity, and in turn also the vertical transport of for example, heat and nutrients in the ocean. Key Points: Arctic Ocean vertical diffusivity (Kz) in the upper halocline in winter is O(10−6) m2 s−1Diffusivity estimates from 7Be measurements and ocean microstructure profiling agree within a factor of 2Kz estimates from turbulent dissipation rate profiles are sensitive to the averaging method [ABSTRACT FROM AUTHOR]

The selection of the planetary boundary layer (PBL) parameterisation schemes is crucial for the simulation of local meteorology, especially under extreme weather events. A suitable PBL scheme is one of the key conditions to accurately reproduce the real weather conditions. In this study, the performance of all the three available PBL schemes that can be coupled with the multi-layer urban canopy model in the Weather Research and Forecasting (WRF) model: Yonsei University (YSU), Mellor-Yamada-Janjic (MYJ), and Bougeault-Lacarrere (BL), is evaluated for a cold spell event with Guangdong-Hong Kong-Macao Greater Bay Area (GBA) as the study region. It is found that accurate prediction of the strength of the vertical mixing and the change in wind speeds over the upper atmosphere is critical for reproducing the nighttime temperatures during the cold spell. The simulations of the BL scheme are consistent with observations that wind speeds do not vary much during daytime and nighttime, thus exhibiting strong vertical mixing and slowing down the near-surface temperature decrease during nighttime. The MYJ and YSU schemes cause significant underestimation of the nighttime 2-m temperature due to significant overestimation of the nighttime upper-air wind speeds, which leads to more heat transport through horizontal advection and hinders vertical mixing and convective activities. The results confirm that selection of a suitable PBL scheme can effectively reduce the uncertainty of the simulation results, and the coupling of the BL scheme with the multi-layer urban canopy model can better simulate the cold spell events occurring in the GBA, which also provides valuable references for the simulation of other similar coastal cities. • Evaluate the performance of PBL schemes in WRF/BEP-BEM to simulate cold spells. • Three experiments, including the local BL, MYJ and nonlocal YSU schemes. • Near-surface variables and vertical profiles are compared with observations. • BL scheme is the preferred option for the Guangdong-Hong Kong-Macao Bay Area during cold spells. • Vertical mixing and horizontal wind speeds are responsible for the optimality of BL scheme. [ABSTRACT FROM AUTHOR]

Insufficient diapycnal oxygen transport through the halocline is the key reason for the anoxic conditions typically observed in the deeper parts of the Baltic Sea. The variability of turbulent oxygen fluxes through the halocline was investigated seasonally and in space during three cruises (summer, fall, winter). Turbulence dissipation rates and oxygen fluxes showed a pronounced seasonal pattern, with the lowest values during summer. At the basin boundaries, halocline oxygen fluxes increased by an order of magnitude as a result of boundary mixing. A simple box model, distinguishing between interior, intermediate, and boundary regions, was used to extrapolate the locally observed fluxes to the central Baltic Sea. The boundary area, covering less than 23% of the total basin area, contributed more than 80% of the basin‐scale oxygen transport through the halocline. According to this model, the annual oxygen flux through the halocline is of the order of 20–28 Gmol per year. Plain Language Summary: Oxygen deficits in natural waters are caused by an imbalance of the oxygen demand within a water mass and the transport of oxygen into it. In the Baltic Sea two main water bodies can be distinguished, the surface water and the deep water starting at 80 m. Oxygen is produced by primary production in the upper part while in the deep water the bio‐material sink down and require oxygen for their degradation. The strong salinity difference between the deep and the surface water, which is much fresher, creates a transport barrier, that inhibits an oxygen transport from the top to the deeper parts, the halocline. We investigate the seasonal timing and spatial location of the mixing and the transport through the halocline and found the closer the halocline is to the seafloor, the stronger is the mixing and the oxygen flux. There is also a strong seasonality: During the summer months almost no oxygen transport has been observed. In contrast to the fall and winter season with fluxes being at least 10 time larger. With the data available we could quantify the total amount of oxygen being transported through the halocline into the deep parts of the central Baltic Sea. Key Points: First direct observation of the seasonality of vertical oxygen fluxes by boundary mixing in one of the largest anoxic systems (Baltic Sea)Oxygen fluxes through the halocline are increased by an order of magnitude at the basin boundaryBoundary mixing contributes more than 80% of the total vertical oxygen transport [ABSTRACT FROM AUTHOR]

A detailed picture of internal solitary waves (ISWs) in the White Sea is presented based on an analysis of historical spaceborne synthetic aperture radar (SAR) data and field measurements. The major hot-spot of ISW generation locates in the southwestern (SW) Gorlo Strait (GS), characterized by the presence of strong tides, complex topography, and two distinct fronts. Here, pronounced high-frequency isopycnal depressions of 5–8 m were regularly observed during flood and flood/ebb slackening. Other regions of pronounced ISW activity are found near Solovetsky Islands and in the northwestern Onega Bay. The spatial and kinematic properties of the observed ISWs are linked to water depth, with larger wave trains and higher propagation speeds being observed over the deep regions. Direct estimates of ISW propagation speeds from sequential and single SAR images agree well, while theoretical ones obtained using a two-layer model overestimate the observed values by 2–3 times. This is explained by the effective modulation of ISW propagation speed during the tidal cycle by background currents that are not accounted for in the model. Enhanced values of diapycnic diffusion coefficient in the pycnocline layer were registered near the frontal zones, where intense 14–17 m high ISWs were regularly observed. [ABSTRACT FROM AUTHOR]

The North China Plain (NCP), known for its dense population, extensive urbanization, and developed industry and agriculture, faces one of the foremost ozone (O 3 ) pollution issues nationwide and even globally. Currently, most studies focus on daytime peak O 3 levels, with insufficient understanding of the increase in nighttime O 3 concentrations. Based on data from 204 national atmospheric composition monitoring sites in the NCP from 2015 to 2023, we investigated the characteristics of nocturnal surface O 3 enhancement (NSOE) events and explored potential formation mechanisms. The mean annual frequencies of single-site and regional NSOE event in the NCP between 2015 and 2023 are 42 % and 21 %, respectively. The daytime peak O 3 concentrations before and after NSOE events exceeded those during the corresponding periods of non-NSOE events by 84 ± 19 and 32 ± 15 μg/m 3 , respectively. The overall effect of the NSOE events was to decelerate the rate of decline in nighttime O 3 concentrations and resulted in a reduction of NO 2 and CO concentrations from 22:00 onwards. Low level jet (LLJ) and vertical mixing were the main factors affecting NSOE events in the NCP. The proportion of NSOE events affected by LLJ in four representative cities ranged from 57.6 % to 79.5 %. Furthermore, the high concentration of O 3 in the residual layer before the NSOE event and the reduction of atmospheric stability during the NSOE event favored downward mixing of upper layer O 3 . The primary weather systems influencing the four most severe regional NSOE events were LLJ, typhoon, and cold fronts. The first two events were dominated by vertical mixing of O 3 , while the latter two events were mainly affected by horizontal transport. Our findings provide the first overview of NSOE events in the NCP from characteristics to mechanisms, emphasizing the necessity for future detailed studies based on nocturnal vertical O 3 observations., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier B.V.)

Based on the meteorological, ozone (O3), and vertical observation data of 2020, this study sought to evaluate the daily variation in O3, particularly the characteristics of nocturnal ozone pollution. We also discuss the effect of local and mesoscale horizontal transport and vertical mixing on the formation of nocturnal O3 pollution. Distinct seasonal characteristics of the daily O3 variation in Shenzhen were identified. In particular, significant nocturnal peaks were found to regularly occur in the winter and spring (November–December and January–April). The monthly average of daily variation had a clear bimodal distribution. During the period, O3 pollution frequently occurred at night, with the maximum hourly O3 concentration reaching 203.5 μg/m3. Nocturnal O3 pollution was closely associated with horizontal transport and vertical mixing. During the study period, the O3 maximum values were recorded on 68 nights, primarily between 23:00 and 03:00, with occasional observation of two peaks. The impact of horizontal transport and vertical mixing on the nocturnal secondary O3 maximum values was elaborated in two case studies, where vertical mixing was mainly associated with low-level jets, with strong wind shear enhancing turbulent mixing and transporting O3 from the upper layers to the surface. [ABSTRACT FROM AUTHOR]

It is widely accepted that the signature of anticyclonic (cyclonic) eddies on the sea surface temperature corresponds to a warm (cold) core anomaly. Nevertheless, this statement has been put to question by recent regional studies showing the existence of inverse eddy SST anomalies: Cold Core anticyclones and, respectively, Warm Core cyclones. This study shows that the emergence of these inverse anomalies is a seasonal phenomenon that affects the life cycle of mesoscale eddies in the Mediterranean Sea. We use remote sensing observations and in situ data to analyse the eddy-induced SST anomaly over a 3-year period (2016–2018). We build an eddy core SST anomaly index to quantify the amount of Cold Core anticyclones and Warm Core cyclones all over the year and especially during the spring re-stratification period. We find that 70 % of eddy anomalies are inverse in May and June both for cyclones and anticyclones. Regular temperature anomalies could reach 1.5 °C, while inverse ones are only present in the first 50 m of the oceanic layer and hardly exceed 1 °C. In order to understand the underlying dynamical processes, we construct a simple vertical column model to study the impact of the seasonal air–sea fluxes on the surface stratification inside and outside eddies. It is only by taking into account a differential diapycnal eddy mixing—increased in anticyclones and reduced in cyclones—that we reproduce correctly, in agreement with the observations, the surface temperature inversion in the eddy core. This simplified model suggests that vertical mixing modulation by mesoscale eddies might be the key mechanism that leads to the eddy–SSTA seasonal inversion in the ocean. [ABSTRACT FROM AUTHOR]