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The Ocean Heat Uptake Efficiency (OHUE) quantifies the ocean's ability to mitigate surface warming through deep heat sequestration. Despite its importance, the main controls on OHUE, and on its two‐fold spread across contemporary climate models, remain unclear. We argue that OHUE is primarily controlled by mid‐latitude ventilation strength in the background climate, itself related to pycnocline depth and stratification. This hypothesis is supported by a strong correlation between mid‐latitude (30–60°) OHUE and the near‐global average (60°S–60°N) pycnocline depth in CMIP5 and CMIP6 AOGCMs under RCP85/SSP585, and in a parameter perturbation ensemble of ocean GCM (MITgcm) experiments. This correlation explains about 70% of the CMIP5‐6 spread in global OHUE. The relationship provides a pathway toward observationally constraining OHUE, and thus reducing uncertainty in projections of future global climate change and sea level rise. Plain Language Summary The ocean absorbs most of the excess heat in the climate system. How effectively this process reduces surface warming depends on how deeply this heat is stored in the ocean, which varies widely across contemporary climate models. Our study shows that around 70% of the variation in deep heat storage across models is explained by differences is how water from the mid‐latitude surface ocean is transported to deeper ocean layers by ocean currents. This difference can be measured by the stratification of seawater density in the base‐state climate, a feature that can be observed in the modern ocean. We show this relationship using a new regional decomposition method. The results of this study can be leveraged with ocean observations to reduce uncertainty in future climate and sea level rise projections. Key Points Pycnocline depth correlates strongly with Ocean Heat Uptake Efficiency (OHUE) in CMIP5/CMIP6 and MITgcm A regional OHUE decomposition shows that mid‐latitude heat uptake and sequestration drives the correlation between OHUE of pycnocline depth Inter‐model differences in pycnocline depth explain around 70% of the spread in OHUE across CMIP5 and CMIP6

The influence of ocean stratification and heat content on the timing of sea ice formation and its subsequent growth remains an open question. Here we investigate the thermohaline conditions prior to fall sea ice formation as well as the roles of stratification and heat content on sea ice growth rates through the analysis of in situ observations and numerical simulations from a one‐dimensional ocean‐ice‐column model. We find that the simulated time series of sea ice concentration are highly correlated with observations. We identify two clusters of sea ice concentration growth rate, which we name Early Slow and Late‐Fast. We find that cold, shallow mixed layers promote early sea ice freeze‐up. Salinity stratification within the upper pycnocline slows the release of heat into the deepening mixed layer, leading to slower ice growth. However, where salinity stratification above the upper pycnocline is absent, sea ice growth occurs later and, once started, progresses faster.

The inversion of remote sensing signatures of internal solitary waves (ISWs) can retrieve dynamic characteristics in the ocean interior. However, the presence of ubiquitous large‐amplitude ISWs poses challenges to the commonly used weakly nonlinear methods for parameter retrieval. Through laboratory experiments, we establish a relationship between surface features and internal characteristics of ISWs by the remote sensing imaging mechanism. The results demonstrate that strong nonlinearity significantly influences the retrieval of ISWs, primarily manifested in the calculation of wave‐induced velocities and the applicability of ISW solutions. A fully nonlinear model Dubreil–Jacotin–Long equation is used in the retrieval and has been tested under different conditions. Mooring observations indicate that the determination of ISW parameters from satellite images is affected by the complexity of in situ stratification, but additional remote sensing information such as surface velocities enables us to perform retrievals even if the real‐time measurement of pycnocline depth is not available. Plain Language Summary Internal solitary waves (ISWs), as nonlinear internal waves, play an essential role in oceanic human activities and ocean mixing. The surface current induced by ISWs can create rough and smooth regions on the sea surface due to the modulated roughness, hence presenting alternating bright and dark stripes in satellite images. Satellites can observe ISWs over a wide range via surface manifestations, and the internal dynamics can be calculated from surface features using retrieval methods. However, the availability of retrieval methods still needs to be verified, facing the difficulty of matching mooring observations and satellite images of the same ISW in a short time interval. According to the proportional relation of remote sensing signatures and wave‐induced velocities, this study establishes the relationship between surface features and internal characteristics of ISWs in laboratory experiments. Strong nonlinearity significantly influences the retrieval of ISWs and a fully nonlinear model is well applied in retrieval. Then we test the retrieval in oceanic environments, mooring observations show the critical role of stratification in retrieval. This work provides a reliable dynamics model for the inversion of remote sensing signatures of ISWs into characteristics in the ocean interior. Key Points The relationship between surface features and internal parameters of internal solitary waves is established in laboratory experiments Strong nonlinearity significantly impacts the determination of wave parameters from the surface. A fully nonlinear model is well applied Accurate parameter determination is constrained by the complex oceanic stratification, but more remote sensing information can overcome it

Pycnocline is a widespread marine phenomenon across the world, which plays an important role in ocean engineering, oceanic meteorology, and biological applications. In this study, based on actual density data, statistical characteristics of complex density profiles are analyzed. It is found that the pycnocline interval d in multilayer structures conforms to the three-parameter Weibull distribution. To describe the multilayer structure more accurately, a dimensionless parameter α is proposed, which takes into account scale effects and the relative position of layers. Then, the probability density distribution of α is analyzed, from which the clustering phenomena are found. Therefore, clustering analysis is carried out to obtain a method for determining the location of pycnocline in complex density profiles. Based on this method, the statistical research of pycnocline distribution in the northern South China Sea is carried out including the key properties of pycnocline depth, pycnocline thickness, and pycnocline intensity. The result shows that the pycnocline distribution changes periodically with seasons, in which heat flux and monsoon are the main influencing factors. As the temperature increases, the formation of the pycnocline is encouraged, with the consequent decrease in pycnocline depth and increase in pycnocline thickness and intensity, vice versa. The monsoon influences the distribution of the pycnocline by affecting vertical mixing and Ekman wind-driven mechanism.

Instability within internal solitary waves (ISWs), featured by temperature inversions with vertical lengths of dozens of meters and current reversals in the upper shoreward velocity layer, was observed in the northern South China Sea at a water depth of 982 m by using mooring measurements between June 2017 and May 2018. Regions of shear instability satisfying Ri < 1/4 were found within those unstable ISWs, and some large ISWs were even possibly in the breaking state, indicated by the ratio of L x (wave width satisfying Ri < 1/4) to λ η /2 (wavelength at half amplitude) larger than 0.86. Wave stability analyses revealed that the observed wave shear instability was induced by strong background current shear associated with multiscale dynamic processes, which greatly strengthened wave shear by introducing sharp perturbations to the fine-scale vertical structures of ISWs. During the observational period, wave shear instability was strong in summer (July–September) while weak in winter (January–March). Sensitivity experiments revealed that the observed shear instability was prone to be triggered within large ISWs by the background current shear and sensitive to the pycnocline depth in the background stratification. However, shear instability within ISWs was observed to be promoted during mid-January, as the near-inertial waves trapped inside an anticyclonic eddy resulted in enhanced background current shear between 150 and 300 m. This work emphasizes the notable impacts of multiscale background processes on ISWs in the oceans.

Primary production dynamics are strongly associated with vertical density profiles in shelf waters. Variations in the vertical structure of the pycnocline in stratified shelf waters are likely to affect nutrient fluxes and hence the vertical distribution and production rate of phytoplankton. To understand the effects of physical changes on primary production, identifying the linkage between water column density and Chlorophyll a (Chl a) profiles is essential. Here, the vertical distributions of density features describing three different portions of the pycnocline (the top, centre, and bottom) were compared to the vertical distribution of Chl a to provide auxiliary variables to estimate Chl a in shelf waters. The proximity of density features with deep Chl a maximum (DCM) was tested using the Spearman correlation, linear regression, and a major axis regression over 15 years in a shelf sea region (the northern North Sea) that exhibits stratified water columns. Out of 1237 observations, 78 % reported DCM above the bottom mixed layer depth (BMLD: depth between the bottom of the pycnocline and the mixed layer underneath) with an average distance of 2.74 ± 5.21 m from each other. BMLD acts as a vertical boundary above which subsurface Chl a maxima are mostly found in shelf seas (depth ≤ 115 m). Overall, DCMs were correlated with the halfway pycnocline depth (HPD) (ρS = 0.56) which, combined with BMLD, were better predictors of the locations of DCMs than surface mixed layer indicators and the maximum squared buoyancy frequency. These results suggest a significant contribution of deep mixing processes in defining the vertical distribution of subsurface production in stratified waters and indicate BMLD as a potential indicator of the Chl a spatiotemporal variability in shelf seas. An analytical approach integrating the threshold and the maximum angle method is proposed to extrapolate BMLD, the surface mixed layer, and DCM from in situ vertical samples.

The pathway of dense river inflows into lakes, which affects the lake water quality, is not accurately predicted by existing models. The pathway of a dense riverine inflow in a lake with a submerged canyon is analyzed based on measurements during a 4‐month period of weakening lake stratification and weakening density excess between river and epilimnion. In line with models, the dense riverine inflow plunges upon entering the lake, continues as an underflow on the sloping lake bottom, and finally intrudes at its level of neutral buoyancy. Underflow and interflow velocities are O(0.1 m s−1). The river inflow is finally trapped in the pycnocline most of the time, even when the river's density excess and the lake's stratification are marginal. This trapping in the pycnocline is explained by the reduction of the inflow density excess due to the intense plunging mixing, which is an order of magnitude larger than that obtained in confined laboratory flumes. The pathway of the dense riverine inflow is affected by interactions of the underflow with the lake bottom and sedimentary processes. A canyon carved by the underflows confines and accelerates the underflow, which enhances its capacity to entrain and carry sediment. The entrainment of sediment that was previously deposited on the canyon bottom accelerates the underflow. Due to both effects, the underflow can temporarily break through the pycnocline and reach the hypolimnion. Existing models explain these observations qualitatively, but a quantitative prediction would require better parameterizations of the plunging mixing and the sedimentary processes. Plain Language Summary The pathway of river inflows into lakes is not accurately predicted by existing models. We investigate the physical processes affecting the pathway of dense riverine inflow (i.e., inflow with a density higher than lake water) into a stratified lake. We investigate the conditions under which a dense riverine inflow get trapped in the pycnocline (the layer that separates warmer surface waters from cold deep waters) or break through it. Unprecedented long records of the temporal evolution of the pathway of the riverine flow into the lake during a period of weakening riverine density excess and lake stratification are conceptualized in a model, which extends existing concepts for dense riverine inflows. The entrainment of lake waters into the riverine inflow in the plunging region is larger than predicted by laboratory studies. This explains why the riverine inflow is trapped in the pycnocline most of the time. Flow confinement by a canyon carved by the riverine inflow into the lake bottom accelerates the riverine inflow and enhance sediment entrainment capacity causing short‐lived self‐accelerating turbidity currents along the lake bottom that break through the pycnocline and reach deep waters. Our results allow improved estimates of oxygen replenishment or sediment deposition from riverine water. Key Points Plunging mixing into an unconfined ambient is an order of magnitude larger than in a confined ambient Pronounced plunging mixing reduces the initial density excess explaining why the inflow is mostly trapped in the pycnocline Resuspension of lake bottom sediment can cause short‐lived self‐accelerating turbidity currents that break through the pycnocline

Ocean circulation supplies the surface ocean with the nutrients that fuel global ocean productivity. However, the mechanisms and rates of water and nutrient transport from the deep ocean to the upper ocean are poorly known. Here, we use the nitrogen isotopic composition of nitrate to place observational constraints on nutrient transport from the Southern Ocean surface into the global pycnocline (roughly the upper 1.2 km), as opposed to directly from the deep ocean. We estimate that 62 ± 5% of the pycnocline nitrate and phosphate originate from the Southern Ocean. Mixing, as opposed to advection, accounts for most of the gross nutrient input to the pycnocline. However, in net, mixing carries nutrients away from the pycnocline. Despite the quantitative dominance of mixing in the gross nutrient transport, the nutrient richness of the pycnocline relies on the large-scale advective flow, through which nutrient-rich water is converted to nutrient-poor surface water that eventually flows to the North Atlantic. Much of the nutrient transport from the deep ocean into the ocean’s upper water column occurs through the Southern Ocean, with mixing and advection playing complementary roles, according to a box model analysis of the isotopic composition of ocean nitrate.

This model study investigates summer hydrographic changes in response to climate projections following the CMIP5 RCP8.5 scenario. We use the high resolution regional coupled ocean–sea ice–atmosphere model RCA4–NEMO to downscale an ensemble of five global climate projections with a main focus on the Baltic Sea and neighboring shelf basins to the west. We find consistently across the ensemble a northward shift in the mean summer position of the westerlies at the end of the twenty-first century compared to the twentieth century. Associated with this is an anomalous precipitation pattern marked by increased rainfall over northern Europe and dryer conditions over the continental central part. In response to these large-scale atmospheric changes, a strong freshening mainly resulting from a higher net precipitation over the year combined with higher annual mean runoff is registered for the Baltic Sea and adjacent seas. The strongest freshening takes place in the southern Skagerrak region where stronger winds enhance the cyclonic circulation and by this, recirculation of fresher waters from the Baltic Sea strengthens. In the Baltic Sea freshening leads to a reduction in basin averaged salinities between 0.6 and 2.3 g kg −1 throughout the ensemble. Likewise, the sea surface temperature response in the Baltic Sea varies between + 2.5 and + 4.7 K depending on the applied global model scenario. The climate induced changes in atmospheric forcing have further consequences for the large-scale circulation in the Baltic Sea. All ensemble members indicate a strengthening of the zonal, wind driven near surface overturning circulation in the southwestern Baltic Sea towards the end of the twenty-first century whereas the more thermohaline driven overturning at depth is reduced by ~ 25%. In the Baltic Proper, the meridional overturning shows no clear climate change signal. However, three out of five ensemble members indicate at least a northward expansion of the main overturning cell. In the Bothnian Sea, all ensemble members show a significant weakening of the meridional overturning. The entire ensemble consistently indicates a basin-wide intensification of the pycnocline (9–35%) for the Baltic Sea and a shallowing of the pycnocline depth in most regions as well. In the Baltic Sea, which is dominated by mesohaline conditions under the historical period, the changes in salinity at the end of the twenty-first century have turned wide areas to be dominated by oligohaline conditions as a result of climate change. Potential consequences for biogeochemical conditions and implications for biodiversity are discussed.

Subduction associated with mesoscale eddies is an important but difficult-to-observe process that can efficiently export carbon and oxygen to the mesopelagic zone (100–1000 dbar). Using a novel BGC-Argo dataset covering the western North Pacific (20–50∘ N, 120–180∘ E), we identified imprints of episodic subduction using anomalies in dissolved oxygen and spicity, a water mass marker. These subduction patches were present in 4.0 % (288) of the total profiles (7120) between 2008 and 2019, situated mainly in the Kuroshio Extension region between March and August (70.6 %). Roughly 31 % and 42 % of the subduction patches were identified below the annual permanent pycnocline depth (300 m vs. 450 m) in the subpolar and subtropical regions, respectively. Around half (52 %) of these episodic events injected oxygen-enriched waters below the maximum annual permanent thermocline depth (450 dbar), with >20 % occurring deeper than 600 dbar. Subduction patches were detected during winter and spring when mixed layers are deep. The oxygen inventory within these subductions is estimated to be on the order of 64 to 152 g O2/m2. These mesoscale events would markedly increase oxygen ventilation as well as carbon removal in the region, both processes helping to support the nutritional and metabolic demands of mesopelagic organisms. Climate-driven patterns of increasing eddy kinetic energies in this region imply that the magnitude of these processes will grow in the future, meaning that these unexpectedly effective small-scale subduction processes need to be better constrained in global climate and biogeochemical models.