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Future sea-level change is characterized by both quantifiable and unquantifiable uncertainties. Effective communication of both types of uncertainty is a key challenge in translating sea-level science to inform long-term coastal planning. Scientific assessments play a key role in the translation process and have taken diverse approaches to communicating sea-level projection uncertainty. Here we review how past IPCC and regional assessments have presented sea-level projection uncertainty, how IPCC presentations have been interpreted by regional assessments and how regional assessments and policy guidance simplify projections for practical use. This information influenced the IPCC Sixth Assessment Report presentation of quantifiable and unquantifiable uncertainty, with the goal of preserving both elements as projections are adapted for regional application.

Sharp fronts and eddies that are ubiquitous in the world ocean, as well as features such as shelf seas and under-ice-shelf cavities, are not captured in climate projections. Such small-scale processes can play a key role in how the large-scale ocean and cryosphere evolve under climate change, posing a challenge to climate models.

Oceanic motions across meso‐, submeso‐, and turbulent scales play distinct roles in vertical heat transport (VHT) between the ocean's surface and its interior. While it is commonly understood that during summertime the enhanced stratification due to increased solar radiation typically results in an reduced upper‐ocean vertical exchange, our study reveals a significant upward VHT associated with submesoscale fronts (<30 km) through high‐resolution observations in the eddy‐active South China Sea. The observation‐based VHT reaches ∼100 W m−2and extends to ∼150 m deep at the fronts between eddies. Combined with microstructure observations, this study demonstrates that mixing process can only partly offset the strong upward VHT by inducing a downward heat flux of 0.5–10 W m−2. Thus, the submesoscale‐associated VHT is effectively heating the subsurface layer. These findings offer a quantitative perspective on the scale‐dependent nature of VHT, with crucial implications for the climate system. Understanding the upper‐ocean heat budget is of great importance for gaining insight into how oceanic processes modulate the climate system, yet vertical heat transport (VHT) by submesoscale processes remains rarely studied using observations. Recently, scientists have identified the potential importance of submesoscale instabilities to enhance upward VHT within the mixed layer. However, the vertical pathways of heat from the ocean interior to the surface and the underlying mechanisms remain unclear, largely due to the limitations in observing such small, fast scales. To elucidate these questions, we conducted high‐resolution (a horizontal resolution of ∼0.6 km), synoptic in‐situ observations targeted at submesoscale phenomena near mesoscale eddies. Our study reveals substantial contributions of submesoscale processes to upward VHT in the stratified subsurface layer. This causes a notable imbalance in VHT by mesoscale, submesoscale, and mixing processes. These findings provide valuable insights for enhancing our understanding of heat uptake in the ocean. Quasi‐synoptic submesoscale‐resolving observations reveal strong upward vertical heat transport (VHT) in the ocean interior (over nearly 10x the mixed layer depth (MLD))Submesoscale fronts (<30 km) between eddies act as the primary driver for enhanced vertical heat transport >100 W m−2There is a significant imbalance in VHT associated with oceanic mesoscale, submesoscale, and mixing processes Quasi‐synoptic submesoscale‐resolving observations reveal strong upward vertical heat transport (VHT) in the ocean interior (over nearly 10x the mixed layer depth (MLD)) Submesoscale fronts (<30 km) between eddies act as the primary driver for enhanced vertical heat transport >100 W m−2 There is a significant imbalance in VHT associated with oceanic mesoscale, submesoscale, and mixing processes

Ocean model simulations show variability due to intrinsic chaos and external forcing (air‐sea fluxes, river input, etc.). It is important to estimate their contributions to total variability for attribution. Using variance to estimate variability might be unreliable due to non‐Gaussian higher statistical moments. We show the use of non‐parametric information theory metrics, Shannon entropy and mutual information, for measuring internal and forced variability in ocean models. These metrics are applied to spatially and temporally averaged data. The metrics delineate relative intrinsic to total variability in a wider range of circumstances than previous approaches based on variance ratios. The metrics are applied to (a) a synthetic ensemble of random vectors, (b) ocean component of a global climate (GFDL‐ESM2M) large ensemble, (c) ensemble of a realistic coastal ocean model. The information theory metric qualitatively agrees with the variance‐based metric and possibly identifies regions of nonlinear correlations. In application (2)–the climate ensemble–the information theory metric detects higher temperature intrinsic variability in the Arctic region compared to the variance metric illustrating that the former is robust in a skewed probability distribution (Arctic sea surface temperature) resulting from sharply nonlinear behavior (freezing point). In application (3)–coastal ensemble–variability is dominated by external forcing. Using different selective forcing ensembles, we quantify the sensitivity of the coastal model to different types of external forcing: variations in the river runoff and changes in wind product do not add information (i.e., variability) during summer. Information theory enables ranking how much each forcing type contributes across multiple variables. It is important in models to distinguish variability caused by external forces versus variability that arises within the system. Within the ocean component of a climate model, disturbances from the atmosphere such as wind, solar heating and cooling, and anthropogenic emissions are external disturbances and variations due to swirling motions (eddies) are internal chaotic disturbances. We use statistical concepts to quantify the amount of variations in these models. Here, we study multiple simulations made using a coastal ocean model and a climate model. We find that measuring internal and external variability depends on the statistical method you choose. Some statistical methods make a priori assumptions about the data, some do not. Choosing the method that makes less assumptions is better for estimating variability, particularly when considering temperature in the Arctic and salinity in an estuary. We show that for the estuarine model simulations with altered forcings, analyzing the experiments with statistical methods that use fewer assumptions about the underlying data helps prioritize parts of the design of a forecast model. We use metrics from information theory to estimate intrinsic and extrinsic variability in ensemble global and coastal modelIn the Arctic region, metric reveals high intrinsic variability and low to moderate extrinsic forcing for temperatureIn the coastal model experiments with altered forcings, metrics quantified their effects on temperature and salinity for forecasting application We use metrics from information theory to estimate intrinsic and extrinsic variability in ensemble global and coastal model In the Arctic region, metric reveals high intrinsic variability and low to moderate extrinsic forcing for temperature In the coastal model experiments with altered forcings, metrics quantified their effects on temperature and salinity for forecasting application

Mesoscale and submesoscale processes have crucial impacts on ocean biogeochemistry, importantly enhancing the primary production in nutrient‐deficient ocean regions. Yet, the intricate biophysical interplay still holds mysteries. Using targeted high‐resolution in situ observations in the South China Sea, we reveal that isopycnal submesoscale stirring serves as the primary driver of vertical nutrient transport to sustain the dome‐shaped subsurface chlorophyll maximum (SCM) within a long‐lived cyclonic mesoscale eddy. Density surface doming at the eddy core increased light exposure for phytoplankton production, while along‐isopycnal submesoscale stirring disrupted the mesoscale coherence and drove significant vertical exchange of tracers. These physical processes play a crucial role in maintaining the elevated phytoplankton biomass in the eddy core. Our findings shed light on the universal mechanism of how mesoscale and submesoscale coupling enhances primary production in ocean cyclonic eddies, highlighting the pivotal role of submesoscale stirring in structuring marine ecosystems. Both physical and biogeochemical processes affect marine ecosystems. Mesoscale cyclonic eddies are known to boost biological productivity by lifting the water in the center of the eddy up to where light is plentiful, which results in a subsurface chlorophyll maximum (SCM) layer. However, the SCM persists long after this eddy lifting process has completed. This study utilizes high‐resolution observations across some eddies to see if smaller, submesoscale processes replenish nutrients by stirring water along density surfaces into the SCM. The results indicate that isopycnal submesoscale stirring plays a significant role, helping the SCM persist within the eddy. We propose that this process might be universal and enhance primary production in all eddy‐rich oceans. Isopycnal stirring serves as the primary driver of nutrient supply to sustain subsurface chlorophyll maximum in ocean cyclonic eddiesFrontogenesis and submesoscale centrifugal–symmetric instabilities are the most likely dynamical mechanisms for isopycnal stirring Isopycnal stirring serves as the primary driver of nutrient supply to sustain subsurface chlorophyll maximum in ocean cyclonic eddies Frontogenesis and submesoscale centrifugal–symmetric instabilities are the most likely dynamical mechanisms for isopycnal stirring

The eddy‐abundant circulation in the northern South China Sea (NSCS) tends to be dynamically complex due to monsoon forcing, Kuroshio intrusion, and emitted internal waves from the Luzon Strait. This study uses 13‐year shipboard acoustic Doppler current profiler measurements (2004–2016), orbital altimeter data, and high‐resolution model output to perform wave‐vortex decompositions and investigate the scale of transition from dominantly geostrophic flows to internal wave motions in the northern South China Sea. The upper ocean kinetic energy spectra transition on scales exceeds 200 km. This large scale of transition is attributed to the energetic low‐mode internal waves (e.g., internal tides and inertia‐gravity waves). However, inconsistencies in the decomposition reveal that the assumptions of homogeneity and isotropy required for the 1‐D decomposition (Bühler et al., 2014) are sufficiently violated at smaller scales to affect the subdominant member of the decomposition on scales below 100 km. A method for direct quantification of the consequences by degree of violation using bootstrapping of the 2‐D model data is developed and illustrated. Observed and modeled sea surface height spectra flatten at scales smaller than 125 km, which is found in the model to be due to the coherent, semidiurnal internal waves. The large scale of transition between geostrophic and wave motions in the South China Sea is an irreducible uncertainty for altimeter velocities (e.g., the Surface Water and Ocean Topography mission). The scale transition from balanced geostrophic flows to unbalanced motions is of great importance for understanding the circulation system of the South China Sea where both eddies and internal waves are particularly active. Based on observations and high‐resolution simulation, this paper investigates the wave number spectra of kinetic energy and sea surface height variance in the northern South China Sea. The large transition length scale (>200 km) from balanced geostrophic flows to unbalanced motions is due to strong low‐mode internal waves in the upper ocean. The violations of the assumptions for the 1‐D decomposition (stationarity, homogeneity, and horizontal isotropy) are also assessed using the 2‐D model data. With the model output, the low‐mode semidiurnal internal tides are found to have significant impacts on the sea surface height variance at scales of ~125 km. These results are directly relevant to the efficiency of diagnosing geostrophic flows from altimeters (e.g., the upcoming Surface Water and Ocean Topography mission). The transition of KE from geostrophic flows to unbalanced motions is found to be larger than 200 km in the upper ocean of the NSCSThe frequency‐wavenumber analysis reveals the dominated processes for the transitionThe semidiurnal internal tides are found to have a significant contribution to the variance of sea surface height in the NSCS

Speleothem oxygen isotopes have been shown to exhibit a close relationship with summer insolation in the Northern Hemisphere, leading to the hypothesis that East Asian monsoon intensity is proportional to the summer insolation. This hypothesis, however, has been questioned because previous climate model simulations have been unable to simulate the observed large variation in precipitation or the precipitation isotope values, about a half of the variation in the entire modern tropical regions, in response to the insolation change due to the precession cycle. Here we show new results, using the fully coupled Geophysical Fluid Dynamics Laboratory model, that it is dynamically possible to have much higher precipitation during the high summer insolation period compared with the low summer insolation period in the East Asian monsoon region. We conclude that past East Asian monsoon intensity probably increased with increasing northern hemispheric insolation, given a large change in speleothem oxygen isotopes. Speleothems from calcite caves draw attention because precise dating is possible using the U‐Th method, opening up a unique chance of studying the past hydrological cycle. The isotopic composition of Chinese caves shows a remarkably coherent pattern following the summer solar radiative flux in the Northern Hemisphere, particularly responding to the precesssional cycle, leading to the hypothesis that East Asian monsoon intensity is proportional to the summer insolation. However, because climate models were not able to simulate the direct precipitation response following the insolation over the East Asian monsoon region or the isotope variation, this interpretation of the speleothem data has been challenged. Our new results show that it is dynamically possible to have much higher precipitation (more than 100% increase locally) during the high summer insolation period compared with the low summer insolation period in the East Asian monsoon region, but unlike previous studies, our simulations use a fully coupled ocean model, emphasizing the importance of including the ocean dynamics in studying climate response. It is dynamically possible to have much higher precipitation when summer insolation is high in the East Asian monsoon regionFully accounting for ocean circulation is important in simulating East Asian monsoon changeEast Asian monsoon intensity probably increases with increasing northern hemispheric insolation

Sea ice is a heterogeneous, evolving mosaic of individual floes, varying in spatial scales from meters to tens of kilometers. Both the internal dynamics of the floe mosaic (floe‐floe interactions), and the evolution of floes under ocean and atmospheric forcing (floe‐flow interactions), determine the exchange of heat, momentum, and tracers between the lower atmosphere and upper ocean. Climate models do not represent either of these highly variable interactions. We use a novel, high‐resolution, discrete element modeling framework to examine ice‐ocean boundary layer (IOBL) turbulence within a domain approximately the size of a climate model grid. We show floe‐scale effects could cause a marked increase in the production of fine‐scale three‐dimensional turbulence in the IOBL relative to continuum model approaches, and provide a method of representing that turbulence using bulk parameters related to the spatial variance of the ice and ocean: the floe size distribution and the ocean kinetic energy spectrum. Sea ice is a complex broken mosaic of individual pieces, called floes. These floes control how heat and momentum move between the atmosphere and ocean. But these floes interact with each other as well as with the upper ocean and lower atmosphere, and this means that these exchanges can be complexly related to both types of processes: floe‐floe and floe‐flow. Using experiments that explicitly evolve sea ice floes interacting with each other and the upper ocean, we develop a formulation for how momentum is transferred between the ice and ocean as a function of simple parameters of the ice‐ocean system that may be available to climate models. Discrete element model results show floe‐size‐dependent ice‐ocean boundary layer (IOBL) stress‐driven turbulenceIOBL turbulence is driven both by floe collisional velocity and by floe‐scale ocean variabilityWe present a framework for a scale‐aware parameterization of IOBL turbulence using bulk parameters Discrete element model results show floe‐size‐dependent ice‐ocean boundary layer (IOBL) stress‐driven turbulence IOBL turbulence is driven both by floe collisional velocity and by floe‐scale ocean variability We present a framework for a scale‐aware parameterization of IOBL turbulence using bulk parameters

This paper introduces two methods for dynamically prescribing eddy‐induced diffusivity, advection, and viscosity appropriate for primitive equation models with resolutions permitting the forward potential enstrophy cascade of quasi‐geostrophic dynamics, such as operational ocean models and high‐resolution climate models with O(25)km horizontal resolution and finer. Where quasi‐geostrophic dynamics fail (e.g., the equator, boundary layers, and deep convection), the method reverts to scalings based on a matched two‐dimensional enstrophy cascade. A principle advantage is that these subgrid models are scale‐aware, meaning that the model is suitable over a range of grid resolutions: from mesoscale grids that just permit baroclinic instabilities to grids below the submesoscale where ageostrophic effects dominate. Two approaches are presented here using Large Eddy Simulation (LES) techniques adapted for three‐dimensional rotating, stratified turbulence. The simpler approach has one nondimensional parameter, Λ, which has an optimal value near 1. The second approach dynamically optimizes Λ during simulation using a test filter. The new methods are tested in an idealized scenario by varying the grid resolution, and their use improves the spectra of potential enstrophy and energy in comparison to extant schemes. The new methods keep the gridscale Reynolds and Péclet numbers near 1 throughout the domain, which confers robust numerical stability and minimal spurious diapycnal mixing. Although there are no explicit parameters in the dynamic approach, there is strong sensitivity to the choice of test filter. Designing test filters for heterogeneous ocean turbulence adds cost and uncertainty, and we find the dynamic method does not noticeably improve over setting Λ = 1. A subgrid closure for mesoscale ocean LES is introducedThe new closure is flow and scale‐aware and specifies the Gent and McWilliams transport coefficientThe new closure compares favorably to extant closures in truncating the cascades of energy and potential enstrophy

We study the impact of the Ekman currents and Stokes drift on the surface horizontal transport and mixing properties of the Mediterranean Sea. Finite Size Lyapunov Exponents (FSLE) at the ocean surface are computed over the whole basin using 25 years of satellite altimetry derived geostrophic currents, 10‐m wind velocity and wave fields. We find that the transport pathways unveiled by the geostrophic Lagrangian Coherent Structures (LCS) are significantly modified by the ageostrophic currents (i.e., Ekman and Stokes induced velocities), often leading to a decrease of the retention capacity of the eddies. An exhaustive assessment of the regional and temporal variability of the FSLE shows an increase of the horizontal mixing processes, due to the ageostrophic component, up to 37% in regions such as the Gulf of Lion or the Aegean Sea, during the seasons where wind and waves are intense and persistent. Positive trends in the total FSLE (up to 1.2% of the value of FSLE per year in some regions) suggest that Mediterranean Sea has experienced a significant increase in the intensity of the mixing processes over the last decades. Ageostrophic features are considered to play a role in determining the properties of the relative dispersion. Through the analysis of the Lagrangian Anisotropy Index (LAI) using virtual and real pair of drifters, we observe that the particle dispersion is mainly dominated by the zonal flow, and that the ageostrophic currents induce meridional dispersion, particularly in regions where wind and wave are intensified. The knowledge of surface oceanic transport is crucial to understand the distribution of quantities of physical, biological, and chemical interest in the marine environment. Here, using available data from satellite altimetry and from reanalysis product during 25 years we analyze the impact of the wind and waves on the ocean transport properties. We show that, at the ocean surface, wind and waves modify the transport barriers, with large impact in the retention capacity of eddies and in the particle dispersion. We find that the horizontal mixing processes' intensity in the Mediterranean Sea increased on the last two decades, up to 37% in the Gulf of Lion and the Aegean Sea areas. The predominant direction in the relative dispersion over the whole Mediterranean Sea is zonal, although it can be observed a higher meridional dispersion in areas where intense regional wind and waves are relevant. These transport properties have profound consequences on the regional distribution of drifting organisms, pollutants and, more generally, any tracer that is transported by the flow. Ageostrophy modifies drastically the direction of the tracer spreading and the retention capacity of mesoscale geostrophic eddiesThere is an increase in the intensity of the mixing processes in the Mediterranean Sea over the last decades (up to 1.2%/yr in some regions)Mesoscale ageostrophic currents induce meridional dispersion in regions of the Mediterranean where wind and wave are intense Ageostrophy modifies drastically the direction of the tracer spreading and the retention capacity of mesoscale geostrophic eddies There is an increase in the intensity of the mixing processes in the Mediterranean Sea over the last decades (up to 1.2%/yr in some regions) Mesoscale ageostrophic currents induce meridional dispersion in regions of the Mediterranean where wind and wave are intense

The proxy record of global temperature shows that the dominant periodicity of the glacial cycle shifts from 40 kyr (obliquity) to 100 kyr (eccentricity) about a million years ago. Using climate model simulations, here we show that the pace of the glacial cycle depends on the pattern of hemispheric sea ice growth. In a cold climate the sea ice grows asymmetrically between two hemispheres under changes to Earth's orbital precession, because sea ice growth potential outside of the Arctic Circle is limited. This difference in hemispheric sea ice growth leads to an asymmetry in absorbed solar energy for the two hemispheres, particularly when eccentricity is high, even if the annual average insolation is similar. In a warmer climate, the hemispheric asymmetry of the sea ice decreases as mean Arctic and Antarctic sea ice decreases, diminishing the precession and eccentricity signals and explaining the dominant obliquity signal (40 kyr) before the mid‐Pleistocene transition. The proxy record shows that the dominant periodicity of the glacial cycle shifts from 40 kyr to 100 kyr about a million years agoIn a cold climate the sea ice grows asymmetrically between two hemispheres with changing Earth's orbital precession and eccentricityIn a warmer climate, the hemispheric asymmetry of the sea ice decreases, diminishing the precession and eccentricity signatures

The wave‐averaged, or Craik‐Leibovich, equations describe the dynamics of upper ocean flow interacting with nonbreaking, not steep, surface gravity waves. This paper formulates the wave effects in these equations in terms of three contributions to momentum: Stokes advection, Stokes Coriolis force, and Stokes shear force. Each contribution scales with a distinctive parameter. Moreover, these contributions affect the turbulence energetics differently from each other such that the classification of instabilities is possible accordingly. Stokes advection transfers energy between turbulence and Eulerian mean‐flow kinetic energy, and its form also parallels the advection of tracers such as salinity, buoyancy, and potential vorticity. Stokes shear force transfers energy between turbulence and surface waves. The Stokes Coriolis force can also transfer energy between turbulence and waves, but this occurs only if the Stokes drift fluctuates. Furthermore, this formulation elucidates the unique nature of Stokes shear force and also allows direct comparison of Stokes shear force with buoyancy. As a result, the classic Langmuir instabilities of Craik and Leibovich, wave‐balanced fronts and filaments, Stokes perturbations of symmetric and geostrophic instabilities, the wavy Ekman layer, and the wavy hydrostatic balance are framed in terms of intuitive physical balances. Wave effects can be formulated as Stokes advection, Stokes Coriolis force, and Stokes shear forceEach force is mapped to a distinctive type of energy transfer and, thereby, instability mechanismsStokes shear force affects dynamics on scales from Langmuir turbulence to the mesoscale

This paper provides a detailed analysis of momentum, angular momentum, vorticity, and energy budgets of a submesoscale front undergoing frontogenesis driven by an upper‐ocean, submesoscale eddy field in a Large Eddy Simulation (LES). The LES solves the wave‐averaged, or Craik‐Leibovich, equations in order to account for the Stokes forces that result from interactions between nonbreaking surface waves and currents, and resolves both submesoscale eddies and boundary layer turbulence down to 4.9 m × 4.9 m × 1.25 m grid scales. It is found that submesoscale frontogenesis differs from traditional frontogenesis theory due to four effects: Stokes forces, momentum and kinetic energy transfer from submesoscale eddies to frontal secondary circulations, resolved turbulent stresses, and unbalanced torque. In the energy, momentum, angular momentum, and vorticity budgets for the frontal overturning circulation, the Stokes shear force is a leading‐order contributor, typically either the second or third largest source of frontal overturning. These effects violate hydrostatic and thermal wind balances during submesoscale frontogenesis. The effect of the Stokes shear force becomes stronger with increasing alignment of the front and Stokes shear and with a nondimensional scaling. The Stokes shear force and momentum transfer from submesoscale eddies significantly energize the frontal secondary circulation along with the buoyancy. Overturning circulation of a submesoscale, down‐Stokes front is energized by Stokes shear force.Overturning circulation extracts momentum and kinetic energy from surrounding submesoscale eddies.Stokes shear force appears alongside buoyancy and modifies the hydrostatic balance.

Seasonal thermal fronts associated with wind‐driven coastal downwelling/upwelling in the northern South China Sea are investigated using satellite measurements and three repeated fine‐resolution mapping surveys in winter, spring, and summer. The results show that vigorous thermal fronts develop over the broad shelf with variable widths and intensities in different seasons, which tend to be approximately aligned with the 20–100 m isobaths. Driven by the prevailing winter/summer monsoon, the band‐shaped fronts were observed with a magnitude exceeding 0.1°C/km in the subsurface, and accompanied by energetic coastal downwelling/upwelling due to shoreward/offshore Ekman transport. The downward/upward tilting of seasonal thermoclines across the shelf exceeds 20 m, significantly contributing to the development of thermal fronts over the shelf. In addition, the diagnostic analysis of Potential Vorticity (PV) suggests that the summer frontal activities induced by the coastal upwelling are more stable to convection and symmetric instabilities in comparison to the winter fronts associated with downwelling‐favorable monsoon forcing. This is primarily due to their essential differences in the upper ocean stratification and horizontal buoyancy gradients arising from wind forcing. At the same time, the coastal currents are substantially regulated by the seasonal winds. An expected lag correlation between the velocity from mooring measurements and alongshore wind stress is detected near the frontal region. These results indicate that seasonal wind forcing plays an important role in the frontal activities and coastal water transport over the shelf. Seasonal band‐shaped thermal fronts were observed in the northern SCSThe thermal fronts are associated with coastal downwelling/upwellingSummer fronts are more stable than in winter

A process study using large‐eddy simulations is carried out to explore the dominant 1‐D processes that affect mixed layer (ML) properties during an event of summer Monsoon Intra‐seasonal Oscillations (MISO) in the Bay of Bengal (BOB). These simulations use realistic air‐sea fluxes and initial conditions that were collected during the summer 2018 MISO‐BOB field experiment to explore the roles of thermal inversion layer (TIL) and Langmuir turbulence (LT) in modulating ML properties. The simulations span an active period with heavy rain and strong winds and a break period with strong solar heat flux and little rain. The mixed layer depth (MLD), sea surface temperature (SST) and sea surface salinity (SSS) are most affected by the presence of near‐inertial oscillations, solar heating and precipitation, all of which occur at different timescales. The subsurface warming induced by the TIL reduces the SST variability at the MISO timescale when compared with the simulation without TIL. Comparison of simulations with and without LT indicates that LT enhances subsurface warming during the active phase and reduces diurnal SST modulation during the break phase. Simulations with 1‐D mixing models show a wide disparity in the evolution of MLD, SST, and SSS. Monsoon rain affects the lives of the more than a billion people in the Indian subcontinent. Increasing the prediction skill of monsoon forecasting models requires understanding how different weather events mix heat in the upper ocean. This study evaluates the different roles of observed rain, cooling, wind, solar warming and waves on mixing for two different ocean conditions. Motivated by recent observations in the Bay of Bengal, one condition includes warm water near the surface and cooler at depth and the other condition includes anomalous heat at depth. During the monsoon onset, surface cooling, winds and waves can release the anomalous heat, causing sea surface temperature to increase rather than decrease. Warmer water at the air‐sea interface leads to stronger evaporation and thus affects the intensity of monsoon rain. Our large‐eddy simulation study suggests that resolving subsurface processes and including the enhanced mixing effect of surface waves can potentially improve the prediction skill of forecast models, and thus, warrant further attention in the future studies. Near‐inertial oscillation, solar heating and precipitation are dominant 1D processes that set mixed layer properties during the 2018 MISO eventPreconditioning by a thermal inversion inside a barrier layer during the Monsoon Intra‐seasonal Oscillations (MISO) event can reduce the sea surface temperature (SST) variability at the MISO timescaleLarge‐eddy simulations with and without Langmuir effects show enhanced subsurface warming and reduced SST variability in the mixed layer in the former Near‐inertial oscillation, solar heating and precipitation are dominant 1D processes that set mixed layer properties during the 2018 MISO event Preconditioning by a thermal inversion inside a barrier layer during the Monsoon Intra‐seasonal Oscillations (MISO) event can reduce the sea surface temperature (SST) variability at the MISO timescale Large‐eddy simulations with and without Langmuir effects show enhanced subsurface warming and reduced SST variability in the mixed layer in the former

Variations of the sea surface temperature (SST) and primary productivity in the northeast Pacific have far‐reaching implications. In addition to influencing the regional and global temperature and hydroclimate, these conditions also control marine ecosystems and their services, which subsequently impact regional economies. Yet, our understanding of the variability and controls of northeast Pacific SST and productivity on timescales exceeding observational records remains limited. Here, we use marine sediment records from seven locations, spanning 25.2°N–59.6°N, in the northeast Pacific to characterize the millennial‐scale variability of SST and productivity from 9,000 to 1,000 years BP. We explore the dynamics of their spatiotemporal evolution and compare these data with transient climate model outputs to identify potential drivers. Through a heat budget analysis and optimal fingerprinting analysis, we characterize the spatial pattern of forcings. We find that SST varied spatially in the northeast Pacific, with higher latitudes exhibiting greater magnitude changes than lower latitudes, which differs from previous work suggesting regional synchronicity and coherence during the Holocene. Our analysis did not find evidence for coherent variability of primary producer community nor carbon export, highlighting the difficulty of identifying the complex interactions between environmental conditions, producers, and carbon export. Model‐proxy disagreement demonstrates the need for higher resolution model frameworks, but shows nonetheless that observed variability in the proxy records can be explained by a combination of greenhouse gas and orbital forcing. We suggest that the complex SST variations and marine ecosystem responses to forced changes are important factors that can drive disagreements in model projections. Holocene temperature and productivity changes in the northeast Pacific are spatially variableTemperature change in proxies is amplified poleward, and productivity is decoupled from temperatureComparison with TraCE21ka model simulations suggests that multiple forcings and dynamical processes are responsible for these changes Holocene temperature and productivity changes in the northeast Pacific are spatially variable Temperature change in proxies is amplified poleward, and productivity is decoupled from temperature Comparison with TraCE21ka model simulations suggests that multiple forcings and dynamical processes are responsible for these changes

Ocean tracers such as carbon dioxide, nutrients, plankton, and oil advect, diffuse, and react primarily in the oceanic mixed layer where air‐sea gas exchange occurs and light is plentiful for photosynthesis. There can be substantial heterogeneity in the spatial distributions of these tracers due to turbulent stirring, particularly in the submesoscale range where partly geostrophic fronts and eddies and small‐scale three‐dimensional turbulence are simultaneously active. In this study, a large eddy simulation spanning horizontal scales from 20 km down to 5 m is used to examine the effects of multiscale turbulent mixing on nonreactive passive ocean tracers from interior and sea‐surface sources. The simulation includes the effects of both wave‐driven Langmuir turbulence and submesoscale eddies, and tracers with different initial and boundary conditions are examined in order to understand the respective impacts of small‐scale and submesoscale motions on tracer transport. Tracer properties are characterized using spatial fields and statistics, multiscale fluxes, and spectra, and the results detail how tracer mixing depends on air‐sea tracer flux rate, tracer release depth, and flow regime. Although vertical fluxes of buoyancy by submesoscale eddies compete with mixing by Langmuir turbulence, vertical fluxes of tracers are often dominated by Langmuir turbulence, particularly for tracers that are released near the mixed‐layer base or that dissolve rapidly through the surface, even in regions with pronounced submesoscale activity. Early in the evolution of some tracers, negative eddy diffusivities occur co‐located with regions of negative potential vorticity, suggesting that symmetric instabilities or other submesoscale phenomenon may act to oppose turbulent mixing. A large eddy simulation is used to study the effects of multiscale turbulence on ocean tracersPassive tracer mixing depends on air‐sea tracer flux rate, tracer release depth, and flow regimeSuppressed turbulent mixing by submesoscale eddies leads to reduced entrainment of tracers

Clarifying contributions to the surface mixed layer (SML) dissipation from dynamic processes including winds, waves, buoyancy forcing and submesoscales is of significance for quantifying exchanges between the atmosphere and the ocean. Based on two observation sections across an anticyclonic eddy in the South China Sea, the contributions from different dynamic processes to the SML dissipation rate of turbulence are quantified. The potential vorticity indicates instability events including symmetric instability (SI), gravitational instability and centrifugal instability at the eddy. Despite of a dominant role of wind‐ and wave‐induced dissipation rates, SI is highlighted by a mean estimated depth‐integrated dissipation rate of 4.3 × 10−6W m kg−1with a maximum up to 3.2 × 10−5W m kg−1. The SI dissipation is believed to play a role in the eddy kinetic energy budget by extracting energy from the vertical geostrophic shear at the eddy. Active dynamic processes generated by air‐sea interactions drive a highly turbulent and dissipative ocean surface mixed layer (SML). It is important to clarify the contributions from these dynamic processes to the SML dissipation. Mesoscale eddies are rotating vortices with spatial scales of O(100) km, which are ubiquitous over the global ocean. Strong currents and fronts at mesoscale eddies exert profound effects on the SML dynamics. Based on in situ observation sections across an anticyclonic mesoscale eddy in the South China Sea, dissipation rates from winds and waves, symmetric instability (SI) and gravitational instability are characterized at the eddy using observations combined with recent theories. Consistent with the traditional assumptions, the SML dissipation is dominated by the energy produced from winds and waves. Nonetheless, the eddy‐induced vertical current shear favors a kind of smaller‐scale instability in the SML, that is, SI, which feeds on the current circulating within the eddy. The dissipation due to the SI process at the eddy can reach 4.3 × 10−6W m kg−1within the SML, which is a non‐negligible contribution to the energy budget. Turbulent dissipation in the surface mixed layer (SML) from different dynamic processes is quantitatively clarified at an anticyclonic eddyNegative potential vorticity values are observed along the eddy sections and are associated with symmetric, gravitational, and centrifugal instabilitiesSymmetric instability contributes to the integrated dissipation rate in the SML by a mean magnitude of 4.3 × 10−6W m kg−1 Turbulent dissipation in the surface mixed layer (SML) from different dynamic processes is quantitatively clarified at an anticyclonic eddy Negative potential vorticity values are observed along the eddy sections and are associated with symmetric, gravitational, and centrifugal instabilities Symmetric instability contributes to the integrated dissipation rate in the SML by a mean magnitude of 4.3 × 10−6W m kg−1

The Ocean State Ocean Model (OSOM) is an application of the Regional Ocean Modeling System spanning the Rhode Island waterways, including Narragansett Bay, Mt. Hope Bay, larger rivers, and the Block Island Shelf circulation from Long Island to Nantucket. This study discusses the physical aspects of the estuary (Narragansett and Mount Hope Bays and larger rivers) to evaluate physical circulation predictability. This estimate is intended to help decide if a forecast and prediction system is warranted, to prepare for coupling with biogeochemistry and fisheries models with widely disparate timescales, and to find the spin‐up time needed to establish the climatological circulation of the region. Perturbed initial condition ensemble simulations are combined with metrics from information theory to quantify the predictability of the OSOM forecast system–i.e., how long anomalies from different initial conditions persist. The predictability timescale in this model agrees with readily estimable timescales such as the freshwater flushing timescale evaluated using the total exchange flow (TEF) framework, indicating that the estuarine dynamics rather than chaotic transport is the dominant model behavior limiting predictions. The predictability of the OSOM is ∼7–40 days, varying with parameters, region, and season. A new model of waterways near Rhode Island is introduced and examined. The model is intended for studying the physical circulation of this region and its ecosystem changes. This study uses a variety of metrics to assess for how long a forecast with this model might be useful (i.e., how long the model's initial state determines its behavior) and relatedly how long to run (or spin up) the model to have poorly known initial conditions not affect the result systematically. Article introduces the Ocean State Ocean Model (OSOM), a Regional Ocean Modeling System implementation focused on the Rhode Island regionThe predictability of the OSOM is evaluated using information theory and initial condition ensembles in summer and winter conditionsThe flushing time scales (freshwater and salinity) of Narragansett and Mt. Hope Bays are calculated and resemble the predictability timescales, indicating that predictability is largely governed by the estuarine circulation in this model Article introduces the Ocean State Ocean Model (OSOM), a Regional Ocean Modeling System implementation focused on the Rhode Island region The predictability of the OSOM is evaluated using information theory and initial condition ensembles in summer and winter conditions The flushing time scales (freshwater and salinity) of Narragansett and Mt. Hope Bays are calculated and resemble the predictability timescales, indicating that predictability is largely governed by the estuarine circulation in this model

Surface drifter observations from the LAgrangian Submesoscale ExpeRiment (LASER) campaign in the Gulf of Mexico are paired with Eulerian (ship‐borne X‐band radar) data to demonstrate that velocity structure functions from drifters differ systematically from Eulerian structure functions over scales from 0.4 to 7 km. These differences result from drifters oversampling surface convergences and regions of intense vorticity. The first‐, second‐, and third‐order structure functions are calculated using quasi‐Lagrangian (drifter) and Eulerian data from approximately the same location and time. Differences between quasi‐Lagrangian and Eulerian structure functions are attributed to two forms of bias. The first bias results from the mean divergence or vorticity of the background flow creating nonzero first‐order structure functions. This background bias affects both quasi‐Lagrangian and Eulerian data when insufficiently time‐averaged. It severely biases the drifter third‐order structure functions but is smaller in Eulerian structure functions at both second and third order. This bias can be corrected for using lower‐order structure functions. The second form of bias results from drifters accumulating in regions with flow statistics that differ from undersampled regions. This accumulation bias is diagnosed by identifying the dependence of the Eulerian structure functions on divergence and vorticity as well as scale. Together, both biases suggest that caution is needed when interpreting second‐order drifter statistics and that linking raw third‐order drifter statistics to energy fluxes is often erroneous in ocean data: Even with background correction and sufficient time‐averaging, drifters overestimate the Eulerian estimate of the third‐order structure function by up to a factor of 5 when signs are consistent. Structure functions are a statistic used to measure the spreading of material floating in the ocean, such as plastics or spilled oil, as well as the transfer of properties like energy across scales. Their calculation requires knowledge of velocities of nearby particles. These can be measured either by (nearly) stationary instruments, such as a radar, or by tracking drifters. Offshore drifter tracking is generally easier, but they are known to be attracted to specific flow features, such as fronts, windrows, and vortices, leading to less sampling of other areas. By considering a unique data set of nearly simultaneous velocity measurements from both radar and drifters, this paper investigates how the uneven sampling by drifters, as well as the limited area coverage of radar measurements, impacts the structure function statistics and their interpretation. Structure functions calculated from observed drifters are subject to accumulation bias when compared to those from Eulerian observationsStructure functions calculated from either drifters or Eulerian data may be subject to a background bias due to mean gradients in the flowThese biases preclude inferences about energy cascades or fluxes from structure functions from drifters or localized Eulerian data

Submesoscale processes in the upper ocean vary seasonally, in tight correspondence with mixed layer thickness variability. Based on a global high‐resolution MITgcm simulation, seasonal evaluation of strong vorticity and spectral analysis of the kinetic energy in the Kuroshio Extension System show the strongest submesoscales occur in March, implying a lag of about a month behind mixed layer thickness maximum in February. An analysis of spectral energy sources and transfers indicates that the seasonality of the submesoscale energy content is a result of the competition between the conversion of available potential energy into submesoscale kinetic energy via a buoyancy production/vertical buoyancy flux associated with mixed layer instability and nonlinear energy transfers to other scales associated with an energy cascade. The buoyancy production is seasonally in phase with the mixed layer depth, but the transfers of energy across scales makes energizing the reservoir of submesoscale kinetic energy lag behind by a month. Submesoscale processes have spatial and temporal scales of O(1–10) km and O(1) day, linking the meso‐ and micro‐scales. Previous works reveal that mixed layer instability, which has extraction rates scaled with mixed layer depth, releases potential energy for the generation of submesoscale kinetic energy in the upper ocean. However, the seasonality of submesoscale kinetic energy in the Kuroshio Extension System region is determined not only by the potential energy releasing due to the mixed layer instability, but also by the energy transfers between different scales. Strongest submesoscales in the Kuroshio Extension System occur in March with a month lag behind mixed layer thickness maximum in FebruaryThe buoyancy production is in phase with the mixed layer depth, but the nonlinear transfers of energy across scales lag a few months behindThe seasonality of the submesoscale energy is a result of the competition between the buoyancy production and nonlinear energy cascade

The observational record of deep ocean variability is short, which makes it difficult to attribute the recent rise in deep ocean temperatures to anthropogenic forcing. Here we test a new proxy—the oxygen isotopic signature of individual benthic foraminifera—to detect rapid (i.e., monthly to decadal) variations in deep ocean temperature and salinity in the sedimentary record. We apply this technique at 1000 m water depth in the Eastern Equatorial Pacific during seven 200 year Holocene intervals. Variability in foraminifer δ18O over the past 200 years is below the detection limit, but δ18O signatures from two mid‐Holocene intervals indicate temperature swings >2°C within 200 years. More vigorous transport between the surface and deep ocean or stronger eddy variability than that observed in the historical record are potential explanations. Distinguishing externally forced climate trends in deep ocean properties from unforced variability should be possible with systematic analysis of suitable deep sea cores. Oxygen isotopes of individual benthic foraminifera provide a new tool for detecting rapid deep ocean variability in the sedimentary recordTemperature at 1000 m water depth in the Eastern Equatorial Pacific varied by more than 2°C during two 200 year Holocene intervalsDeep water variability was larger during two 200 year Holocene intervals in the early‐ and mid‐Holocene than over the past 200 years