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.
Plain Language Summary
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.
Key Points
Strongest submesoscales in the Kuroshio Extension System occur in March with a month lag behind mixed layer thickness maximum in February
The buoyancy production is in phase with the mixed layer depth, but the nonlinear transfers of energy across scales lag a few months behind
The seasonality of the submesoscale energy is a result of the competition between the buoyancy production and nonlinear energy cascade
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−2 and 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.
Plain Language Summary
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.
Key Points
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
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 × 10−6 W m kg−1 with a maximum up to 3.2 × 10−5 W 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.
Plain Language Summary
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 × 10−6 W m kg−1 within the SML, which is a non‐negligible contribution to the energy budget.
Key Points
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 × 10−6 W m kg−1
Abstract
Large-eddy simulations (LESs) with various constant wind, wave, and surface destabilizing surface buoyancy flux forcing are conducted, with a focus on assessing the impact of Langmuir ...turbulence on the entrainment buoyancy flux at the base of the ocean surface boundary layer. An estimate of the entrainment buoyancy flux scaling is made to best fit the LES results. The presence of Stokes drift forcing and the resulting Langmuir turbulence enhances the entrainment rate significantly under weak surface destabilizing buoyancy flux conditions, that is, weakly convective turbulence. In contrast, Langmuir turbulence effects are moderate when convective turbulence is dominant and appear to be additive rather than multiplicative to the convection-induced mixing. The parameterized unresolved velocity scale in the
K
-profile parameterization (KPP) is modified to adhere to the new scaling law of the entrainment buoyancy flux and account for the effects of Langmuir turbulence. This modification is targeted on common situations in a climate model where either Langmuir turbulence or convection is important and may overestimate the entrainment when both are weak. Nevertheless, the modified KPP is tested in a global climate model and generally outperforms those tested in previous studies. Improvements in the simulated mixed layer depth are found, especially in the Southern Ocean in austral summer.
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.
Plain Language Summary
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.
Key Points
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
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.
Key Points:
Wave effects can be formulated as Stokes advection, Stokes Coriolis force, and Stokes shear force
Each force is mapped to a distinctive type of energy transfer and, thereby, instability mechanisms
Stokes shear force affects dynamics on scales from Langmuir turbulence to the mesoscale
Oceanic mesoscale eddies are known to diffuse and stir tracers, and the development of skillful eddy closures is aided considerably by the accurate diagnosis of these processes from eddy‐resolving ...model statistics. In this work a multiple‐tracers inversion method is applied to a global mesoscale eddy‐resolving simulation, with the intent to solve for the eddy transport tensor that describes the eddy diffusion (symmetric part) and stirring (antisymmetric part). Special emphasis is placed on diagnosing the anisotropy of the horizontal transport, which is described by the eigenvalues and eigenvectors of the
2×2 horizontal symmetric subtensor. Global diagnoses of these quantities, along with an examination of their vertical structures, are used to recommend an algorithm for extending the Gent and McWilliams and Redi parameterizations to include anisotropic effects.
Plain Language Summary
Tracer transport by ocean mesoscale eddies is usually parameterized using a flux‐gradient relationship with a scalar transport coefficient for the horizontal fluxes. Using a scalar coefficient implies that the transport is horizontally isotropic. There exist many mechanisms that may lead to anisotropic transport, however, and parameterizing this anisotropy requires a (symmetric) tensor coefficient. We diagnose this tensor coefficient using a high‐resolution global ocean model with an inversion method that uses multiple passive tracers. The statistics from the diagnosis allow us to explore the properties of the flow that govern the anisotropy, and to recommend an anisotropic extension of extant tracer parameterization schemes.
Key Points
Anisotropic eddy diffusivity is diagnosed with a multiple tracers inversion technique
An eigendecomposition of the horizontal diffusion tensor determines the 3‐D parameterized transport
This diagnosis informs an anisotropic extension to the GM‐Redi parameterization
The turbulent mixing in thin ocean surface boundary layers (OSBL), which occupy the upper 100 m or so of the ocean, control the exchange of heat and trace gases between the atmosphere and ocean. Here ...we show that current parameterizations of this turbulent mixing lead to systematic and substantial errors in the depth of the OSBL in global climate models, which then leads to biases in sea surface temperature. One reason, we argue, is that current parameterizations are missing key surface‐wave processes that force Langmuir turbulence that deepens the OSBL more rapidly than steady wind forcing. Scaling arguments are presented to identify two dimensionless parameters that measure the importance of wave forcing against wind forcing, and against buoyancy forcing. A global perspective on the occurrence of wave‐forced turbulence is developed using re‐analysis data to compute these parameters globally. The diagnostic study developed here suggests that turbulent energy available for mixing the OSBL is under‐estimated without forcing by surface waves. Wave‐forcing and hence Langmuir turbulence could be important over wide areas of the ocean and in all seasons in the Southern Ocean. We conclude that surface‐wave‐forced Langmuir turbulence is an important process in the OSBL that requires parameterization.
Key Points
Climate models have biases in the depth of the ocean surface boundary layer
Langmuir turbulence is a key process mixing the ocean surface boundary layer
Langmuir turbulence deepens the layer more quickly than wind‐forced turbulence
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.
Plain Language Summary
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.
Key Points
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
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