Submesoscale Instabilities Research Articles

Submesoscale Processes in the Northern Red Sea: Insights From Underwater Glider Observations

AbstractThe critical role of oceanic submesoscale currents in promoting energy cascade and modulating biogeochemical processes as well as the heat budget in the upper ocean has gained wide recognition. Although high‐resolution numerical simulations have enabled qualitative investigation of the spatiotemporal variability of submesoscale processes in the north Red Sea (NRS), observational evidence remains scarce. This study investigated the submesoscale processes in the NRS through field observations of underwater gliders. High‐resolution glider and satellite observation data sets reveal the spatiotemporal variation characteristics of submesoscale fronts and deepen mixed layer depth during winter. Diagnosis of potential vorticity (PV) and classifications of submesoscale instabilities demonstrate conducive conditions for the mixed layer baroclinic, gravitational, and symmetric instability. The significant negative PV induced by atmospheric cooling associated with robust fronts promotes the development of submesoscale processes. Combining the Omega equation with biogeochemical observations suggests that coherent pathways via submesoscale processes lead to the vertical transport of biomass and oxygen patches, supplying nutrients into the euphotic layer and ventilating hypoxic waters at depths. These results demonstrate the fundamental role of submesoscale processes in the ocean dynamics of the NRS.

Asymmetry of Submesoscale Instabilities in Anticyclonic and Cyclonic Eddies

AbstractThe upper‐ocean relative vorticity has been found to be cyclonically skewed, but altimetry observations indicate that long‐lifespan mesoscale eddies tend to be anticyclonic. We are thus interested in whether cyclonic or anticyclonic eddies are more unstable under similar circumstances. Here we use submesoscale‐resolving simulations of idealized mesoscale eddies, incorporating theoretical analyses, to investigate asymmetries of submesoscale instabilities within the anticyclones and cyclones. It is found that submesoscale filaments initiate at regions with the largest horizontal buoyancy gradients for both anticyclones and cyclones, but these filaments subsequently rotate outward in anticyclones while inward in cyclones. Hence submesoscales are more vigorous at anticyclone peripheries and the cyclone center. Such differing distributions and evolutions of submesoscale processes are primarily caused by changes in the background stratification associated with the decaying of mesoscale eddies. The active submesoscales near the cyclone center eventually distort its core structure radically, whereas the anticyclone remains largely unaffected.

Characterization of physical properties of a coastal upwelling filament with evidence of enhanced submesoscale activity and transition from balanced to unbalanced motions in the Benguela upwelling region

Abstract. We combine high-resolution in situ data (acoustic Doppler current profiler (ADCP), Scanfish, and surface drifters) and remote sensing to investigate the physical characteristics of a major filament observed in the Benguela upwelling region. The 30–50 km wide and about 400 km long filament persisted for at least 40 d. Mixed-layer depths were less than 40 m in the filament and over 60 m outside of it. Observations of the Rossby number Ro from the various platforms provide the spatial distribution of Ro for different resolutions. Remote sensing focuses on geostrophic motions of the region related to the mesoscale eddies that drive the filament formation and thereby reveals |Ro|<0.1. Ship-based measurements in the surface mixed layer reveal 0.5<|Ro|<1, indicating the presence of unbalanced, ageostrophic motions. Time series of Ro from triplets of surface drifters trapped within the filament confirm these relatively large Ro values and show a high variability along the filament. A scale-dependent analysis of Ro, which relies on the second-order velocity structure function, was applied to the latter drifter group and to another drifter group released in the upwelling zone. The two releases explored the area nearly distinctly and simultaneously and reveal that at small scales (<15 km) Ro values are twice as large in the filament in comparison to its environment with Ro>1 for scales smaller than ∼500 m. This suggests that filaments are hotspots of ageostrophic dynamics, pointing to the presence of a forward energy cascade. The different dynamics indicated by our Ro analysis are confirmed by horizontal kinetic energy wavenumber spectra, which exhibit a power law k−α with α∼5/3 for wavelengths 2π/k smaller than a transition scale of 15 km, supporting significant submesoscale energy at scales smaller than the first baroclinic Rossby radius (Ro1∼30 km). The detected transition scale is smaller than those found in regions with less mesoscale eddy energy, consistent with previous studies. We found evidence for the processes which drive the energy transfer to turbulent scales. Positive Rossby numbers 𝒪(1) associated with cyclonic motion inhibit the occurrence of positive Ertel potential vorticity (EPV) and stabilize the water column. However, where the baroclinic component of EPV dominates, submesoscale instability analysis suggests that mostly gravitational instabilities occur and that symmetric instabilities may be important at the filament edges.

Instabilities Across the Agulhas Current Enhance Upward Nitrate Supply in the Southwest Subtropical Indian Ocean

AbstractThe Agulhas Current, like other western boundary currents (WBCs), transports nutrients laterally from the tropics to the subtropics in a subsurface “nutrient stream.” These nutrients are predominantly supplied to surface waters by seasonal convective mixing, to fuel a brief period of productivity before phytoplankton become nutrient‐limited. Episodic mixing events characteristic of WBC systems can temporarily alleviate nutrient scarcity by vertically entraining deep nutrients into surface waters. However, our understanding of these nutrient fluxes is lacking because they are spatio‐temporally limited, and once they enter the sunlit layer, the nutrients are rapidly consumed by phytoplankton. Here, we use a novel application of nitrate Δ(15–18), the difference between the nitrogen and oxygen isotope ratios of nitrate, to characterize three (sub)mesoscale events of upward nitrate supply across the Agulhas Current in winter: (1) mixing at the edges of an anticyclonic eddy, (2) inshore upwelling associated with a submesoscale meander of the Agulhas Current, and (3) overturning at the edge of the current core driven by submesoscale instabilities. All three events manifest as upward injections of high‐Δ(15–18) nitrate into the thermocline and surface where nitrate Δ(15–18) is otherwise low; these entrainment events are not always apparent in the other co‐collected data. The dynamics driving the nitrate supply events are common to all WBCs, implying that nutrient entrainment facilitated by WBCs is quantitatively significant and supports productivity in otherwise oligotrophic subtropical surface waters. A future rise in energy across WBC systems may increase these nutrient fluxes, partly offsetting the predicted stratification‐induced decrease in subtropical ocean fertility.

Cessation of Labrador Sea Convection Triggered by Distinct Fresh and Warm (Sub)Mesoscale Flows

Abstract By ventilating the deep ocean, deep convection in the Labrador Sea plays a crucial role in the climate system. Unfortunately, the mechanisms leading to the cessation of convection and, hence, the mechanisms by which a changing climate might affect deep convection remain unclear. In winter 2020, three autonomous underwater gliders sampled the convective region and both its spatial and temporal boundaries. Both boundaries are characterized by higher subdaily mixed layer depth variability sampled by the gliders than the convective region. At the convection boundaries, buoyant intrusions—including eddies and filaments—instead of atmospheric warming primarily trigger restratification by bringing buoyancy with a comparable contribution from either fresh or warm intrusions. At the edges of these intrusions, submesoscale instabilities, such as symmetric instabilities and mixed layer baroclinic instabilities, seem to contribute to the decay of the intrusions. In winter, enhanced lateral buoyancy gradients are correlated with strong destabilizing surface heat fluxes and alongfront winds. Consequently, winter atmospheric conditions and buoyant intrusions participate in halting convection by triggering restratification while surface fluxes are still destratifying. This study reveals freshwater anomalies in a narrow area offshore of the Labrador Current and near the convective region; this area has received less attention than the more eddy-rich West Greenland Current, but is a potential source of freshwater in closer proximity to the region of deep convection. Freshwater fluxes from the Arctic and Greenland are expected to increase under a changing climate, and our findings suggest that they may play an active role in the restratification of deep convection.

Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders

Abstract The western boundary current system off southeastern Brazil is composed of the poleward-flowing Brazil Current (BC) in the upper 300 m and the equatorward flowing Intermediate Western Boundary Current (IWBC) underneath it, forming a first-baroclinic mode structure in the mean. Between 22° and 23°S, the BC-IWBC jet develops recurrent cyclonic meanders that grow quasi-stationarily via baroclinic instability, though their triggering mechanisms are not yet well understood. Our study, thus, aims to propose a mechanism that could initiate the formation of these mesoscale eddies by adding the submesoscale component to the hydrodynamic scenario. To address this, we perform a regional 1/50° (∼2 km) resolution numerical simulation using CROCO (Coastal and Regional Ocean Community model). Our results indicate that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instability that can trigger the meanders’ formation. Subsequently, this process generates submesoscale cyclones that contribute, along with baroclinic instability, to the meanders’ growth, resulting in a submesoscale-to-mesoscale inverse cascade. Last, as the mesoscale cyclones grow, they interact with the slope, generating inertially and symmetrically unstable anticyclonic submesoscale vortices and filaments. Significance Statement Off southeastern Brazil, the Brazil Current develops recurrent cyclonic meanders. Such meanders enhance the open-ocean primary productivity and are of societal importance as they are located in a region rich in oil and gas where oil-spill accidents have already happened. This study aims to explore the processes responsible for triggering the formation of these mesoscale eddies. We find that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instabilities that eject submesoscale filaments and vortices and can trigger the meanders’ formation. Such results show that topographically generated submesoscale instabilities can play an important role in the dynamics of mesoscale meanders off southeastern Brazil. Moreover, this may indicate that resolving the submesoscale dynamics in operational numerical models may contribute to an increase in the predictability of the regional eddies.

Spatial Variability of Turbulent Mixing From an Underwater Glider in a Large, Deep, Stratified Lake

AbstractRecent efforts using microstructure turbulence measurements have contributed to our understanding of the overall energy budget in lakes and linkages to vertical fluxes. A paucity of lake‐wide turbulence measurements hinders our ability to assess how representative such budgets are at the basin scale. Using an autonomous underwater glider equipped with a microstructure payload, we explored the spatial variability of turbulence in pelagic and near‐shore regions of Lake Geneva. Dissipation rates of kinetic energy and thermal variance were estimated by fitting temperature gradient fluctuations spectra to the Batchelor spectrum. In deep waters, turbulent dissipation rates in the surface and thermocline were mild (∼10−8 W kg−1) and weakened toward the hypolimnion (∼10−11 to 10−10 W kg−1). The seasonal thermocline exhibited inhibited interior mixing, with extremely low values of mixing efficiency (Rif ≪ 0.1). In contrast, in the slope zone, a band of significantly enhanced energy dissipation (∼5 × 10−8 W kg−1) extended well above the bottom boundary layer and was associated with strong, efficient mixing (Rif > 0.17). The resulting contribution of the slope region to basin‐scale mixing was large, with 90% of the basin‐wide mixing—and only 30% energy dissipation—occurring within 4 km of the shoreline. This boundary mixing will modify overturning circulation and the transport pathways of dissolved compounds exchanged with the sediments. The dynamics responsible for the shift in the mixing regime, which appears crucial for the mixing budget of lakes, could not be fully unraveled with the collected observations. Additional model data analyses hint at the role of submesoscale instabilities.

Assessing the Spatio-Temporal Features and Mechanisms of Symmetric Instability Activity Probability in the Central Part of the South China Sea Based on a Regional Ocean Model

Symmetric instability (SI) is credited with one of the important submesoscale instabilities. However, due to its small scales, it is challenging to capture using current observational measurements and ocean models. Estimates of SI activity are useful for assessing whether SI should be parameterized. Based on a high-resolution ocean model, we use a criterion to assess the spatio-temporal features of SI activity without directly solving SI in the Xisha–Zhongsha waters. An Ertel potential vorticity (PV) analysis is performed, and the negative PV injection and frontal tendency are calculated to analyze the generation mechanisms. The results show that the activity of SI is strongly seasonal. In comparison, SI is active in winter, but it is inactive in summer. In addition, it is mainly found within the ocean surface mixed layer (SML), and it almost disappears in the base of the surface mixed layer (BML). Moreover, the vertical component of the Ertel PV leads to the vertical spatial difference of SI activity, and both the vertical component of the Ertel PV and the sea surface buoyancy flux play an important role in the seasonality of SI activity. The stronger frontogenesis around the Xisha Islands partially accounts for the horizontal distribution difference of SI. This work implies that the parameterization of SI may have potential value in practical application in this region in winter due to the high probability of SI activity.

Scale‐Dependent Temperature‐Salinity Compensation in Frontal Regions of the Taiwan Strait

AbstractBased on high‐resolution, cross‐frontal towed measurements, this study investigates temperature‐salinity (T‐S) compensation and its scale dependence in the Taiwan Strait. In winter and spring when the northeasterly monsoon is prevailing, colder and fresher waters flow southward along the coast of the Chinese mainland, which encounter the northward‐flowing, warmer and saltier waters in the Taiwan Strait. Two slanted density fronts are generated across the interfacing zone forming a cone‐shaped isopycnal distribution in the cross‐frontal direction. Analyses based on the density ratio and Turner angle suggest the expected S‐dominated and T‐dominated types of density variations at the western and eastern density fronts, respectively. Exact T‐S compensation is observed within the interfacing zone. Analysis of the scale dependence indicates that temperature and salinity get more (less) compensated from O(10 km) to O(1 km) scale in the S‐dominated (T‐dominated) frontal zone. Although contrasting density compensating features are observed on the two flanks of the transition zone, they can both be interpreted by the restratification‐cooling mechanism. Specifically, the overturning cells due to submesoscale instabilities in the upper mixed layer slump isopycnals in the frontal zone, inducing shoaling of the mixed layer at both ends of the sampled sections. Surface cooling results in larger temperature drops in the shallower mixed layers, and thus increases (decreases) the degree of T‐S compensation in the S‐dominated (T‐dominated) frontal zone at the scale for submesoscale instabilities to develop.

Evaluating the effects of a symmetric instability parameterization scheme in the Xisha-Zhongsha waters, South China Sea in winter

As one of the important submesoscale instabilities, symmetric instability (SI) widely exists in the ocean surface mixed layer (SML), which enhances the vertical material transport in the SML and also the exchanges between the SML and the ocean interior. Due to the small spatial scales of SI, O (10 m–1 km), which are not resolved by most current ocean models, the application of SI parameterization is an alternative choice in the coming decades to include the SI effects in ocean models and improve the model performance. In this study, we evaluate the impacts of SI in a realistic configuration with the SI parameterization scheme applied in the Xisha-Zhongsha waters, South China Sea in winter by using the Coastal and Regional Ocean Community Model (CROCO) version of the Regional Ocean Modeling System. Compared to the SI-lacking case, the SI energy source, the geostrophic shear production, is increased and elimination of anticyclonic potential vorticity is revealed in the SI-parameterized case. According to the energy analysis, multi-scale interactions are also influenced by the SI. The effective wind energy input is reduced, and the potential energy release in the SML is suppressed. Moreover, the SI scheme makes the SML depth shallower and closer to the reanalysis one. This work demonstrates a good performance of the SI scheme applied in regional models in representing SI effects.

Off‐Coast Phytoplankton Bloom in the Taiwan Strait During the Northeasterly Monsoon Wind Relaxation Period

AbstractThis study applied cruise, model, and satellite data to analyze the off‐coast phytoplankton blooming during the late fall to early spring monsoon period in the Taiwan Strait when northeasterly wind prevails. Based on the composite and self‐organizing map analyses, the three data sets consistently show high chlorophyll‐a concentration near the along‐shore front during the down‐front northeasterly wind relaxation period while lower concentration when relatively strong wind is persistent. Meanwhile, the off‐coast blooming always coincides with intense near‐surface stratification when the northeasterly wind relaxes. Diagnoses of balanced Richardson number, Ertel potential vorticity and instability energy budget from high‐resolution cruise observations and model results demonstrate that vigorous submesoscale symmetric and baroclinic instabilities can develop near the along‐shore front under the down‐front NE wind. Diagnoses of modeled buoyancy and chlorophyll‐a budget equations further suggest the submesoscale instabilities lead to rapid near‐surface restratification and offshore stretching of the along‐shore front within the upper 10‐m of the mixed layer when the down‐front NE wind relaxes, favoring the surface 10‐m phytoplankton growth. As comparison, contribution of the larger‐scale advection related with geostrophic adjustment and Ekman transport to the chlorophyll‐a increment reached beyond the middle layer of ∼20‐m depth.

Submesoscale frontal waves and instabilities driven by sheared flows

Submesoscale processes at the frontal region are frequently observed in the form of submesoscale fronts, eddies, and filaments due to a variety of submesoscale instabilities. Previous studies tended to emphasize baroclinic conversions (e.g. mixed-layer instability) and few efforts have focused on the contribution of barotropic instability. In the present study, a series of idealized numerical simulations are implemented to examine how submesoscale structures are generated and evolve on conditions of different (horizontal and vertical) flow shear. The results show that barotropic instability would slow down or even restrain the growth of propagating submesoscale frontal waves, but instead, generate slower, larger phase-locked frontal waves. These waves with large amplitude significantly advect the denser, colder water to the warmer, lighter side. This provides active available potential energy and subsequently causes strong buoyancy production. In that context, barotropic instability also greatly contributes to the generation of submesoscale features, restratification, and vertical transport in the upper ocean. But this process cannot be identified in most time-averaged analyses. In this paper, the energy variation with time is also studied based on the spatial mean by Reynolds decomposition, the results of which suggest that the energy cascade paths caused by baroclinic instability and barotropic instability are different, but their contributions to downscale energy transition are equally important.

The Role of Freshwater Forcing on Surface Predictability in the Gulf of Mexico

AbstractThe potential predictability of fields at the ocean surface in the northern Gulf of Mexico (GoM) is investigated through five ensembles of regional ocean simulations between 2014 and 2016. The ensembles explore two horizontal resolutions and different representations of the riverine inflow, and focus on the interactions between the Loop Current system (LCS) and the riverine system. The potential predictability of the surface fields is high when simulated by an ocean‐only model forced by appropriate atmospheric forcing and boundary conditions, and the ensembles simulate similar LCS behavior up to 5 months. The ensemble spread provides a mean to quantify the potential predictability. The ensembles confirm that LCS‐riverine interactions are modulated by the LC mesoscale variability. The relationship is two‐ways, with the LCS being influenced by—And not only influencing—The freshwater plume. Whenever the freshwater flux is strong, the northward extension of the LCS is constrained by the intensified salinity fronts. This influence is slightly stronger if the riverine inflow is simulated in an active fashion with a meridional velocity component proportional to the flux. Sea surface temperature (SST) and salinity (SSS) predictability have opposite seasonality in their signal, with the SST (SSS) being more predictable in summer (winter). Partially resolving submesoscale instabilities and improving the realism of the riverine fluxes’ representation causes the spread to increase, especially in SST. When increasing resolution, the spread increases also for surface vorticity due to feedbacks between the mesoscale and submesoscale circulations. The intraseasonal and interannual signal in vorticity is however similar among ensembles.

Frontal Dynamics in the Alboran Sea: 2. Processes for Vertical Velocities Development

AbstractSignificant lateral density gradients occur throughout the year in the Alboran Sea, giving rise to two main fronts: the Western Alboran Gyre Front (WAGF) and Eastern Alboran Gyre Front (EAGF), where large vertical velocities often develop. To improve the understanding of the processes that underlie the development of the vertical velocities in the fronts, the periods of development were analyzed in the perspective of the frontogenesis, instabilities, non‐linear Ekman, and filamentogenesis mechanisms, using multi‐platform in‐situ observations and a high‐resolution realistic simulation in spring 2018. The spatio‐temporal characteristics of the WAGF indicate a wider, deeper, and longer‐lasting front than the EAGF. Additionally, the WAGF shows stronger and deeper upwelling and downwelling regions. The WAGF vertical velocities (up to |55| m/day) are amplified by an across‐front ageostrophic secondary circulation generated by: (a) frontal intensification explained by frontogenesis, which shows a sharpening of buoyancy gradients associated with the Atlantic Jet, (b) nonlinear Ekman effects, that are enhanced by the persistent western wind blowing along the frontal direction, and (c) submesoscale instabilities (symmetric and ageostrophic baroclinic instabilities). The EAGF vertical velocities (up to |38| m/day) are amplified by two asymmetrical ageostrophic cells developed across the front with a narrow upwelling region in the middle. The cell's circulation is explained by frontal intensification produced by filamentogenesis through a cold filament advection to the Mediterranean Sea interior, that is characterized by pointy isopycnals at the center of the filament. This mechanism is observed in both the model and glider observations.

Seasonal to Intraseasonal Variability of the Upper Ocean Mixed Layer in the Gulf of Oman

AbstractHigh‐resolution underwater glider data collected in the Gulf of Oman (2015–2016), combined with reanalysis data sets, describe the spatial and temporal variability of the mixed layer during winter and spring. We assess the effect of surface forcing and submesoscale processes on upper ocean buoyancy and their effects on mixed layer stratification. Episodic strong and dry wind events from the northwest (Shamals) drive rapid latent heat loss events which lead to intraseasonal deepening of the mixed layer. Comparatively, the prevailing southeasterly winds in the region are more humid, and do not lead to significant heat loss, thereby reducing intraseasonal upper ocean variability in stratification. We use this unique data set to investigate the presence and strength of submesoscale flows, particularly in winter, during deep mixed layers. These submesoscale instabilities act mainly to restratify the upper ocean during winter through mixed layer eddies. The timing of the spring restratification differs by three weeks between 2015 and 2016 and matches the sign change of the net heat flux entering the ocean and the presence of restratifying submesoscale fluxes. These findings describe key high temporal and spatial resolution drivers of upper ocean variability, with downstream effects on phytoplankton bloom dynamics and ventilation of the oxygen minimum zone.

Sound speed, submesoscale, and spice: The role of rapidly evolving instabilities in setting upper ocean properties

Sound speed characteristics in the upper ocean are sometimes thought to be set by a combination of mostly one-dimensional vertical mixing processes and (slow) mesoscale advection and stirring. However, a series of recent projects have showcased the role of small-scale (tens of meters to tens of km horizontally), rapidly evolving (hours to days) submesoscale instabilities in setting both stratification and sound speed properties. The observations also reveal that sound speed structure set by such processes is fundamentally three dimensional, with significant lateral gradients at a broad range of scales. Some relevant processes are frontogenetic and create sometimes rapid secondary circulations near lateral density gradients. Some are associated with wind-driven near-inertial oscillations or internal waves. Some of the more interesting recent observations concern the intersection of submesoscale and internal wave processes. Here, I will review a few highlights from the last decade of observations on unbalanced, nonlinear processes that conspire to set density, spice, and sound speed profiles in several contrasting oceans.

Submesoscale Vorticity and Divergence in the Alboran Sea: Scale and Depth Dependence

The statistics of submesoscale divergence and vorticity (kinematic properties, KPs) in the Alboran Sea (Mediterranean Sea) are investigated, using data from drifters released during two experiments in June 2018 and April 2019 in the framework of the Coherent Lagrangian Pathways from the Surface Ocean to Interior (CALYPSO) project. Surface drifters sampling the first meter of water (CARTHE and CODE) and 15 m drifters (SVP) are considered. The area of interest is dominated by processes of strong frontogenesis and eddy formation as well as mixing, related to the high lateral gradients between Mediterranean and Atlantic waters. Drifter coverage and distribution allow to investigate the dependence of KPs on horizontal scales in a range between 1 and 16 km, that effectively bridges submesoscale and mesoscale processes, and at two depths, of 1 and 15 m. For both experiments, the surface flow is highly ageostrophic at 1 km scale, with positive vorticity skewness indicating the presence of submesoscale features. Surface divergence quickly decreases at increasing scales with a slope compatible with a turbulent process with broadband wavenumber spectrum, suggesting the influence of surface boundary layer processes such as wind effects, waves and Langmuir cells at the smaller scales. Vorticity, on the other hand, has a significantly slower decay, suggesting interaction between submesoscale and mesoscale dynamics. Results at 15 m are characterized by reduced ageostrophic dynamics with respect to the surface, especially for divergence. Submesoscale processes are present but appear attenuated in terms of KP magnitude and skewness. The results are generally consistent for the two experiments, despite the observed differences in the mixed layer stratification, suggesting that submesoscale instabilities occur mostly at surface fronts associated with filaments of Atlantic and Mediterranean waters that are present in both cases. The results are compared with previous literature results in other parts of the world ocean and a synthesis is provided. Good agreement with previous surface results is found, suggesting some general properties for divergence and vorticity scale dependence. The importance of further investigating very high resolution frontal processes at scales of tens of meters, as well as processes of interaction with high wind effects is highlighted.

Parameterization of Submesoscale Symmetric Instability in Dense Flows Along Topography

AbstractWe develop a parameterization for representing the effects of submesoscale symmetric instability (SI) in the ocean interior. SI may contribute to water mass modification and mesoscale energy dissipation in flow systems throughout the World Ocean. Dense gravity currents forced by surface buoyancy loss over shallow shelves are a particularly compelling test case, as they are characterized by density fronts and shears susceptible to a wide range of submesoscale instabilities. We present idealized experiments of Arctic shelf overflows employing the GFDL‐MOM6 in z* and isopycnal coordinates. At the highest resolutions, the dense flow undergoes geostrophic adjustment and forms bottom‐ and surface‐intensified jets. The density front along the topography combined with geostrophic shear initiates SI, leading to onset of secondary shear instability, dissipation of geostrophic energy, and turbulent mixing. We explore the impact of vertical coordinate, resolution, and parameterization of shear‐driven mixing on the representation of water mass transformation. We find that in isopycnal and low‐resolution z* simulations, limited vertical resolution leads to inadequate representation of diapycnal mixing. This motivates our development of a parameterization for SI‐driven turbulence. The parameterization is based on identifying unstable regions through a balanced Richardson number criterion and slumping isopycnals toward a balanced state. The potential energy extracted from the large‐scale flow is assumed to correspond to the kinetic energy of SI which is dissipated through shear mixing. Parameterizing submesoscale instabilities by combining isopycnal slumping with diapycnal mixing becomes crucial as ocean models move toward resolving mesoscale eddies and fronts but not the submesoscale phenomena they host.

Submesoscale Eddies in the Upper Ocean of the Kuroshio Extension from High-resolution Simulation: Energy Budget

AbstractThe submesoscale energy budget is complex and remains understood only in region-by-region analyses. Based on a series of nested numerical simulations, this study investigated the submesoscale energy budget and flux in the upper ocean of the Kuroshio Extension, including some innovations for examining submesoscale energy budgets in general. The highest-resolution simulation on a ~500 m grid resolves a variety of submesoscale instabilities allowing an energetic analysis in the submesoscale range. The frequency–wavenumber spectra of vertical vorticity variance (i.e., enstrophy) and horizontal divergence variance were used to identify the scales of submesoscale flows as distinct from those of inertia-gravity waves but dominating horizontal divergence variance. Next, the energy transfers between the background scales and the submesoscale were examined. The submesoscale kinetic and potential energy (SMKE and SMPE) were mainly contained in the mixed layer and energized through both barotropic (shear production) and baroclinic (buoyancy production) routes. Averaged over the upper 50 m of ROMS2, the baroclinic transfers amounted to approximately 75% of the sources for the SMKE (3.42 × 10−9 W/kg) versus the remaining 25% (1.12 × 10−9 W/kg) via barotropic downscale KE transfers. The KE field was greatly strengthened by energy sources through the boundary—this flux is larger than the mesoscale-to-submesoscale transfers in this region. Spectral energy production, importantly, reveals upscale KE transfers at larger submesoscales and downscale KE transfers at smaller submesoscales (i.e., a transition from inverse to forward KE cascade). This study seeks to extend our understanding of the energy cycle to the submesoscale and highlight the forward KE cascade induced by upper-ocean submesoscale activities in the research domain.

Structure of Submesoscale Fronts of the Mississippi River Plume

AbstractSubmesoscale currents (SMCs), in the forms of fronts, filaments, and vortices, are studied using a high-resolution (~150 m) Regional Oceanic Modeling System (ROMS) simulation in the Mississippi River plume system. Fronts and filaments are identified by large horizontal velocity and buoyancy gradients, surface convergence, and cyclonic vertical vorticity with along-coast fronts and along-plume-edge filaments notably evident. Frontogenesis and arrest/destruction are two fundamental phases in the life cycle of fronts and filaments. In the Mississippi River plume region, the horizontal advective tendency induced by confluence and convergence plays a primary role in frontogenesis. Confluent currents sharpen preexisting horizontal buoyancy gradients and initiate frontogenesis. Once the fronts and filaments are formed and the Rossby number reaches O(1), they further evolve frontogenetically mainly by convergent secondary circulations, which can be maintained by different cross-front momentum balance regimes. Confluent motions and preexisting horizontal buoyancy gradients depend on the interaction between wind-induced Ekman transport and the spreading plume water. Consequently, the direction of wind has a significant effect on the temporal variability of SMCs, with more active SMCs generated during a coastally downwelling-favorable wind and fewer SMCs during an upwelling-favorable wind. Submesoscale instabilities (~1–3 km) play a primary role in the arrest and fragmentation of most fronts and filaments. These instabilities propagate along the fronts and filaments, and their energy conversion is a mixed barotropic–baroclinic type with horizontal-shear instabilities dominating.