We 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
Submesoscale turbulence in the upper ocean consists of fronts, filaments, and vortices that have horizontal scales on the order of 100 m to 10 km. High-resolution numerical simulations have suggested that submesoscale turbulence is associated with strong vertical motion that could substantially enhance the vertical exchange between the thermocline and mixed layer, which may have an impact on marine ecosystems and climate. Theoretical, numerical, and observational work indicates that submesoscale turbulence is energized primarily by baroclinic instability in the mixed layer, which is most vigorous in winter. This study demonstrates how such mixed layer baroclinic instabilities induce vertical exchange by drawing filaments of thermocline water into the mixed layer. A scaling law is proposed for the dependence of the exchange on environmental parameters. Linear stability analysis and nonlinear simulations indicate that the exchange, quantified by how much thermocline water is entrained into the mixed layer, is proportional to the mixed layer depth, is inversely proportional to the Richardson number of the thermocline, and increases with increasing Richardson number of the mixed layer. The results imply that the tracer exchange between the thermocline and mixed layer is more efficient when the mixed layer is thicker, when the mixed layer stratification is stronger, when the lateral buoyancy gradient is stronger, and when the thermocline stratification is weaker. The scaling suggests vigorous exchange between the permanent thermocline and deep mixed layers in winter, especially in mode water formation regions.
This study examines how instabilities in the surface layer of the ocean bring interior water up from below. This interior–surface exchange can be important for dissolved gases such as carbon dioxide and oxygen as well as nutrients fueling biological growth in the surface ocean. A scaling law is proposed for the dependence of the exchange on environmental parameters. The results of this study imply that the exchange is particularly strong if the well-mixed surface layer is thick, lateral density gradients are strong (such as at fronts), and the stratification below the surface layer is weak. These theoretical findings can be implemented in boundary layer parameterization schemes in global ocean models and improve our understanding of the marine ecosystem and how the ocean mediates climate change.
- NSF-PAR ID:
- 10476011
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 53
- Issue:
- 12
- ISSN:
- 0022-3670
- Format(s):
- Medium: X Size: p. 2701-2716
- Size(s):
- ["p. 2701-2716"]
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract 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‐resolutionz * 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. -
more » « less
Abstract Drake Passage is a key region for transport between the surface and interior ocean, but a mechanistic understanding of this exchange remains immature. Here, we present wintertime, submesoscale‐resolving hydrographic transects spanning the southern boundary of the Antarctic Circumpolar Current and the Polar Front (PF). Despite the strong surface wind and buoyancy forcing, a freshwater lens suppresses surface‐interior exchange south of the PF; ventilation is instead localized to the PF. Multiple lines of the analysis suggest submesoscale processes contribute to ventilation at the PF, including small‐scale, O(10 km), frontal structure in water mass properties below the mixed layer and modulation of a surface eddy diffusivity at sub‐50 km scales. These results show that ventilation is sensitive to both submesoscale properties near fronts and non‐local processes, for example, sea‐ice melt, that set stratification and mixed layer properties. This highlights the need for adaptive observing strategies to constrain Southern Ocean heat and carbon budgets.
-
more » « less
Abstract Over the Texas-Louisiana Shelf in the Northern Gulf of Mexico, the eutrophic, fresh Mississippi/Atchafalaya river plume isolates saltier waters below, supporting the formation of bottom hypoxia in summer. The plume also generates strong density fronts, features of the circulation that are known pathways for the exchange of water between the ocean surface and the deep. Using high-resolution ocean observations and numerical simulations, we demonstrate how the summer land-sea breeze generates rapid vertical exchange at the plume fronts. We show that the interaction between the land-sea breeze and the fronts leads to convergence/divergence in the surface mixed layer, which further facilitates a slantwise circulation that subducts surface water along isopycnals into the interior and upwells bottom waters to the surface. This process causes significant vertical displacements of water parcels and creates a ventilation pathway for the bottom water in the northern Gulf. The ventilation of bottom water can bypass the stratification barrier associated with the Mississippi/Atchafalaya river plume and might impact the dynamics of the region’s dead zone.
-
more » « less
Abstract Lateral intrusions flowing from the boundaries of lakes and oceans might influence basin‐scale distributions of heat, chemical solutes, sediments, and organisms. Here, observations of numerous intrusions from multiple locations in a small lake were used to examine the relation of intrusions to a lakewide internal seiche, and to estimate mean exchange between sloping bottom boundary layers (BBLs) and the lake interior. BBL waters were observed periodically separating from the bed and flowing laterally offshore. Boundary layer separation was initiated where strong downslope flows encountered upslope‐propagating temperature fronts. These separation events propagated upslope, coherent with the vertically propagating internal seiche, resembling previously reported cases of upslope‐propagating internal bores. Following separation, jets flowed offshore immediately above temperature fronts, transporting boundary layer water at least 61 m into the interior. Jet propagation could be traced upwards, from the bed at 9.4 m depth to the base of the surface mixed layer. Mean offshore transport was most intense within the metalimnion (4–7 m depth). Intruding intermediate‐temperature thermocline water was likely supplied both by river inputs and by boundary mixing of cold and warm waters. These findings suggest that periodic jets, resulting from seiche‐induced boundary layer separation, transported significant quantities of water from the boundary layer toward the stratified lake interior, with possible implications for distributions of sediments, solutes, and organisms.
-
more » « less
Abstract Observations from a tidal estuary show that tidal intrusion fronts occur regularly during flood tides near topographic features including constrictions and bends. A realistic model is used to study the generation of these fronts and their influence on stratification and mixing in the estuary. At the constriction, flow separation occurs on both sides of the jet flow downstream of the narrow opening, leading to sharp lateral salinity gradients and baroclinic secondary circulation. A tidal intrusion front, with a V-shaped convergence zone on the surface, is generated by the interaction between secondary circulation and the jet flow. Stratification is created at the front due to the straining of lateral salinity gradients by secondary circulation. Though stratification is expected to suppress turbulence, strong turbulent mixing is found near the surface front. The intense mixing is attributed to enhanced vertical shear due to both frontal baroclinicity and the twisting of lateral shear by secondary circulation. In the bend, flow separation occurs along the inner bank, resulting in lateral salinity gradients, secondary circulation, frontogenesis, and enhanced mixing near the front. In contrast to the V-shaped front at the constriction, an oblique linear surface convergence front occurs in the bend, which resembles a one-sided tidal intrusion front. Moreover, in addition to baroclinicity, channel curvature also affects secondary circulation, frontogenesis, and mixing in the bend. Overall in the estuary, the near-surface mixing associated with tidal intrusion fronts during flood tides is similar in magnitude to bottom boundary layer mixing that occurs primarily during ebbs.
