Leif N. Thomas

Submesoscale processes and dynamics

Increased spatial resolution in recent observations and modeling has revealed a richness of structure and processes on lateral scales of a kilometer in the upper ocean. Processes at this scale, termed submesoscale, are distinguished by order one Rossby and Richardson numbers; their dynamics are distinct from those of the largely quasi-geostrophic mesoscale, as well as fully three-dimensional, small-scale, processes. Submesoscale pro- cesses make an important contribution to the vertical flux of mass, buoyancy, and trac- ers in the upper ocean. They flux potential vorticity through the mixed layer, enhance communication between the pycnocline and surface, and play a crucial role in changing the upper-ocean stratification and mixed-layer structure on a time scale of days. In this review, we present a synthesis of upper-ocean submesoscale processes, arising in the presence of lateral buoyancy gradients. We describe their generation through fron- togenesis, unforced instabilities, and forced motions due to buoyancy loss or down-front winds. Using the semi-geostrophic (SG) framework, we present physical arguments to help interpret several key aspects of submesoscale flows. These include the development of narrow elongated regions with O(1) Rossby and Richardson numbers through fron- togenesis, intense vertical velocities with a downward bias at these sites, and secondary circulations that redistribute buoyancy to stratify the mixed layer. We review some of the first parameterizations for submesoscale processes that attempt to capture their con- tribution to, firstly, vertical buoyancy fluxes and restratification by mixed layer insta- bilities and, secondly, the exchange of potential vorticity between the wind- and buoyancy- forced surface, mixed layer, and pycnocline. Submesoscale processes are emerging as vi- tal for the transport of biogeochemical properties, for generating spatial heterogeneity that is critical for biogeochemical processes and mixing, and for the transfer of energy from the meso to small scales. Several studies are in progress to model, measure, ana- lyze, understand, and parameterize these motions.

Enhanced Turbulence and Energy Dissipation at Ocean Fronts

Energy in surface ocean currents can dissipate into deep water via enhanced turbulence at the boundaries between water masses. The ocean surface boundary layer mediates air-sea exchange. In the classical paradigm and in current climate models, its turbulence is driven by atmospheric forcing. Observations at a 1-kilometer-wide front within the Kuroshio Current indicate that the rate of energy dissipation within the boundary layer is enhanced by one to two orders of magnitude, suggesting that the front, rather than the atmospheric forcing, supplied the energy for the turbulence. The data quantitatively support the hypothesis that winds aligned with the frontal velocity catalyzed a release of energy from the front to the turbulence. The resulting boundary layer is stratified in contrast to the classically well-mixed layer. These effects will be strongest at the intense fronts found in the Kuroshio Current, the Gulf Stream, and the Antarctic Circumpolar Current, all of which are key players in the climate system.

Intensification of Ocean Fronts by Down-Front Winds

Abstract Many ocean fronts experience strong local atmospheric forcing by down-front winds, that is, winds blowing in the direction of the frontal jet. An analytic theory and nonhydrostatic numerical simulations are used to demonstrate the mechanism by which down-front winds lead to frontogenesis. When a wind blows down a front, cross-front advection of density by Ekman flow results in a destabilizing wind-driven buoyancy flux (WDBF) equal to the product of the Ekman transport with the surface lateral buoyancy gradient. Destabilization of the water column results in convection that is localized to the front and that has a buoyancy flux that is scaled by the WDBF. Mixing of buoyancy by convection, and Ekman pumping/suction resulting from the cross-front contrast in vertical vorticity of the frontal jet, drive frontogenetic ageostrophic secondary circulations (ASCs). For mixed layers with negative potential vorticity, the most frontogenetic ASCs select a preferred cross-front width and do not translate with...

Destruction of Potential Vorticity by Winds

The destruction of potential vorticity (PV) at ocean fronts by wind stress–driven frictional forces is examined using PV flux formalism and numerical simulations. When a front is forced by “downfront” winds, that is, winds blowing in the direction of the frontal jet, a nonadvective frictional PV flux that is upward at the sea surface is induced. The flux extracts PV out of the ocean, leading to the formation of a boundary layer thicker than the Ekman layer, with nearly zero PV and nonzero stratification. The PV reduction is not only active in the Ekman layer but is transmitted through the boundary layer via secondary circulations that exchange low PV from the Ekman layer with high PV from the pycnocline. Extraction of PV from the pycnocline by the secondary circulations results in an upward advective PV flux at the base of the boundary layer that scales with the surface, nonadvective, frictional PV flux and that leads to the deepening of the layer. At fronts forced by both downfront winds and a destabilizing atmospheric buoyancy flux F B, the critical parameter that determines whether the wind or the buoyancy flux is the dominant cause for PV

Comment on "Eddy/wind interactions stimulate extraordinary mid-ocean plankton blooms".

McGillicuddy et al. (Reports, 18 May 2007, p. 1021) proposed that eddy/wind interactions enhance the vertical nutrient flux in mode-water eddies, thus feeding large mid-ocean plankton blooms. We argue that the supply of nutrients to ocean eddies is most likely affected by submesoscale processes that act along the periphery of eddies and can induce vertical velocities several times larger than those due to eddy/wind interactions.

Friction, Frontogenesis, and the Stratification of the Surface Mixed Layer

The generation and destruction of stratification in the surface mixed layer of the ocean is understood to result from vertical turbulent transport of buoyancy and momentum driven by air–sea fluxes and stresses. In this paper, it is shown that the magnitude and penetration of vertical fluxes are strongly modified by horizontal gradients in buoyancy and momentum. A classic example is the strong restratification resulting from frontogenesis in regions of confluent flow. Frictional forces acting on a baroclinic current either imposed externally by a wind stress or caused by the spindown of the current itself also modify the stratification by driving Ekman flows that differentially advect density. Ekman flow induced during spindown always tends to restratify the fluid, while wind-driven Ekman currents will restratify or destratify the mixed layer if the wind stress has a component up or down front (i.e., directed against or with the geostrophic shear), respectively. Scalings are constructed for the relative importance of friction versus frontogenesis in the restratification of the mixed layer and are tested using numerical experiments of mixed layer fronts forced by both winds and a strain field. The scalings suggest and the numerical experiments confirm that for wind stress magnitudes, mixed layer depths, and cross-front density gradients typical of the ocean, wind-induced friction often dominates frontogenesis in the modification of the stratification of the upper ocean. The experiments reveal that wind-induced destratification is weaker in magnitude than restratification because the stratification generated by up-front winds confines the turbulent stress to a depth shallower than the Ekman layer, which enhances the frictional force, Ekman flow, and differential advection of density. Frictional destratification is further reduced over restratification because the stress associated with the geostrophic shear at the surface tends to compensate a down-front wind stress.

The Climode Field Campaign: Observing the Cycle of Convection and Restratification over the Gulf Stream

Abstract A major oceanographic field experiment is described, which is designed to observe, quantify, and understand the creation and dispersal of weakly stratified fluid known as “mode water” in the region of the Gulf Stream. Formed in the wintertime by convection driven by the most intense air–sea fluxes observed anywhere over the globe, the role of mode waters in the general circulation of the subtropical gyre and its biogeo-chemical cycles is also addressed. The experiment is known as the CLIVAR Mode Water Dynamic Experiment (CLIMODE). Here we review the scientific objectives of the experiment and present some preliminary results.

Nonlinear stratified spin-up

Both a weakly nonlinear analytic theory and direct numerical simulation are used to document processes involved during the spin-up of a rotating stratified fluid driven by wind-stress forcing for time periods less than a homogeneous spin-up time. The strength of the wind forcing, characterized by the Rossby number e, is small enough (i.e. e[Lt ]1) that a regular perturbation expansion in e can be performed yet large enough (more specifically, e∝ E 1/2 , where E is the Ekman number) that higher-order effects of vertical diffusion and horizontal advection of momentum/density are comparable in magnitude. Cases of strong stratification, where the Burger number S is equal to one, with zero heat flux at the upper boundary are considered. The Ekman transport calculated to O (e) decreases with increasing absolute vorticity. In contrast to nonlinear barotropic spin-up, vortex stretching in the interior is predominantly linear, as vertical advection negates stretching of interior relative vorticity, yet is driven by Ekman pumping modified by nonlinearity. As vertical vorticity is generated during the spin-up of the fluid, the vertical vorticity feeds back on the Ekman pumping/suction, enhancing pumping and vortex squashing while reducing suction and vortex stretching. This feedback mechanism causes anticyclonic vorticity to grow more rapidly than cyclonic vorticity. Strict application of the zero-heat-flux boundary condition leads to the growth of a diffusive thermal boundary layer E −1/4 times thicker than the Ekman layer embedded within it. In the Ekman layer, vertical diffusion of heat balances horizontal advection of temperature by extracting heat from the thermal boundary layer beneath. The flux of heat extracted from the top of the thermal boundary layer by this mechanism is proportional to the product of the Ekman transport and the horizontal gradient of the temperature at the surface. The cooling caused by this heat flux generates density inversions and intensifies lateral density gradients where the wind-stress curl is negative. These thermal gradients make the potential vorticity strongly negative, conditioning the fluid for ensuing symmetric instability which greatly modifies the spin-up process.

Near-Inertial Waves in Strongly Baroclinic Currents

AbstractAn analysis and physical interpretation of near-inertial waves (NIWs) propagating perpendicular to a steady, two-dimensional, strongly baroclinic, geostrophic current are presented. The analysis is appropriate for geostrophic currents with order-one Richardson numbers such as those associated with fronts experiencing strong, wintertime atmospheric forcing. This work highlights the underlying physics behind the properties of the NIWs using parcel arguments and the principles of conservation of density and absolute momentum. Baroclinicity introduces lateral gradients in density and vertical gradients in absolute momentum that significantly modify the dispersion and polarization relations and propagation of NIWs relative to classical internal wave theory. In particular, oscillations at the minimum frequency are not horizontal but, instead, are slanted along isopycnals. Furthermore, the polarization of the horizontal velocity is not necessarily circular at the minimum frequency and the spiraling of th...

The latmix summer campaign: Submesoscale stirring in the upper ocean

AbstractLateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast diff...