Amala Mahadevan

Eddy-Driven Stratification Initiates North Atlantic Spring Phytoplankton Blooms

Early Bloom Trigger Springtime phytoplankton blooms occur when high nutrient concentrations are combined with abundant sunlight and a stratified upper ocean layer. It has been thought that stratification occurs because in the spring, seasonal warming causes the water to expand, making it less dense, which creates a layer resistant to mixing from below. Now, Mahadevan et al. (p. 54; see the Perspective by Martin) have combined observations of the upper water column from the subpolar North Atlantic with ocean model simulations, which demonstrate that the initial stratification can be triggered by the dynamic effects of passing ocean eddies. These eddies can advance the time of the bloom by 20 to 30 days. Oceans eddies can trigger springtime plankton blooms previously attributed to surface heating. Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.

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.

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.

Eddy-driven subduction exports particulate organic carbon from the spring bloom

Down with atmospheric carbon dioxide How does the ocean move carbon from surface waters to its deep interior? Current understanding is that carbon dioxide is removed from the atmosphere by phytoplankton that are eaten, and in turn their predators die and sink into deep water and seafloor sediments. In addition to this route, Omand et al. show that downwelling caused by ocean eddies 1 to 10 km across can deliver much of the carbon produced in spring to the deep sea. The eddies entrain small particles and dissolved organic carbon to augment the flux of large sinking particles. Science, this issue p. 222 Ocean eddies can transport appreciable quantities of organic carbon from the surface to depth. The export of particulate organic carbon (POC) from the surface ocean to depth is traditionally ascribed to sinking. Here, we show that a dynamic eddying flow field subducts surface water with high concentrations of nonsinking POC. Autonomous observations made by gliders during the North Atlantic spring bloom reveal anomalous features at depths of 100 to 350 meters with elevated POC, chlorophyll, oxygen, and temperature-salinity characteristics of surface water. High-resolution modeling reveals that during the spring transition, intrusions of POC-rich surface water descend as coherent, 1- to 10-kilometer–scale filamentous features, often along the perimeter of eddies. Such a submesoscale eddy-driven flux of POC is unresolved in global carbon cycle models but can contribute as much as half of the total springtime export of POC from the highly productive subpolar oceans.

The Impact of Submesoscale Physics on Primary Productivity of Plankton

Life in the ocean relies on the photosynthetic production of phytoplankton, which is influenced by the availability of light and nutrients that are modulated by a host of physical processes. Submesoscale processes are particularly relevant to phytoplankton productivity because the timescales on which they act are similar to those of phytoplankton growth. Their dynamics are associated with strong vorticity and strain rates that occur on lateral scales of 0.1-10 km. They can support vertical velocities as large as 100 m d(-1) and play a crucial role in transporting nutrients into the sunlit ocean for phytoplankton production. In regimes with deep surface mixed layers, submesoscale instabilities can cause stratification within days, thereby increasing light exposure for phytoplankton trapped close to the surface. These instabilities help to create and maintain localized environments that favor the growth of phytoplankton, contribute to productivity, and cause enormous heterogeneity in the abundance of phytoplankton, which has implications for interactions within the ecosystem.

Estimating subsurface horizontal and vertical velocities from sea-surface temperature

We examine a dynamical method for estimating subsurface fields (density, pressure, horizontal and vertical velocities) in the upper ocean using sea-surface temperature (SST) and a climatological estimate of the stratification. The method derives from the “surface quasi-geostrophic” (SQG) approximation. The SST is used to generate a potential vorticity (PV) field which is then inverted for the pressure. We examine first the standard SQG model, in which the PV is assumed trapped in a delta-function layer at the surface. We then modify the model by introducing a subsurface PV which is proportional to the surface density and decays exponentially with depth. We derive the subsurface density from the hydrostatic relation, the horizontal velocities from geostrophy and the subsurface vertical velocities from the quasi-geostrophic omega equation. We compare the predicted densities and velocities with those from a three-dimensional (3D) ocean model, and from in situ measurements in the Mediterranean, Eastern Pacific and the Azores Current. In most cases the standard SQG model predicts the qualitative structure of the subsurface flow. But it also underestimates its strength. The modified model yields better estimates of both the strength and vertical structure of the subsurface flow.

A Nonhydrostatic Mesoscale Ocean Model. Part II: Numerical Implementation

Abstract The nonhydrostatic model with a free surface is numerically implemented in boundary-fitted curvilinear coordinates to model the mesoscale circulation in an ocean basin with natural topography. A semi-implicit numerical scheme is used, and the directional inhomogeneity in the elliptic equation for pressure is exploited to speed up the computation of its solution while using the multigrid method. The model is used to simulate the circulation in the Gulf of Mexico. We observe the formation of the Loop Current and several eddies. The flow is very strongly controlled by the topography and our numerical experiments reveal that in the bottom layers, the flow along topographic contours is in the opposite direction of the anticyclonic circulation in the top layers.

A Nonhydrostatic Mesoscale Ocean Model. Part I: Well-Posedness and Scaling

Abstract The incompressibility and hydrostatic approximations that are traditionally used in large-scale oceanography to make the hydrodynamic equations more amenable to numerical integration result in the primitive equations. These are ill-posed in domains with open boundaries and hence not well-suited to mesoscale or regional modeling. Instead of using the hydrostatic approximation, the authors permit a greater deviation from hydrostatic balance than what exists in the ocean to obtain a system of equations that is well-posed with the specification of pointwise boundary conditions at open or solid boundaries. These equations, formulated with a free-surface boundary, model the mesoscale flow field accurately in all three-dimensions, even the vertical. It is essential to retain the vertical component of the Coriolis acceleration in the model since it is nonhydrostatic.

Reconstructing the Ocean's Interior from Surface Data

A new method is proposed for extrapolating subsurface velocity and density fields from sea surface density and sea surface height (SSH). In this, the surface density is linked to the subsurface fields via the surface quasigeostrophic (SQG) formalism, as proposed in several recent papers. The subsurface field is augmented by the addition of the barotropic and first baroclinic modes, whose amplitudes are determined by matching to the sea surface height (pressure), after subtracting the SQG contribution. An additional constraint is that the bottom pressure anomaly vanishes. The method is tested for three regions in the North Atlantic using data from a high-resolution numericalsimulation. The decomposition yields strikinglyrealistic subsurfacefields. It is particularly successful in energetic regions like the Gulf Stream extension and at high latitudes where the mixed layer is deep, but it also works in less energetic eastern subtropics. The demonstration highlights the possibility of reconstructing three-dimensional oceanic flows using a combination of satellite fields, for example,seasurfacetemperature(SST)andSSH,andsparse(orclimatological)estimatesoftheregionaldepthresolveddensity.Themethodcouldbefurtherelaboratedtointegrateadditionalsubsurfaceinformation,such as mooring measurements.

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...