Submesoscale Coherent Vortices Research Articles

Direct observations of formation and propagation of subpolar eddies into the Subtropical North Atlantic

Subsurface float and moored observations are presented to show for the first time the formation and propagation of anticyclonic submesoscale coherent vortices that transport relatively cold, fresh subpolar water to the interior subtropical North Atlantic. Acoustically tracked RAFOS floats released in the southward-flowing Western Boundary Current at the exit of the Labrador Sea reveal the formation of three of these eddies at the southern tip of the Grand Banks (42°N, 50°W). Using a recently developed method to detect eddies in float trajectories and estimate their kinematic properties, it was found that the eddies had average rotation periods of 5–7 day at radii of 10–25km, with mean rotation speeds of up to 0.3ms−1. One especially long-lived (5.1 months) eddy crossed under the Gulf Stream path and translated southwestward in the subtropical recirculation to at least 35°N, where it hit one of the Corner Rise Seamounts. Velocity, temperature and salinity measurements from a nine-month deployment of two moorings south of the Gulf Stream at 38°N, 50°W reveal the passage of at least two eddies with similar hydrographic and kinematic properties. The core temperature and salinity properties of the eddies imply their formation at intermediate levels of the Labrador Current south of the Tail of the Grand Banks. These observations confirm earlier speculation that eddies form in this region and transport anomalously cold, low-salinity water directly into the subtropical interior. Possible formation mechanisms and potential importance of these eddies to interior ventilation and the equatorward spreading of Labrador Sea Water are discussed.

Baroclinic instability and submesoscale eddy formation in weakly stratified oceans under cooling

Numerical experiments with a three‐dimensional nonhydrostatic model have been performed to investigate baroclinic instability and submesoscale eddy formation in weakly stratified oceans under cooling. Two types of baroclinic instability can exist in a two‐layered ocean where the convectively formed deep mixed layer overlies the weakly stratified lower layer. One rapidly develops in the mixed layer with short wavelengths (shallow mode), and the other occupies the whole ocean depth with long wavelengths (deep mode). When a background flow is deep, the shallow mode develops first, but the deep mode replaces it in several days. In contrast, only the shallow mode is excited in the mixed layer overlying the strongly stratified layer. The linear stability analysis explains these results well. Despite active cooling, subsequently formed eddies restratify the mixed layer and modify water density over the whole depth. When a background flow is shallow, baroclinic instability is confined to the mixed layer and produces a dipole of surface‐intensified cyclonic eddy and middepth‐intensified anticyclonic eddy, both of which are of submesoscale (∼10 km). The anticyclonic eddy consisting of convectively formed but weakly stratified water moves across the front to ventilate the deep layer, where convection does not reach locally. In this way, density modification extends below the mixed layer. Surface cooling as well as baroclinicity enhances restratification of the mixed layer and density modification at depths by activating submesoscale eddy formation. The results can explain the restratification of the deep mixed layer observed during a cooling season and the origins of submesoscale coherent vortices recently detected in polar oceans.

A Greenland Sea Perspective on the Dynamics of Postconvective Eddies*

Abstract Open ocean deep postconvection contributes to the formation of the dense waters that fill the global deep ocean. The dynamics of postconvective vortices are key to understanding the role of convection in ocean circulation. Submesoscale coherent vortices (SCVs) observed in convective regions are likely to be the anticyclonic components of hetons. Hetons are dipoles, consisting of a surface cyclone and a weakly stratified subsurface anticyclone, that can be formed by convection. Here, key postconvective processes are investigated using numerical experiments of increasing sophistication with two primary goals: 1) to understand how the ambient hydrography and topography influence the propagation of hetons and 2) to provide a theoretical context for recent observations of SCVs in the Greenland Sea. It is found that the alignment of hetons is controlled by ambient horizontal density gradients and that hetons self-propagate into lighter waters as a result. This provides a mechanism for transporting convected water out of a cyclonic gyre, but the propagation is arrested if the heton meets large-amplitude topography. Upon interaction with topography, hetons usually separate, and the surface cyclone returns toward denser water. The anticyclone usually remains close to topography and may become trapped for several hundred days. These findings may explain the observed accumulation and longevity of SCVs at the Greenland Fracture Zone, on the rim of the Greenland Sea gyre. The separation and sorting of cyclones from anticyclones have likely implications for the density and vorticity budgets of convective regions.

A submesoscale coherent eddy in the Greenland Sea in 2003

Submesoscale Coherent Vortices (SCVs) have been observed earlier in the Greenland Sea, but their overall characteristics, the formation and the dissolution mechanisms, and the effects on the large‐scale hydrodynamics were not well understood. In order to improve the understanding of these features, a simultaneous investigation of hydrography, chemical tracers, and full‐depth velocity profiles in a SCV was employed in September 2003. The observed eddy had a homogeneous cold core from 500 m to 2500 m depth with a radius of 8∼15 km. The velocity field of the eddy was higher than in the previous years, and the eddy was in strong anticyclonic rotation in the intermediate layer (1000∼2000 m). The high velocity field led to the estimate of eddy vorticity twice as high as previous observations, and this was accounted for the eddy migration while the earlier observed eddies were rather stationary around 75°N 0°E. The eddy migrated northeast ward with a speed of 3 km/day driven by the background mean flow under the strong effects of the background shear, which tilted the rotation axis in the upper layer. The concentrations of sulphur hexafluoride (SF6) and chlorofluorocarbons (CFCs) in the eddy provided firm information about the source water end‐members. The Greenland Sea Arctic Intermediate Water and winter cold surface water were determined as the principal eddy source waters. This differs from the earlier conception of eddies being sourced from intermediate waters at the periphery of the Greenland Basin.

Large-Scale Spreading of Deep Waters in the Western Mediterranean Sea by Submesoscale Coherent Eddies

Abstract Two large-scale free-drifting isobaric-floats experiments, “SOFARGOS”/Marine Science and Technology Programme, phase 2 (MAST2) and Mass Transfer and Ecosystem Response (MATER)/MAST3, undertaken in 1994–95 in the northwestern Mediterranean Sea and in 1997–98 in the Algerian Basin, respectively, have revealed for the first time that Western Mediterranean Deep Water, newly formed by deep convection in the Gulf of Lion (the so-called Medoc site), can be advected several hundreds of kilometers away from the formation area by anticyclonic submesoscale coherent vortices (SCVs). This behavior implies that SCVs participate actively in the large-scale thermohaline circulation and deep ventilation of the western Mediterranean Sea. These SCVs are characterized by small radius (∼5 km), very low potential vorticity, high aspect ratio (∼0.1), and extended lifetime (>0.5 yr).

Linear Instability of Barotropic Submesoscale Coherent Vortices Observed in the Ocean

The linear instability of circular quasigeostrophic vortices with horizontal cross sections of angular velocity identical to those that fit the observations of submesoscale eddies in the ocean is investigated analytically and numerically. The theoretical necessary conditions for instability are formulated as a single condition on the mean potential vorticity or mean angular velocity rather than the streamfunction, which is more readily applicable to oceanic lenses, eddies, and meddies. It is shown that the suggested cross sections that best fit the observed angular velocity of several long-lived vortices are all unstable to small, wavelike perturbations, and that the e-folding time for perturbation growth at all wavenumbers and cross sections is on the order of 1 day. For all cross sections considered, azimuthal wavenumber 1 is stable while all higher azimuthal wavenumbers, as well as all vertical wavenumbers, are unstable. The main contribution to the instability comes from the jump in potential vorticity at the radius of maximum angular velocity. When the potential vorticity is continuous at this radius, the growth rates become smaller and the details of potential vorticity distribution become important. The fast growth rates obtained by the numerical calculations clearly emphasize the insufficient spatial resolution of existing observations for deciphering the exact velocity cross sections of submesoscale oceanic vortices, especially near the radius of maximum angular velocity.

Lagrangian Exploration of the California Undercurrent, 1992–95

Abstract During the period 1992–95, nineteen isobaric RAFOS floats, placed in the California Undercurrent at intermediate depths (150–600 m) off Monterey and San Francisco, California, reveal a region of varying width of subsurface, poleward flow adjacent to the continental margin. The float trajectories exhibit three patterns: poleward flow in the undercurrent; reversing, but predominately alongshore, flow adjacent to the continental margin;and, farther offshore, anticyclonic motion accompanied by slow westward drift. Flow continuity of the undercurrent exists between Pt. Reyes and at least Cape Mendocino with an average speed dependent on the float depth. Speeds were variable, but common features were acceleration occurring to the south of Pt. Arena and deceleration to the north of Cape Mendocino. An important mechanism for floats, and water, to enter the ocean interior from the undercurrent is through the formation of submesoscale coherent vortices.

Lagrangian Observations of Meddy Formation during A Mediterranean Undercurrent Seeding Experiment*

Mediterranean eddies (meddies) play an important role in maintaining the temperature and salinity distributions in the North Atlantic, but relatively little is known about their early life histories, including where, how often, and by what mechanism they form. A major field program, called A Mediterranean Undercurrent Seeding Experiment, has been carried out to directly observe meddy formation and the spreading pathways of Mediterranean Water into the North Atlantic. Between May 1993 and March 1994, 49 RAFOS floats were deployed sequentially in the Mediterranean Undercurrent south of Portugal and tracked acoustically for up to 11 months. The float deployments were accompanied by high-resolution XBT sections across the undercurrent. Nine meddy formation events were observed in the float trajectories, six near Cape St. Vincent, at the southwestern corner of the Iberian Peninsula, and three near the Estremadura Promontory, along the western Portuguese continental slope. Meddy formation thus occurs where the continental slope turns sharply to the right (when facing in the downstream direction of the undercurrent). After conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent, the authors have estimated a meddy formation rate of 15–20 meddies per year. The timescale for meddy formation at Cape St. Vincent was found to be 3–7 days, shorter than previous estimates based on the volume of larger meddies. Meddies were observed to form most frequently when the speed of the Mediterranean Undercurrent was relatively fast. The meddy formation process at Cape St. Vincent resembles the conceptual model of E. A. D’Asaro, whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer (with negative relative vorticity) at a sharp corner. Comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent. This confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices, by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean, may not be the principle mechanism at work in the formation of meddies at Cape St. Vincent.

Frontogenesis, subduction, and cross‐front exchange at upper ocean fronts

The subduction of water at upper ocean fronts driven by internal instabilities is investigated. A simple analytic model which assumes conservation of potential density and potential vorticity is used to characterize the expected structure of subducted parcels as a function of the surface layer depth and the length scale of the parcel. Parcels subducted from a deep surface layer will be characterized by anomalously low potential vorticity and anticyclonic circulation, while shallow surface layers result in subducted parcels with high potential vorticity and cyclonic circulation. A nonlinear isopycnal primitive equation model is used to demonstrate how baroclinic instability and the resulting frontogenesis force the subduction of parcels below and across the front. Frontogenesis generates vertical velocities of 𝒪(30 m d−1) and ageostrophic cross front flows of 3 to 5 cm s−1. The subduction is achieved by the deep ageostrophic flow which carries water from the surface layer below and across the front. The subducted parcels quickly develop a circulation through balanced adjustment, anticyclonic for deep surface layers and cyclonic for shallow surface layers. The characteristics of parcels subducted in winter conditions closely resemble a class of eddies called submesoscale coherent vortices. The horizontal and vertical scales of the vortices are determined by the frontogenetic mechanism and the depth of the surface layer. The total permanent subduction rate is estimated to be 20 m yr−1 for frontal subduction driven by frontogenesis, although local (and temporary) subduction can be much larger. The subducted anticyclonic vortices are often coupled to upper ocean cyclonic vortices, thus forming a baroclinic dipole that propagates at approximately 3.5 cm s−1 and is capable of transporting the anomalous water properties far from the region of formation. Similarities with observations in the North Atlantic Subtropical Convergence Zone and near the Gulf Stream are discussed.

An Upper Bound on the Size of Sub-mesoscale Coherent Vortices

Laboratory experiments show that ageostrophic instability can “break up” a parallel flow into a sequence of axisymmetric eddies. This is a plausible scenario for the generation of sub-mesoscale coherent Vortices (SCVs). Here we show that conservation of mass, energy and potential vorticity enables one to very simply calculate the radius of the axisymmetric eddy and the wavelength of the nonlinear instability. The latter agrees more closely with laboratory experiments than does the wavelength predicted by linearized stability theory. It energy is not conserved, say because of wave radiation into the lower layer, then the preceding calculation establishes an upper bound on the radius of the eddy. We offer this as an explanation of the observed small size of SCVs.

Vortex Generation Through Balanced Adjustment

Abstract The problem of geostrophic adjustment, originally considered by C.G. Rossby, is solved in an axisymmetric geometry for a continuously stratified fluid, where the adjusted final state is in hydrostatic, gradient-wind balance. This problem is relevant to the generation of submesoscale coherent vortices in the ocean: diapycnal mixing events can create a local anomaly of less strongly stratified fluid, which then develops a balancing circulation through adjustment. An analytical solution is obtained for a few uniform-density layers, and this is compared with numerical solutions for continuous stratification. In both representations, two-dimensional solutions are compared with axisymmetric ones.

The Vertical Motion of Submesoscale Coherent Vortices across Neutral Surfaces

Abstract Submesoscale coherent vortices (SCVs, “meddies” or “bullets”) are shown not to move along either potential density surfaces or neutral surfaces, due to the compressibility of sea water being a function of potential temperature and salinity. While it has been recognized that SCVs may make an important contribution to lateral property fluxes in the ocean, it is shown in this paper that they can also cause significant vertical fluxes across neutral surfaces. Since they represent the slow vertical translation of macroscopic water masses, these fluxes are not measurable with microstructure measurements of the dissipation of kinetic energy, but are nonetheless real. These vertical fluxes of heat and salt can be either down-gradient or up-gradient, corresponding to either a positive or a negative vertical diffusivity. The magnitude of the effective diffusivity for salt is different from that for heat. Formulae are derived for estimating the mean vertical velocity of SCVs through neutral surfaces.