Eddy Decay Research Articles

Influence on phytoplankton of different developmental stages of mesoscale eddies off eastern Australia

The influence of oceanic mesoscale eddies on their internal phytoplankton levels varies at different developmental stages. Based on 12 years of satellite ocean color observations, we investigated the variability of phytoplankton inside 4564 anticyclonic eddies and 3675 cyclonic eddies off eastern Australia in different development stages using the method of composited average analysis. The results indicated that the lowest level of chlorophyll was observed in the forth (decay) development stage for anticyclonic eddies, which was associated with the warmest SST, largest eddies amplitude, rotation speeds, angular velocities and surface water convergence. It is indicated that downwelling induced by anticyclonic eddies dominates the chlorophyll variations. Because the convergence induced by the ageostrophic velocity components occurred throughout the eddy's development stage, relaxation of the density perturbations (upwelling) associated with eddy decay was not observed in this study. Chlorophyll concentration near the center of cyclonic eddies decreased from the first to the middle stage, and then increased to the largest levels at the last stage. Although vertical motions induced by the ageostrophic velocity components varied from divergence to convergence during the development of an eddy, a higher eddy-ambient water exchange occurred and dominated the increases in total divergence and chlorophyll concentration in the last stage.

The Lateral Eddy Viscosity Derived from the Decay of Oceanic Mesoscale Eddies

The relationship of lateral eddy viscosity depending on length scale is estimated with the decay rate of mesoscale eddies identified from sea level anomaly of satellite observations. The eddy viscosity is expressed in terms of the mesoscale eddy parameters according to vortex dynamics. The census of mesoscale eddies shows, in general, that the eddy numbers obey the e-folding decay laws in terms of their amplitude, area and lifetime. The intrinsic values in the e-folding laws are used to estimate the lateral eddy viscosity. Dislike the previous theory that diffusivities are proportional to the length square, the eddy mixing rates (diffusivity and viscosity) from satellite mesoscale eddy datasets are proportional to rs to power of 1.8 (slightly less than 2), where rs is the radius of eddy with radius larger than the Batchelor scale. Additionally, the extrapolation of the eddy mixing to the molecule scale implies that the above power laws may hold until the value of rs is less than O (1 m). These mixing rates with the new parameterizations are suggested to use in numerical schemes. Finally, the climatological distributions of eddy viscosity are calculated.

Evolution of turbulence through a broadened preheat zone in a premixed piloted Bunsen flame from conditionally-averaged velocity measurements

This work assesses two hypotheses that predict how turbulence properties vary within premixed turbulent flames that lie in the regime of Broadened Preheat-Thin Reaction layers. There have been few prior measurements describing flames in this regime. The authors previously found that very broad preheat layers were achieved for turbulence levels (u′/SL) up to 243. Surprisingly, the reaction layer thickness did not increase, despite having Kolmogorov scales smaller than the laminar reaction layer thickness. A first hypothesis is that the turbulence decays in the preheat layer (as the temperature rises and viscous forces increase), so that the reaction layer sees only a small fraction of the initial turbulence. It follows that this turbulence decay might be responsible for the observed non-linear bending of the curve of turbulent burning velocity versus turbulence level. A second hypothesis is that the total turbulent kinetic energy does not decrease significantly in the preheat zone; instead, the small eddies decay and cause the integral scale to increase. Conditional averages are required to assess these two hypotheses. Fluorescence imaging identified the reaction zone boundary and particle image velocimetry diagnostics were applied simultaneously. The velocity measurements were conditioned on η, the distance to the upstream boundary of the reaction zone in each image. Conditioned measurements of turbulent kinetic energy, average eddy rotational velocity, strain rate, enstrophy, and integral length scale were computed through the flame. Results indicate that the turbulence level does not decrease within the broad preheat layers, and therefore the first hypothesis is not valid. In fact, the turbulence level within the entire burner core does not vary appreciably. However, the second hypothesis was supported by the measurements, since the integral scale increased by 50% across the preheat layer. The total turbulent kinetic energy did not decrease significantly. One explanation for this result is that small eddies are dissipated in the preheat zone.

A three-dimensional modeling study on eddy-mean flow interaction between a Gaussian-type anticyclonic eddy and Kuroshio

The Princeton ocean model is employed to study the energy balance of a fast-moving anticyclonic eddy (AE) during eddy-mean flow interaction. The AE is initialized with an axisymmetric Gaussian-type temperature profile and is placed to the east of the Philippine Islands. An energy analysis suggests that the advection term, pressure work and friction term play dominant roles in the initial eddy decay. During the strong interaction stage, barotropic instability (BTI) becomes the main force for the eddy kinetic energy (EKE) production, with the largest positive BTI in the interaction zone, which means that the eddy always obtains kinetic energy from the Kuroshio during this stage. Most of the EKE dissipation, the large conversion from the eddy available potential energy to the EKE and that from the mean kinetic energy to the EKE all occur at the upper layer during the strong interaction stage. When the AE interacts with the mean flow on the eastern side of the Kuroshio, whether the AE gains kinetic energy from the Kuroshio or loses kinetic energy to the Kuroshio is mainly determined by its shape in the interaction zone.

Erosion of a California Undercurrent eddy by bottom topography

AbstractSubsurface eddies are ubiquitous features in eastern boundary current systems. These phenomena tend to modulate the across‐shore distribution of heat and biogeochemical tracers. A California Undercurrent eddy was detected by shipboard observations in October 2009 off the northern Baja California continental slope. The spatiotemporal variation in the California Undercurrent eddy is investigated by using a mesoscale‐resolving hindcast ocean simulation. A poleward coastal current that is driven by an upwelling‐wind relaxation event and the coastline geometry instigated the separation of the California Undercurrent from the slope, forming a meander‐like structure, which evolved as a mesoscale eddy‐like structure. The latter structure evolved as a subsurface eddy with a warm anomaly core, a distinctive feature of eddies that form from the California Undercurrent. During the initial stage, the subsurface eddy presented a cone‐shape form, with the maximum amplitude of the relative vorticity in the upper section. The inviscid effect of the irregular bottom topography altered both the initial direction of propagation and the initial eddy shape: the propagation direction of the eddy changed from north‐south to southwest, and the shape changed from a cone shape to a subsurface lens shape. The strong eddy‐topography interactions triggered a sign change in the heat flux direction from the environment toward the eddy and vice versa through the horizontally divergent component of the velocity field, which accelerated the eddy's decay. This study shows the relevance of the synoptic wind stress events and the irregularity of the bottom topography on the mesoscale eddy activity in the southern portion of the California Current.

Numerical study on the eddy–mean flow interaction between a cyclonic eddy and Kuroshio

A three-dimensional primitive equations ocean model (POM) is employed to study the momentum and energy balance of a moving cyclonic eddy (CE) during eddy–mean flow interaction. The CE generated by an idealized typhoon forms to the east of the Philippine islands. A momentum balance analysis shows that the dynamics of the CE are generally dominated by the geostrophic current throughout the life cycle of the CE. An energy analysis suggests that the eddy kinetic energy (EKE) and the eddy potential energy (EPE) decay rapidly after generation. The maximum EPE initially appears at the surface of the eddy center and gradually appears in the subsurface layer. The largest baroclinic instability (BCI) initially occurs at the surface. For a CE moving along a trajectory, the conversion from mean potential energy (MPE) to the EPE is positive (negative) in the front (rear) part of the trajectory, and then the eddy transfers its EPE forward along its trajectory by means of the front (rear) part of the eddy obtaining (losing) EPE from (to) the mean flow. During the interaction stage, the northward flowing Kuroshio interacts with the southward flow on the western side of the eddy and the inverse velocity shear between the Kuroshio and the eddy causes the EKE to gradually develop east–west asymmetry. The largest barotropic instability (BTI) is found in the interaction zone. Advection term, pressure work, and friction term play the dominating role in eddy decay in the eddy zone, while BTI only dominates in the interaction zone.

Generation of Internal Waves by Eddies Impinging on the Western Boundary of the North Atlantic

AbstractDespite the major role played by mesoscale eddies in redistributing the energy of the large-scale circulation, our understanding of their dissipation is still incomplete. This study investigates the generation of internal waves by decaying eddies in the North Atlantic western boundary. The eddy presence and decay are measured from the altimetric surface relative vorticity associated with an array of full-depth current meters extending ~100 km offshore at 26.5°N. In addition, internal waves are analyzed over a topographic rise from 2-yr high-frequency measurements of an acoustic Doppler current profiler (ADCP), which is located 13 km offshore in 600-m deep water. Despite an apparent polarity independence of the eddy decay observed from altimetric data, the flow in the deepest 100 m is enhanced for anticyclones (25.2 cm s−1) compared with cyclones (−4.7 cm s−1). Accordingly, the internal wave field is sensitive to this polarity-dependent deep velocity. This is apparent from the eddy-modulated enhanced dissipation rate, which is obtained from a finescale parameterization and exceeds 10−9 W kg−1 for near-bottom flows greater than 8 cm s−1. The present study underlines the importance of oceanic western boundaries for removing the energy of low-mode westward-propagating eddies to higher-mode internal waves.

An examination of double‐diffusive processes in a mesoscale eddy in the Arctic Ocean

AbstractTemperature and salinity measurements of an Atlantic Water mesoscale eddy in the Arctic Ocean's Canada Basin are analyzed to understand the effects of velocity shear on a range of double‐diffusive processes. Double‐diffusive structures in and around the eddy are examined through the transition from low shear (outside the eddy and within its solid body core) to high geostrophic shear zones at the eddy flanks. The geostrophic Richardson number takes large values where a double‐diffusive staircase is observed and lowest values at the eddy flanks where geostrophic velocity is largest and a well‐formed staircase is not present. A Thorpe scale analysis is used to estimate turbulent diffusivities in the flank regions. Double‐diffusive and turbulent heat, salt, and buoyancy fluxes from the eddy are computed, and used to infer that the eddy decays on time scales of around 4–9 years. Fluxes highlight that Atlantic Water heat within the eddy can be fluxed downward into deeper water layers by means of both double‐diffusive and turbulent mixing. Estimated lateral variations in vertical fluxes across the eddy allow for speculation that double diffusion speeds up the eddy decay, having important implications for the transfer of Atlantic Water heat in the Arctic Ocean.

Gulf Stream Dynamics along the Southeastern U.S. Seaboard

AbstractThe Gulf Stream strongly interacts with the topography along the southeastern U.S. seaboard, between the Straits of Florida and Cape Hatteras. The dynamics of the Gulf Stream in this region is investigated with a set of realistic, very high-resolution simulations using the Regional Ocean Modeling System (ROMS). The mean path is strongly influenced by the topography and in particular the Charleston Bump. There are significant local pressure anomalies and topographic form stresses exerted by the bump that retard the mean flow and steer the mean current pathway seaward. The topography provides, through bottom pressure torque, the positive input of barotropic vorticity necessary to balance the meridional transport of fluid and close the gyre-scale vorticity balance. The effect of the topography on the development of meanders and eddies is studied by computing energy budgets of the eddies and the mean flow. The baroclinic instability is stabilized by the slope everywhere except past the bump. The flow is barotropically unstable, and kinetic energy is converted from the mean flow to the eddies following the Straits of Florida and at the bump with regions of eddy-to-mean conversion in between. There is eddy growth by Reynolds stress and downstream development of the eddies. Interaction of the flow with the topography acts as an external forcing process to localize these oceanic storm tracks. Associated time-averaged eddy fluxes are essential to maintain and reshape the mean current. The pattern of eddy fluxes is interpreted in terms of eddy life cycle, eddy fluxes being directed downgradient in eddy growth regions and upgradient in eddy decay regions.

Time-resolved evolution of coherent structures in turbulent channels: characterization of eddies and cascades

AbstractA novel approach to the study of the kinematics and dynamics of turbulent flows is presented. The method involves tracking in time coherent structures, and provides all of the information required to characterize eddies from birth to death. Spatially and temporally well-resolved DNSs of channel data at $\mathit{Re}_{{\it\tau}}=930{-}4200$ are used to analyse the evolution of three-dimensional sweeps, ejections (Lozano-Durán et al., J. Fluid Mech., vol. 694, 2012, pp. 100–130) and clusters of vortices (del Álamo et al., J. Fluid Mech., vol. 561, 2006, pp. 329–358). The results show that most of the eddies remain small and do not last for long times, but that some become large, attach to the wall and extend across the logarithmic layer. The latter are geometrically and temporally self-similar, with lifetimes proportional to their size (or distance from the wall), and their dynamics is controlled by the mean shear near their centre of gravity. They are responsible for most of the total momentum transfer. Their origin, eventual disappearance, and history are investigated and characterized, including their advection velocity at different wall distances and the temporal evolution of their size. Reinforcing previous results, the symmetry found between sweeps and ejections supports the idea that they are not independent structures, but different manifestations of larger quasi-streamwise rollers in which they are embedded. Spatially localized direct and inverse cascades are respectively associated with the splitting and merging of individual structures, as in the models of Richardson (Proc. R. Soc. Lond. A, vol. 97(686), 1920, pp. 354–373) or Obukhov (Izv. Akad. Nauk USSR, Ser. Geogr. Geofiz., vol. 5(4), 1941, pp. 453–466). It is found that the direct cascade predominates, but that both directions are roughly comparable. Most of the merged or split fragments have sizes of the order of a few Kolmogorov viscous units, but a substantial fraction of the growth and decay of the larger eddies is due to a self-similar inertial process in which eddies merge and split in fragments spanning a wide range of scales.

Subthermocline Eddies over the Washington Continental Slope as Observed by Seagliders, 2003–09

Abstract In the California Current System, subthermocline, lenslike anticyclonic eddies generated within the California Undercurrent (CU) are one mechanism for lateral transport of the warm, saline waters of the CU. Garfield et al. established the name “Cuddies” for eddies of this type and hypothesized that they account for a significant fraction of the offshore transport of CU water. This study presents observations of subthermocline eddies collected from a time series of Seaglider surveys in the northern California Current System. Gliders made 46 crossings of subthermocline anticyclones and 17 crossings of subthermocline cyclones over 5.5 yr. Close inspection grouped these into 20 distinct anticyclones and 10 distinct cyclones. Water properties at the core of anticyclonic eddies were similar to those in the core of the CU over the continental slope; these anticyclones are examples of Cuddies. Anticyclonic (cyclonic) eddies had average radii of 20.4 (20.6) km, peak azimuthal current speeds of 0.25 (0.23) m s−1, and average core anomalies of potential vorticity 65% below (125% above) ambient values. Anticyclones contained an order of magnitude greater available heat and salt anomaly relative to background conditions than cyclones on average. Circumstantial evidence of eddy decay through lateral intrusions was found although this was not observed consistently. Observed eddy properties and the geometry of flow over the continental slope were consistent with eddy formation due to frictional torque acting on the CU. Loss of heat and salt from the CU due to subthermocline eddies is estimated to account for 44% of the freshening and cooling of the CU as it flows poleward.

Catalina Eddy as revealed by the historical downscaling of reanalysis

Climatological properties, dynamical and thermodynamical characteristics of the Catalina Eddy are examined from the 61 years NCEP/NCAR Reanalysis downscaled to hourly 10 km resolution. The eddy is identified as a mesoscale cyclonic circulation confined to the Southern California Bight. Pattern correlation of wind direction against the canonical Catalina Eddy is used to extract cases from the downscaled analysis. Validation against published cases and various observations confirmed that the downscaled analysis accurately reproduces Catalina Eddy events. A composite analysis of the initiation phase of the eddy indicates that no apparent large-scale cyclonic/anti-cyclonic large-scale forcing is associated with the eddy formation or decay. The source of the vorticity is located at the coast of the Santa Barbara Channel. It is generated by the convergence of the wind system crossing over the San Rafael Mountains and the large-scale northwesterly flow associated with the subtropical high. This vorticity is advected towards the southeast by the northwesterly flow, which contributes to the formation of the streak of positive vorticity. At 6 hours prior to the mature stage, there is an explosive generation of positive vorticity along the coast, coincident with the phase change of the sea breeze circulation (wind turning from onshore to offshore), resulting in the convergence all along the California coast. The generation of vorticity due to convergence along the coast together with the advection of vorticity from the north resulted in the formation of southerly flow along the coast, forming the Catalina Eddy. The importance of diurnal variation and the lack of large-scale forcing are new findings, which are in sharp contrast to prior studies. These differences are due to the inclusion of many short-lived eddy events detected in our study which have not been included in other studies.

Decay of eddies at the South-West Indian Ridge

The South-West Indian Ridge in the Indian sector of the Southern Ocean is a region recognised for the creation of particularly intense eddy disturbances in the mean flow of the Antarctic Circumpolar Current. Eddies formed at this ridge have been extensively studied over the past decade using hydrographic, satellite, drifter and float data and it is hypothesised that they could provide a vehicle for localised meridional heat and salt exchange. The effectiveness of this process is dependent on the rate of decay of the eddies. However, in order to investigate eddy decay, logistically difficult hydrographic monitoring is required. This study presents the decay of cold eddies at the South-West Indian Ridge, using outputs from a high-resolution ocean model. The model’s representation of the dynamic nature of this region is fully characteristic of observations. On average, 3–4 intense and well-defined cold eddies are generated per year; these eddies have mean longevities of 5.0±2.2 months with average advection speeds of 5±2 km/day. Most simulated eddies reach their peak intensity within 1.5–2.5 months after genesis and have depths of 2000 m – 3000 m. Thereafter they dissipate within approximately 3 months. The decay of eddies is generally characterised by a decrease in their sea surface height signature, a weakening in their rotation rates and a modification in their temperature–salinity characteristics. Subantarctic top predators are suspected to forage preferentially along the edges of eddies. The process of eddy dissipation may thus influence their feeding behaviour.

Eddy-Mean Flow Interactions in the Along-Stream Development of a Western Boundary Current Jet: An Idealized Model Study

A theoretical study on the role of eddy-mean flow interactions in the time-mean dynamics of a zonally evolving, unstable, strongly inertial jet in a configuration and parameter regime that is relevant to oceanic western boundary current (WBC) jets is described. Progress is made by diagnosing the eddy effect on the time-mean circulation, examining the mechanism that permits the eddies to drive the time-mean recirculation gyres, and exploring the dependence of the eddy effect on system parameters. It is found that the nature of the eddy-mean flow interactions in this idealized configuration is critically dependent on along-stream position, in particular relative to the along-stream evolving stability properties of the time-mean jet. Just after separation from the western boundary, eddies act to stabilize the jet through downgradient fluxes of potential vorticity (PV). Downstream of where the time-mean jet has (through the effect of the eddies) been stabilized, eddies act to drive the time-mean recirculations through the mechanism of an upgradient PV flux. This upgradient flux is permitted by an eddy enstrophy convergence downstream of jet stabilization, which results from the generation of eddies in the upstream region where the jet is unstable, the advection of that eddy activity along stream by the jet, and the dissipation of the eddies in the region downstream of jet stabilization. It is in this region of eddy decay that eddies drive the time-mean recirculations through the mechanism of nonlinear eddy rectification, resulting from the radiation of waves from a localized region. It is found that similar mechanisms operate in both barotropic and baroclinic configurations, although differences in the background PV gradient on which the eddies act implies that the recirculation-driving mechanism is more effective in the baroclinic case. This study highlights the important roles that eddies play in the idealized WBC jet dynamics considered here of stabilizing the jet and driving the flanking recirculations. In the absence of eddy terms, the magnitude of the upper-ocean jet transport would be significantly less and the abyssal ocean recirculations (and their significant enhancement to the jet transport) would be missing altogether.

An infinity of possible invariants for decaying homogeneous turbulence

The von Kárman–Howarth equation implies an infinity of invariants corresponding to an infinity of different asymptotic behaviours of the double and triple velocity correlation functions at infinite separations. Given an asymptotic behaviour at infinity for which the Birkhoff–Saffman invariant is not infinite, there are either none, or only one or only two finite invariants. If there are two, one of them is the Loitsyansky invariant and the decay of large eddies cannot be self-similar. We examine the consequences of this infinity of invariants on a particular family of exact solutions of the von Kárman–Howarth equation.

Effect of Eddy Length Scale on Mechanical Loading of Blood Cells in Turbulent Flow

Non-physiological turbulent blood flow is known to occur in and near implanted cardiovascular devices, but its effects on blood are poorly understood. The objective of this work is to investigate the effect of turbulent eddy length scale on blood cell damage, and in particular to test the hypothesis that only eddies similar in size to blood cells can cause damage. The microscale flow near a red blood cell (RBC) in an idealized turbulent eddy is modeled computationally using an immersed boundary method. The model is validated for the special case of a tank-treading RBC. In comparisons between turbulent flow fields, based on Kolmogorov theory, the model predicts that damage due to the smallest eddies is almost independent of the Kolmogorov length scale. The model predicts that within a given flow field, however, eddies of sub-cellular scale are less damaging than larger eddies. Eddy decay time and the turbulent energy spectral density are highlighted as important factors. The results suggest that Kolmogorov scale is not an adequate predictor of flow-induced blood trauma, and highlights the need for deeper understanding of the microscale structure of turbulent blood flow.

Dissipation of intense vortices

In this article, we extend the space-time analogy shared by viscous vortices to generalize the dissipation of intense vortices Vatistas and Aboelkassem (Am. Inst. Aeronaut. Astronaut. J. 44, 912 (2006)). The main objective is to layout a mathematical algorithm that can be used to transform vortices into their dissipative phase or into a steady-state counterpart via a similarity transformation to Navier–Stokes equations. The method is applied to examine the time decay of eddies that belong to the Oseen-like concentrated type of viscous vortices. The steady-state Burgers’ vortex model was used to validate the transformation approach. This analysis shows that, upon variable transformation, Burgers’ vortex model has changed into the Oseen–Lamb class of decaying vortices. The approach is simple, general, and applicable to all kinds of viscous incompressible vortices like those of Sullivan, Rott, Bellamy-Knights, Vatistas, and Scully.

Synoptic eddies of the Subantarctic and Agulhas fronts and generation of the Antarctic Intermediate Water in the Atlantic Ocean

The concept developed in [9] and [10], according to which the Antarctic Intermediate Water is generated owing to the formation and subsequent decay of cyclonic eddies of the Subantarctic Front, is tested for the Atlantic Ocean. The cross frontal water and salt transport by the Subantarctic Front eddies and the Agulhas Front anticyclones is estimated by developing cinematic eddy models and analyzing satellite altimetry maps. Calculations of the water and salt balance for a box in the South Atlantic based on these results and various experimental data confirmed the above-mentioned concept.

Ocean and Atmosphere Storm Tracks: The Role of Eddy Vorticity Forcing

Abstract Signatures of eddy variability and vorticity forcing are diagnosed in the atmosphere and ocean from weather center reanalysis and altimetric data broadly covering the same period, 1992–2002. In the atmosphere, there are localized regions of eddy variability referred to as storm tracks. At the entrance of the storm track the eddies grow, providing a downgradient heat flux and accelerating the mean flow eastward. At the exit and downstream of the storm track, the eddies decay and instead provide a westward acceleration. In the ocean, there are similar regions of enhanced eddy variability along the extension of midlatitude boundary currents and the Antarctic Circumpolar Current. Within these regions of high eddy kinetic energy, there are more localized signals of high Eady growth rate and downgradient eddy heat fluxes. As in the atmosphere, there are localized regions in the Southern Ocean where ocean eddies provide statistically significant vorticity forcing, which acts to accelerate the mean flow eastward, provide torques to shift the jet, or decelerate the mean flow. These regions of significant eddy vorticity forcing are often associated with gaps in the topography, suggesting that the ocean jets are being locally steered by topography. The eddy forcing may also act to assist in the separation of boundary currents, although the diagnostics of this study suggest that this contribution is relatively small when compared with the advection of planetary vorticity by the time-mean flow.

Mesoscale eddies dominate surface phytoplankton in northern Gulf of Alaska

The HNLC waters of the Gulf of Alaska normally receive too little iron for primary productivity to draw down silicate and nitrate in surface waters, even in spring and summer. Our observations of chlorophyll sensed by SeaWiFS north of 54°N in pelagic waters (>500 m depth) of the gulf found that, on average, more than half of all surface chlorophyll was inside the 4 cm contours of anticyclonic mesoscale eddies (the ratio approaches 80% in spring months), yet these contours enclosed only 10% of the total surface area of pelagic waters in the gulf. Therefore, eddies dominate the chlorophyll and phytoplankton distribution in surface pelagic waters. We outline several eddy processes that enhance primary productivity. Eddies near the continental margin entrain nutrient – (and Fe) – rich and chlorophyll-rich coastal waters into their outer rings, advecting these waters into the basin interior to directly increase phytoplankton populations there. In addition, eddies carry excess nutrients and iron in their core waters into pelagic regions as they propagate away from the continental margin. As these anticyclonic eddies decay, their depressed isopycnals relax upward, injecting nutrients up toward the surface layer. We propose that this transport brings iron and macro-nutrients toward the surface mixed layer, where they are available for wind-forced mixing to bring them to surface. These mesoscale eddies decay slowly, but steadily, perhaps providing a relatively regular upward supply of macro-nutrients and iron toward euphotic layers. They might behave as isolated oases of enhanced marine productivity in an otherwise iron-poor basin. We note that much of this productivity might be near or just below the base of the surface mixed layer, and therefore poorly sampled by colour-sensing satellites. It is possible, then, that eddies enrich phytoplankton populations to a greater extent than noted from satellite surface observations only.