Eddy Regime Research Articles

Latitudinal storm track shift in a reduced two-level model of the atmosphere

Latitudinal storm track shift in a reduced two-level model of the atmosphere

Modulation of Phytoplankton Uptake by Mesoscale and Submesoscale Eddies in the California Current System

AbstractEddies play a crucial role in shaping ocean dynamics by affecting material transport, and generating spatio‐temporal heterogeneity. However, how eddies at different scales modulate biogeochemical transformation rates remains an open question. Applying a multi‐scale decomposition to a numerical simulation, we investigate the respective impact of mesoscale and submesoscale eddies on nutrient transport and biogeochemical cycling in the California Current System. First, the non‐linear nature of nutrient uptake by phytoplankton results in a 50% reduction in primary production in the presence of eddies. Second, eddies shape the vertical transport of nutrients with a strong compensation between mesoscale and submesoscale. Third, the eddy effect on nutrient uptake is controlled by the covariance of temperature, nutrient and phytoplankton fluctuations caused by eddies. Our results shed new light on the tight interaction between non‐linear fluid dynamics and ecosystem processes in realistic eddy regimes, which remain largely under‐resolved by global Earth system models.

ПРОСТРАНСТВЕННО-ВРЕМЕННАЯ ИЗМЕНЧИВОСТЬ ГИДРОФИЗИЧЕСКИХ И ЭНЕРГЕТИЧЕСКИХ ХАРАКТЕРИСТИК ЦИРКУЛЯЦИИ ЧЕРНОГО МОРЯ ПРИ ДОМИНИРОВАНИИ ДВИЖЕНИЙ РАЗНЫХ МАСШТАБОВ

Simulation results of the Black Sea dynamics for two periods when the annual mean circulation corresponded to the basin-scale and eddy regimes (2011 and 2016) are considered in the paper. Numerical experiments are carried out using the MHI model and considering the realistic atmospheric forcing from SKIRON. The seasonal variability of dynamic and thermohaline fields, as well as the kinetic and available potential energy, and their conversion rates are estimated. According to the model data on the seasonal mean distribution of currents velocity, it is found that in 2011 the RIM Current is detected in all seasons, and the most intense mesoscale eddies developed on its periphery over the continental slope in the warm period of the year; in 2016, separate cyclonic jets in the area of the continental slope are observed in the northern and southwestern parts of the basin during cold seasons, and mesoscale eddies are propagated in the central part of the sea throughout the year. The change in the mean current kinetic energy is determined by the circulation regime: energy maxima are revealed in the spring of 2011 and in the winter of 2016, when the mean current was the most intense. The distribution of the mean available potential energy is predominantly seasonal, the time variability is qualitatively similar for both modes and is provided by an increase in the density anomaly due to seawater heating. The eddy kinetic energy characterizing the mesoscale variability depends both on the circulation regime and on the season. In the spring 2011, the mean current and eddy kinetic energies are comparable; in 2016, the maximum eddy energy exceeded the mean current kinetic energy. In autumn and winter, for both calculations, the increase in eddy energy occurs due to the energy transfer from the wind and the mean current through the barotropic instability mechanism. In summer when wind activity weakens, in the basin-scale circulation mode, mesoscale variability is supported by commensurate contributions from barotropic and baroclinic instability; in the eddy circulation mode – mainly due to the conversion of available potential energy through baroclinic instability.

ANALYSIS OF THE LORENZ ENERGY CYCLE FOR DIFFERENT REGIMES OF THE BLACK SEA CIRCULATION

The Lorentz energy cycle components are calculated from numerically simulated data on the Black Sea dynamics for three regimes: climatic circulation, basin-scale (2011), and eddy (2016) circulations. The some common features have been found between climatic and realistic energy. The mean annual conversion of energy from mean motion to eddy motion is observed for all circulation regimes. The annual mean buoyancy work increases the mean current for all experiments, which indicates that the slope of the isopycnal surfaces remains such that the condition for converting the available potential energy into kinetic energy is realized. The qualitative difference is that the conversion of eddy kinetic energy into available potential energy is observed only for climatic circulation. For basin-scale circulation, the variability of the eddy kinetic energy is determined by the flux formed by the transformation of kinetic energy due to the barotropic instability of the mean current. For the eddy regime, the growth in the eddy kinetic energy is a result of the available potential energy conversion into kinetic energy due to baroclinic instability. The circulation regime, as well as the qualitative and quantitative difference in the magnitudes and directions of energy conversion, are primarily associated with atmospheric forcing.

Analysis of the annual mean energy cycle of the Black Sea circulation for the climatic, basin-scale and eddy regimes

Analysis of the annual mean energy cycle of the Black Sea circulation for the climatic, basin-scale and eddy regimes

Seasonal Variability of the Dynamics and Energy Transport in the Black Sea by Simulation Data

This paper presents an assessment of the seasonal variability of the velocity fields, mean and eddy kinetics, and available potential energies, and the energy conversion rates for the eddy and basin-scale circulation regimes. The data were obtained through the numerical modeling of the Black Sea circulation for 2011 and 2016. It revealed significant differences in the current structure in the southern and central sea parts for 2011 and 2016. In 2011, the Rim Current was an almost continuous cyclonic basin-scale gyre, while in 2016 a system of mesoscale anticyclones was observed in the southern part. The variability of the mean kinetic energy depends more on the circulation regime than on the season of the year, while the distribution of the mean available potential energy is predominantly seasonal. The eddy kinetic energy depends on both the circulation regime and the season. In winter, the energy transport from the mean current via a barotropic instability mechanism sustains the mesoscale eddy generation. In summer, the mesoscale variability in the basin-scale regime is provided by commensurate contributions of barotropic and baroclinic instability, and, in the eddy regime, mainly by the energy transport from the available potential energy through the baroclinic instability.

Editorial: Zooplankton and Nekton: Gatekeepers of the Biological Pump

Editorial on the Research Topic Zooplankton and Nekton: Gatekeepers of the Biological Pump Zooplankton and nekton organisms create and destroy particles in manifold ways. They feed on the diverse components of the plankton community and on detrital matter. They disaggregate these components, but also repackage them into fecal pellets. Zooplankton and nekton thereby contributes to the attenuation, but also to the export of vertically settling particles. Many zooplankton and nekton organisms also ascend to the surface layer of the ocean at dusk to feed during the dark hours, and return to midwater at the break of dawn. This diurnal vertical migration (DVM) shuttles organic matter from the surface ocean to deeper layers, where it is metabolized and excreted. This active flux (as opposed to the passive flux of sinking particles) can contribute substantially to the biological pump, the downward export of carbon and nutrients into the oceans interior. Due to their multiple roles in oceanic particle dynamics, zooplankton and nekton organisms can actually be considered the gatekeepers of the biological pump. Several articles in this Research Topic deal with the contribution of zooplankton and nekton-mediated active flux to the total export of organic matter. Using biomass and enzyme transport system (ETS) assessments of respiratory flux for both mesozooplankton and micronekton communities, Hernandez-Leon et al. estimated the total active transport of carbon (respiration, excretion, mortality, and egestion) along a transect in the Atlantic from the Canary Islands to Brazil. They found that active flux by these communities ranged from 25 to 80% of the total particulate organic carbon export at 150 m depth and that the importance of active flux increased with increasing surface productivity. Kwong et al. compared biomass, diel vertical migration, and active flux of mesozooplankton and micronekton across a range of mesoscale eddy structures along the east-coast of Australia during winter and spring. They found that although all eddy regimes had similar integrated biomass of mesozooplakton and micronekton, the organisms in the individual eddies had different migratory behavior, which resulted in contrasting importance of active flux. Kiko et al. assessed the impact of mesozooplankton DVM on elemental cycling at three stations in the Eastern Tropical North Atlantic. They found that approximately 31 to 41% of the total nitrogen loss from the upper 200 m of the water column was attributable to DVM mediated fluxes. They also suggest that gut flux-the flux created by migrators when they evacuate their gut at DVM-depth-and migrator mortality at DVM-depth contribute to an Intermediate Particle Maximum. In their study conducted in the Peruvian upwelling system (which features a severe midwater oxygen minimum zone), Kiko and Hauss concluded that the metabolic suppression

An experimental study of flow regimes of a gravity current over a Cape in a stratified environment

Properties of the flow generated by a buoyancy source are investigated by a number of laboratory experiments. Experiments are carried out in a tank with a Cape to simulate the features of a gravity current when moving and separating over a delta Cape as in a real marine flow. Of the many experiments performed, 63 are recorded and analyzed in detail. Most of the experiments are carried out under 0.01 < Fr < 3 and 0.03 < Ro < 0.45 (where Fr and Ro are the flow Froude and Rossby numbers respectively). Dimensionless parameters of the simulations of the flow in a laboratory are comparable with those of the real flow in the Caspian Sea. Based on the behavior of flow upstream of the Cape, three regimes can occur in these experiments: laminar, eddy, and laminar-eddy regimes. The eddy regime is found for values of the Rossby number less than 0.05. For the moving flow on the bottom slope, an empirical relation between CNof, the Nof speed, and u, the current speed, is found from which CNof is predicted from u using oceanographic data. Based on the Rossby number of the flow in the Caspian Sea, the laminar-eddy regime is more likely to occur. The flow shows a different behavior when moving over the Cape which is categorized by the Cape upstream behavior of the flow. Under g´ = 0.02 m/s2, f = 0.72 s−1 (T = 17 s) Ro = 0.17, and Fr = 0.24, one cyclone eddy and one anticyclone eddy are formed similar to those seen in nature (here the Caspian Sea). In the eddy regime, the cascade process occurs with a timescale of longevity tL < 6T for each eddy. The results indicate that the geometry of the Cape and the features of the flow (g´, Fr, and Ro) upstream can be effective in the shape, size, and location of eddy formation. The experimental results also show that the radius of the eddy is about two times larger than the Rossby deformation radius of the flow in upstream of the Cape, while having a timescale tf between T/2 and 2T. In nature, the eddy formation and development time scale are about 1 to 2 years with 10 and 15 months for Seddy (cyclone eddy) and Anseddy (anticyclone eddy) respectively. Because of the ability of an eddy to transport and spread pollutants such as oil in the southern Caspian Sea, this work can also be important for the marine dispersion estimations.

Diagnosing the Influence of Mesoscale Eddy Fluxes on the Deep Western Boundary Current in the 1/10° STORM/NCEP Simulation

AbstractUsing a 0.1° ocean model, this paper establishes a consistent picture of the interaction of mesoscale eddy density fluxes with the geostrophic deep western boundary current (DWBC) in the Atlantic between 26°N and 20°S. Above the DWBC core (the level of maximum southward flow, ~2000-m depth), the eddies flatten isopycnals and hence decrease the potential energy of the mean flow, which agrees with their interpretation and parameterization in the Gent–McWilliams framework. Below the core, even though the eddy fluxes have a weaker magnitude, they systematically steepen isopycnals and thus feed potential energy to the mean flow, which contradicts common expectations. These two vertically separated eddy regimes are found through an analysis of the eddy density flux divergence in stream-following coordinates. In addition, pathways of potential energy in terms of the Lorenz energy cycle reveal this regime shift. The twofold eddy effect on density is balanced by an overturning in the plane normal to the DWBC. Its direction is clockwise (with upwelling close to the shore and downwelling further offshore) north of the equator. In agreement with the sign change in the Coriolis parameter, the overturning changes direction to anticlockwise south of the equator. Within the domain covered in this study, except in a narrow band around the equator, this scenario is robust along the DWBC.

On One Estimate of the Boundary of the Rossby Regime Zone in the Atmosphere

A two-zone model of the atmospheric circulation over the hemisphere is considered. The geographic latitude φ of the boundary between the Rossby circulation regime zone at middle and high latitudes and the Hadley circulation regime zone at low latitudes serves as a model variable. The closeness between the actual and reference (exponential) air-mass distribution over the hemisphere, with respect to Ertel’s modified potential vorticity (MPV), is accounted for. The informational entropy of the statistical MPV distribution in the hemispheric atmosphere and the informational entropy of the eddy regime in the basic storm-track zone are used to determine a statistically (climatically) equilibrium value of φ. The question of atmospheric blocking over the hemisphere is considered using the proposed statistical–dynamical model.

Low‐frequency variability in coastal sea level from tide gauges and altimetry

Long sea level observations are fundamental for studies of the low‐frequency changes in ocean circulation and climate. Tide gauges provide records several decades long, but their locations are geographically sparse and confined to the coastline. To assess the extent to which tide gauge point measurements represent broader‐scale conditions in the adjacent ocean, we compare tide gauge records to altimeter data in their vicinity for the period 1993–2008. The analysis is based on detrended annual mean series and focuses on interannual variability. The best agreement between tide gauge and altimeter series is found for mid‐ocean islands in the tropical Pacific and Indian oceans and along the coast of western Australia. Elsewhere, along most continental coasts, root‐mean‐square differences between the two sea level estimates can be substantial when compared to the observed variability or the expected measurement noise, with the worst correspondence found in western South America, Japan, and eastern Australia. Differences in magnitude as well as poor correlations between the tide gauge and altimeter estimates are involved. Although land motions and measurement errors can be a factor, results point to the presence of considerable short spatial structure in coastal sea level at interannual time scales, which can be related to the low‐frequency dynamics of coastally trapped circulations, western boundary currents and associated eddy regimes, or response to river runoff. Knowing whether coastal sea level variability becomes more spatially homogeneous at longer time scales must await longer sea level records.

SeaWiFS sensing of hazardous algal blooms and their underlying mechanisms in shelf-slope waters of the Northwest Pacific during summer

Abstract RCA-chlorophyll (red tide index chlorophyll algorithm — RCA) estimates from the SeaWiFS, sea surface height (SSH) variations/geostrophic currents from the multi-satellite altimeters, sea surface temperature (SST) from the NOAA-AVHRR, and wind speed/direction from the QuikSCAT are used in conjugation with field observation data to first describe comprehensively the occurrences of various hazardous algal blooms (HABs) and their underlying mechanisms and link to nutrient enrichment during the summer (June–September) in shelf-slope waters off the Northwest Pacific (NWP) covering China, Korea, Japan and Russia (perhaps this is the first satellite-based study in Russia). These datasets provide a coherent view of the summertime evolution of HABs and related physical processes in the above regional segments with four common dynamic regions: coastal cold/estuary water zones, upwelling zones next to the coast, repeated meanders/eddies, and frontal regimes induced by the Kuroshio and its tributaries. Summer HABs numerically dominated by dinoflagellates and diatoms (only in few cases) were initiated in these hydrodynamically active coastal regions and subsequently transported throughout their coastal and oceanic ranges by major currents and eddy systems. As a consequence, dense and colossal blooms displayed mean RCA of > 7 mg m− 3 and TBCA (total bloom covered area) of > 20 × 103 km2, which limits the research vessels to observe concomitantly their spatially explicit phases together with physical oceanographic features in such large regions. Less dense and spatially disbanded blooms were characterized by mean RCA of

Annual sea level amphidromes in the South China Sea revealed by merged altimeter data

Annual phase‐amplitude characteristics of sea level anomaly (SLA) in the South China Sea (SCS) are investigated by a merged SLA data set derived from simultaneous measurements of Envisat, Geosat‐Follow‐on (GFO), Jason‐1, and TOPEX/Poseidon (T/P) from January 2004 to December 2005. Four annual amphidromes instead of two are revealed and their locations, surrounding the Vietnam eddy, distinguish two distinctive regimes of annual variations in the SCS, a basin scale monsoon regime and a local Vietnam eddy regime. Their existence suggests that the annual amphidrome is not only a common feature on global scale, but also a phenomenon in regional seas. However, the locations of these amphidromes in the SCS vary considerably from year to year, in contrast to the annual amphidomes found in the tropical ocean basins, which are much more stable.

A Dense Current Flowing down a Sloping Bottom in a Rotating Fluid

A density-driven current was generated in the laboratory by releasing dense fluid over a sloping bottom in a rotating freshwater system. The behavior of the dense fluid descending the slope has been investigated by systematically varying four parameters: the rotation rate, the bottom slope, the flow rate of the dense fluid, and the density of the dense fluid. Over a wide range of parameter values, the following three flow types were found: a laminar regime in which the dense current had a constant thickness behind the head, a wave regime in which wavelike disturbances appeared on the interface between the dense and fresh fluids, and an eddy regime in which periodic formation of cyclonic eddies in the fresh overlying ambient fluid was observed. All of the experiments revealed that increasing the slope angle and the density of the bottom fluid allowed the flow to evolve from the laminar to the wave regime. Furthermore, increasing rotation rate induced the formation of eddies. A theoretical solution for the downslope velocity field has been found using a steady-state model. Comparison between the theoretical and experimental downslope velocities gave good agreement. The wave regime was observed to occur for values of the Froude number greater than 1. The laminar regime was found for values of the Froude number less than 1. The amount of mixing between the dense and the ambient fluids was measured. Mixing increased significantly when passing from the laminar to the wave regime, that is, with increasing Froude number. Good agreement between the amount of mixing observed in the ocean and in the laboratory experiments is encouraging and makes the waves observed in the present experiments a possible candidate for the mixing observed during oceanic dense current overflows.

The 3D Oceanic Mixed Layer Response to Hurricane Gilbert

Upper-ocean heat and mass budgets are examined from three snapshots of data acquired during and after the passage of Hurricane Gilbert in the western Gulf of Mexico. Measurements prior to storm passage indicated a warm core eddy in the region with velocities of O(1) m s21. Based upon conservation of heat and mass, the threedimensional mixed layer processes are quantified from the data. During and subsequent to hurricane passage, horizontal advection due to geostrophic velocities is significant in the eddy regime, suggesting that prestorm oceanic variability is important when background flows have the same magnitude as the mixed layer current response. Storm-induced near-inertial currents lead to large vertical advection magnitudes as they diverge from and converge toward the storm track. Surface fluxes, estimated by reducing flight-level winds to 10 m, indicate a maximum wind stress of 4.2 N m22 and a heat flux of 1200 W m 22 in the directly forced region. The upward heat flux after the passage of the storm has a maximum of 200 W m22 corresponding to a less than 7 m s21 wind speed. Entrainment mixing across the mixed layer base is estimated using three bulk entrainment closure schemes that differ in their physical basis of parameterization. Entrainment remains the dominant mechanism in controlling the heat and mass budgets irrespective of the scheme. Depending on the magnitudes of friction velocity, surface fluxes and/or shear across the mixed layer base, the pattern and location of maximum entrainment rates differ in the directly forced region. While the general area of maximum entrainment is in the right-rear quadrant of the storm, shear-induced entrainment scheme predicts a narrow region of cooling compared to the the stressinduced mixing scheme and observed SST decreases. After the storm passage, the maximum contribution to the mixed layer dynamics is associated with shear-induced entrainment mixing forced by near-inertial motions up to the third day as indicated by bulk Richardson numbers that remained below criticality. Thus, entrainment based on a combination of surface fluxes, friction velocity and shear across the entrainment zone may be more relevant for three-dimensional ocean response studies.

Interbasin deep water exchange in the western Mediterranean

Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean. The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provençal Basin was postulated in the depth range 700–1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero‐Provençal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero‐Provençal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600–1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv = 106 m3 s−1), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero‐Provençal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero‐Provençal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior.

Vorticity analysis of transient shallow water eddy fields at the river plume front of the River Elbe in the German Bight

Abstract The mesoscale variability of the circulation of the German Bight is investigated for selected wind forcing situations with an eddy resolving model. A vorticity equation is derived which considers explicitly both mass distribution and tides. Different shallow water eddy regimes are identified in the German Bight. Using the vorticity equation, the statistical steady state eddy fields are analyzed with respect to the production mechanisms of the gyres and shallow water eddies. The large German Bight gyre is affected by the joint effect of baroclinicity and topography. It is demonstrated that all shallow water eddies which appear at the East-Friesian coast are affected by the interaction between wind stress and the gradient of bottom topography. Small shallow water eddies at the North-Friesian coast formed from the baroclinic forcing, the vortex squeezing and the interaction between wind stress and the gradient of bottom topography. The results further suggest that the contribution of advection or of tidal stress to the vorticity production is of minor relevance.

Surface circulation in the Northern Adriatic Sea as revealed by a drifter experiment

Nine-hours-averaged positions from five satellite-tracked drifting buoys were used to map the surface circulation in the Northern Adriatic sea. The velocity and kinetic-energy fields were calculated on a regular (0.1×0.1)o grid using averaged positions and remapping the obtained field by means of univariate objective analysis techniques. Eddy and mean flow kinetic-energy contributions were also calculated on a (0.1×0.1)o grid. The main features of the surface circulation agree with other representations. The double eddy regime due to the Po river outflow, which spreads in a typical summer regime and the corresponding low-dynamics coastal regime along the Italian littoral south the Po outlet are rather well resolved.

The effect of an arctic polynya on the Northern Hemisphere mean circulation and eddy regime: a numerical experiment

The feedback of an arctic polynya, which is a large ice-free zone within the sea ice, on the hemispheric climate is studied with the ECMWF T21 GCM. For this purpose a control and an anomaly integration, in which a polynya was introduced in the Kara Sea, are compared. As the GCM, like the real atmosphere, shows a high level of low frequency variability, the mean response to the changed boundary conditions is obscured by internal noise. The necessary significance analyses are thus performed to enhance the signal-tonoise ratio within the framework of an a priori chosen guess pattern and a multivariate test statistic. The sensible and latent heat fluxes increased above the polynya, which resulted in a warming of the lower troposphere above and near the polynya. No statistically significant local or global sea-level pressure changes are associated with this heating. However we find a significant change of hemispheric extent of the geopotential fields at 300 hPa, if we use as guess patterns the eigenmodes of the barotropic vorticity equation. The different mean flow field is accompanied by significant changes of the synoptic transient eddy field. We find a significant variation in the barotropic and baroclinic forcing of the mean flow by the eddies, a change in the location and intensity of the storm tracks and in the conversion between eddy available and eddy kinetic energy. The additional heat flux from the polynya results in a reduction of the meridional heat flux by the synoptic eddies on the western Atlantic.

Diabatically Forced, Nearly Inviscid Eddy Regimes

Abstract We use a scaling analysis and numerical solutions of a quasigeostrophic two-level model on a β plane to explore what kind of eddy regimes can occur when the eddies are forced solely by differential diabatic heating, and friction is negligible outside a boundary layer near the lower surface. The eddy regimes all have a convergent eddy momentum flux and a Ferrel cell near the center of the channel flow, and a downgradient eddy heat flux that exceeds the upgradient heat flux by the Ferrel cell. Quantitatively the regimes can be characterized by R, the ratio of the heat transport by the Ferrel cell to the heat transport by the eddies. This ratio depends on a nondimensional parameter, δ, which is proportional to the characteristic relaxation time associated with the diabatic heating and to the convergence of the eddy momentum flux. If δ → ∞, then R → 1, the divergence of the Eliassen-Palm flux approaches zero, and the temperature structure approaches equilibrium; that is, the motions have very little ...