Formation Of Anticyclonic Eddies Research Articles

Characteristics of mesoscale eddies in the Mozambique Channel.

The mesoscale eddy characteristics of the Mozambique Warm Current were investigated by detecting and tracking satellite altimetry data from 2010 to 2019. A total of 1,086 eddies were identified in the Mozambique Channel, comprising 509 cyclonic eddies and 577 anticyclonic eddies. The results revealed that the bay area on the northwest coast of Madagascar was the main hotspot of eddy generation, and the mean amplitude and radius of the anticyclonic eddies in the Mozambique Channel were 24.23 cm and 82.7 km, respectively, which are larger than those of the cyclonic eddies. Local wind forcing had a significant impact on the formation of mesoscale eddies in the Mozambique Channel. In winter, the wind stress in the northern and southern areas of the Mozambique Channel exhibited a strong correlation with the distribution of eddy kinetic energy (EKE), where both monsoonal winds in the north and trade winds in the south could facilitate mesoscale anticyclonic eddy formation. In addition, the variability in the number of anticyclonic and cyclonic eddies in the Mozambique Channel may have exerted a significant influence on the seasonal anomalous fluctuations in local sea surface temperatures (SSTs). This study presented a novel analysis of the mesoscale eddy characteristics in the Mozambique Channel.

Analysis of the Features of Water Circulation in the Northern Part of the Black Sea for Different Seasons on the Basis of the Results of Numerical Modeling with the Assimilation of the Data of Hydrological Surveys in 2016

The analysis of the dynamic and energy characteristics of water circulation in the northern part of the Black Sea was performed on the basis of the assimilation in the numerical model of the data of three hydrological surveys in 2016, carried out on expeditions of 87, 89 and 91 cruises of the R/V Professor Vodyanitsky (summer, autumn and autumn-winter seasons). Numerical experiments were implemented on a horizontal grid (~1.6 km × ~1.6 km) with 27 vertical horizons and an atmospheric effect close to the real one was used. A procedure of assimilation of the observational data was based on the Kalman filter, taking into account the heterogeneity and nonisotropy of the errors of the estimates of the temperature and salinity fields. The integral energy terms in the kinetic and potential energy budget equations for three seasons were estimated. In the summer season, there was a slight weakening of the RC and the main mechanism for the formation of anticyclonic eddies near Sevastopol and near the southeastern shores of Crimea was baroclinic instability of the current (as evidenced by the increase in the slope of isopycnical surfaces and negative values of the work of the buoyancy force). An anticyclonic eddy near Yalta with a radius of about 25 km was generated due to the development of shear instability of the current. In the autumn season, the RC jet was pressed to the shore and there was a decrease in the number of eddies in comparison to the summer season. The formation of anticyclonic eddies with a radius of about 35–40 km in the western part of the region was caused by barotropic instability of the current, the formation of eddies along the Crimean coast – by baroclinic instability. In the autumn-winter season, the RC had a pronounced jet character and there was an increase in the processes of baroclinic instability with the generation of eddies of different scales between the coast and the RC, as well as in the area, located between 31.5 and 33° E, with the weakening of the wind effect. During all seasons, small-scale anticyclonic and cyclonic eddies could be generated along the western and eastern coast of Crimea in the upper layer when current flowed around the coastline and inhomogeneities of the bottom topography under the action of weak winds.

Variation of upper-layer flow structures and water masses observed around the Noto Peninsula and Sado Island, Japan

After classifying various water masses around the Noto Peninsula and Sado Island, Japan and identifying their seasonal variation, we investigated flow fields in the area. Distributions of flow structures and water properties were analyzed based on intensive observations using a conductivity–temperature–depth profiler during 2002–2020. Tide gauge and satellite altimetry data were also analyzed to elucidate the temporal and the more widespread horizontal variation of the sea surface geostrophic flow field. Interannual change of the northeastward baroclinic jet formed offshore of the Noto Peninsula in fall was found. Furthermore, anticyclonic eddy formation around Toyama Bay which is part of the current path transition of the coastal branch of the Tsushima Warm Current was found to result from effects of bathymetric change behind the Noto Peninsula. At that time, the sea-surface current speed of the coastal branch was also high. We propose new methods for predicting current path transition events two weeks in advance.

Variability and Drivers of Ocean Temperature Extremes in a Warming Western Boundary Current

Abstract Western boundary current (WBC) extensions such as the East Australian Current (EAC) southern extension are warming 2–3 times faster than the global average. However, there are nuances in the spatial and temporal variability of the warming that are not well resolved in climate models. In addition, the physical drivers of ocean heat content (OHC) extremes are not well understood. Here, using a high-resolution ocean model run for multiple decades, we show nonuniform warming trends in OHC in the EAC, with strong positive trends in the southern extension region (∼36°–38°S) but negative OHC trends equatorward of 33°S. The OHC variability in the EAC is associated with the formation of anticyclonic eddies, which is modulated by transport ∼880 km upstream (EAC mode) and the westward propagation of Rossby waves (eddy mode). Diagnosing the drivers of temperature extremes has implications for predictability both in the EAC and in WBCs more broadly, where ocean warming is already having considerable ecological impacts.

Structure and Formation of Anticyclonic Eddies in the Iceland Basin

AbstractThe Iceland Basin has the most energetic eddy activities in the subpolar North Atlantic. This study documents the structure for an anticyclonic eddy in the Iceland Basin using high‐resolution hydrographic and velocity observations. The eddy core waters have lens‐like structure with warm and salty features in the upper 1,000 m. The eddy distorts the density surface by doming the upper isopycnals and deepening the ones near the permanent pycnocline. The eddy has a diameter of about 120 km with substantial barotropic component in the velocity profiles. One branch of the North Atlantic Current in the central Iceland Basin is superimposed onto the eddy, leading to asymmetric velocity structure. Satellite maps show that eddy first shows up over the western slope of the Hatton Bank and moves westward to the central Iceland Basin. The waters enclosed in the eddy core share the same properties with Subpolar Mode Waters. Similar anticyclonic eddies are also found in high‐resolution numerical model simulations, which is used to explore eddy formation. The model results reveal that the potential vorticity gradient prior to the eddy event change signs in both horizontal and vertical directions. This potential vorticity gradient structure meets the necessary condition for the barotropic and baroclinic instabilities. Further calculation of the energy conversions suggests that eddies extract mean potential energy from the large‐scale isopycnal slope and gain the mean kinetic energy in the upper ocean. Therefore, both barotropic and baroclinic instabilities are involved to support the eddy growth.

Black Sea thermohaline properties: Long-term trends and variations.

The current knowledge about spatial and temporal dynamics of the Black Sea's thermohaline structure is incomplete because of missing data and sparse distribution of existing measurements in space and time. This study presents 56 year continuous simulations of the Black Sea's hydrodynamics using the 3D General Estuarine Transport Model (GETM), without incorporating any relaxation toward climatological or observational data fields. This property of the model allows us to estimate independent temporal trends, in addition to resolving the spatial structure. The simulations suggest that the intermediate layer temperature is characterized by a weak positive trend (warming), whereas the surface temperature does not show a clear linear trend. Different salinity trends have been established at the surface (negative), upper (weaker negative) and main halocline (positive). Three distinct dynamic periods are identified (1960–1970, 1970–1995, 1995–2015), which exhibit pronounced changes in the Black Sea's thermohaline properties and basin circulation. Strengthening of the main cyclonic circulation, accompanied by intensification of the mesoscale anticyclonic eddy formation is found. Both events strongly affect the sea surface salinity but contribute in opposing directions. Specifically, strong composite large‐scale circulation leads to an increase in sea surface salinity, while enhanced formation of mesoscale anticyclones decreases it. Salinity evolution with time is thus the result of the competition of these two opposing yet interdependent processes.

Lagrangian and Eulerian characterization of two counter‐rotating submesoscale eddies in a western boundary current

AbstractIn recent decades, high‐spatial resolution ocean radar and satellite imagery measurements have revealed a complex tangle of submesoscale filaments and eddies, in the surface velocity, temperature, and chlorophyll a fields. We use a suite of high‐resolution data to characterize two counter‐rotating, short‐lived eddies formed at the front between the warm East Australian Current (EAC) and temperate coastal waters (30°S, Eastern Australia). In this region, submesoscale filaments and short‐lived eddies are dynamically generated and decay at time scales of hours to days. Dominant cyclonic filaments of O(1) Rossby number formed along frontal jets and eddy boundaries, generating localized ageostrophic circulations at the submesoscale. Measurements of over‐ocean wind direction and surface currents from high‐frequency radars reveal the influence of the short‐term, small‐scale wind forcing on the surface circulation, enhancement of the horizontal shear, frontal jet destabilization, and the generation and decay of the cyclonic eddy. By contrast, the anticyclonic eddy formation was most likely associated with EAC mesoscale instability and anticyclonic vorticity. Lagrangian tracks show that surface particles can be temporarily trapped in the eddies and frontal convergent zones, limiting their transport. Mixing between EAC‐derived and coastal waters was increased along the frontal regions, and particles starting at the divergent regions around the eddies experienced significant dispersion at submesoscales. The cyclonic cold‐core eddy entrained high chlorophyll a shelf waters on its convergent side, suggesting spiral eddy cyclogenesis.

Oceanographic influences on Deep Scattering Layers across the North Atlantic

The distribution and density of Deep Scattering Layers (DSLs) were quantified along North Atlantic transits from Ireland to the Grand Banks of Newfoundland in the springs of 2012, 2013 and 2014 employing a calibrated Simrad EK60 echo sounder at 38kHz. Concurrently, Sippican T5 XBTs (eXpendable Bathy Thermographs) were used to profile temperatures to 1800m. In each year the scattering layers spanned the deep basin at depths ranging from near surface to approximately 900m, but annual mean densities differed significantly. Higher DSL densities were recorded during years that exhibited higher sea temperatures at the depths of major DSL concentration (400–600m), higher sea level anomalies and stronger eastward geostrophic currents. The highest concentration of the DSLs in each year was found in the area east of the Grand Banks that corresponded with areas of anticyclonic eddies. In this region DSL densities in 2014 were among the highest recorded worldwide (>7000m2nauticalmile–2). Midwater fishing indicated DSLs were dominated by Myctophids and Sternoptychids. Anticyclonic eddy formation is discussed as a possible means of transport and aggregation of the DSLs in that region, where oceanographic influences may play a dominant role in the distribution and density of the DSLs and upper trophic level fishes.

Simulation of East India Coastal Features and Validation with Satellite Altimetry and Drifter Climatology

The circulation features of western coast of the Bay of Bengal (BOB) have been analyzed using Regional Ocean Modeling System (ROMS) with Comprehensive Ocean-Atmosphere Data Set (COADS) wind and thermal forcing. The model simulation shows that the coastal current is not continuous throughout the year similar to the structure seen from the drifter climatology. The western boundary current (WBC) is formed in February and persists till May. This boundary current is very strong during March and April due to formation of anticyclonic eddies. From July to September, the coastal current is disorganized because of sequential development of anticyclonic and cyclonic eddies. But in October the coastal current starts to flow southward as the East India Coastal Current (EICC) and it prevails till December with the formation of cyclonic eddies along the coast. The simulated sea surface height anomaly (SSHA) is competent to detect the upwelling and downwelling zones in the coastal region as supported by TOPEX/POSEIDON climatology.

Dynamically Induced Anomalies of the Japan/East Sea Surface Temperature

Variability of sea surface temperature (SST) in the Japan/East Sea (JES) was studied using complex empirical orthogonal function (CEOF) analysis. Two daily data sets were analyzed: (1) New Generation 0.05o-gridded SST from Tohoku University, Japan (July 2002-July 2006), and (2) 0.25o-gridded SST from the Japan Meteorological Agency (October 1993-November 2006). Linkages with wind stress curl were revealed using 6-h 1o-gridded surface zonal and meridional winds from ancillary data of the Sea- WiFS Project, a special National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) product (1998-2005). SST anomalies (SSTA) were obtained by removing the seasonal signal, estimated as the leading mode of the CEOF decomposition of the original SST. Leading CEOF modes of residual SSTA obtained from both data sets were consistent with each other and were characterized by annual, semiannual, and quasi-biennial time scales estimated with 95% statistical significance. The Semiannual Mode lagged 2 months behind the increased occurrence of the anticyclonic (AC) wind stress curl over the JES. Links to dynamic processes were investigated by numerical simulations using an oceanic model. The suggested dynamic forcings of SSTA are the inflow of subtropical water into the JES through the Korea Strait, divergence in the surface layer induced by Ekman suction, meridional shifts of the Subarctic Front in the western JES, AC eddy formation, and wind-driven strengthening/weakening of large-scale currents. Events of west-east SSTA movement were identified in July-September. The SSTA moved from the northeastern JES towards the continental coast along the path of the westward branch of the Tsushima Current at a speed consistent with the advective scale.

Phytoplankton variability off the Western Australian Coast: Mesoscale eddies and their role in cross-shelf exchange

Abstract We utilize over 6 years of remotely sensed chlorophyll a estimates from sea-viewing wide-field-of-view sensor (SeaWiFS), along with in situ observations from the targeted sampling of mesoscale eddies during the 2000 “Transport Indian Pacific” (TIP2000) cruise to characterize phytoplankton variability on and off the central Western Australian (WA) shelf. Much of the variability is associated with mesoscale eddies, and we find that, contrary to canonical ideas, satellite and in situ estimates of surface chlorophyll off WA show low phytoplankton biomass in cyclonic eddies and high biomass in anticyclonic eddies. Off-shelf we found two types of phytoplankton communities; modified shelf communities in anticyclonic eddies and offshore subtropical communities concentrated in the deep chlorophyll maximum outside the anticyclonic eddies. The presence of the modified shelf community offshore in anticyclonic eddies supports a shelf-based origin and cross-shelf distribution mechanism for phytoplankton off the central WA coast. Anticyclonic eddies, formed adjacent to the shelf, entrain shelf waters with relatively high chlorophyll a concentrations and, as they propagate westward, export coastal phytoplankton communities offshore. The formation of anticyclonic eddies appears to occur at preferred locations along the WA coast. Our observations suggest that the anticyclonic eddy communities are long-lived (5 months or more) and their maintenance may result from the sub-mesoscale injection of nutrients into the eddy boundary.

Distribution of dissolved forms of iron and manganese in the Black Sea

Dissolved forms of the redox-sensitive elements manganese (MnDISS) and iron (Fe(II) and Fe(III)) were measured in the oxic/anoxic transition zone, or suboxic zone, of the Black Sea. An offset was found between dissolved manganese and iron gradients at all stations. In the western cyclonic gyre of the Black Sea, the onset of dissolved Mn (>0.1 μM) was located at a density of about σt=15.85, where the concentration of dissolved oxygen (DO) was less than 5 μM. The onset of dissolved Fe(II) (>0.025 μM) was at σt=15.90, just above the increases in hydrogen sulfide. These characteristic features were modified in the southern coastal waters by such physical processes as the rim current, formation of anticyclonic eddies, and injection of saline, oxygenated Marmara Sea waters from the Bosporus in the Bosporus Plume.

Interannual variability of Tehuantepec eddies

TOPEX/Poseidon satellite altimeter observations and the Naval Research Laboratory Layered Ocean Model simulations show interannual variability in the number and intensity of Tehuantepec eddies off the Mexican southwest coast. Analysis of the results illustrates that downwelling coastally trapped waves, which are generated in the equatorial Pacific, play a crucial role in the modulation and generation of Tehuantepec eddies and a dominant role in Tehuantepec eddy interannual variability. This introduces a new paradigm in which the generation and modulation of Tehuantepec eddies is not exclusively explained in terms of the strong and intermittent Tehuantepec wind events. In fact, the results show anticyclonic eddy formation during periods of calm Tehuantepec winds. That is specifically exemplified by the formation of two anticyclonic Tehuantepec eddies during a 5‐month period of weak Gulf of Tehuantepec winds during summer of 1997. Furthermore, the satellite‐observed and NLOM‐simulated proliferation of Tehuantepec eddies during El Niño years is explained by the corresponding increase in downwelling coastally trapped waves and a lack of increase in the number and strength of Tehuantepec wind events during El Niño years.

Annual westward propagating anomalies near 26°N and eddy generation south of the Canary Islands: remote sensing (altimeter/SeaWiFS) and in situ measurement

Seasonal changes in altimeter data are derived for the North Atlantic Ocean. Altimeter data are then used to examine annually propagating structure along 26°N. By averaging the altimeter data into monthly values or by Fourier analysis, a positive anomaly can be followed from 17°W to ∼50°W along ∼26°N. The methods give a westward travel speed of 1° of longitude a month and a half-life of one year for the average decaying structure. At ∼50°W 26°N, the average structure is about 2·8 years old with an elevation signal of ∼1 cm, having travelled ∼3300 km westward. The mean positive anomaly results from the formation of anticyclonic eddies which are generally formed annually south of the Canary Islands by late summer and which then travel westward near 26°N. Individual eddy structure along 26°N is examined and related to in situ measurements and anomalies in the annual seasonal concentration cycle of SeaWiFS chlorophyll-a.

Scatterometer and Model Wind and Wind Stress in the Oregon–Northern California Coastal Zone

Measurements of surface wind stress by the SeaWinds scatterometer on NASA's Quick Scatterometer (QuikSCAT) satellite are analyzed and compared with several different atmospheric model products, from an operational model and two high-resolution nested regional models, during two summer periods, June through September 2000 and 2001, in the coastal region west of Oregon and northern California. The mean summer wind stress had a southward component over the entire region in both years. Orographic intensifications of both the mean and fluctuating wind stress occurred near Cape Blanco, Cape Mendocino, and Point Arena. Substantial differences between the model products are found for the mean, variable, and diurnal wind stress fields. Temporal correlations with the QuikSCAT observations are highest for the operational model, and are not improved by either nested model. The highest-resolution nested model most accurately reproduced the mean observed stress fields, but slightly degrades the temporal correlations due to incoherent high-frequency (0.5–2 cpd) fluctuations. The QuikSCAT data reveal surprisingly strong diurnal fluctuations that extend offshore 150 km or more with magnitudes that are a significant fraction of the mean wind stress. Wind stress curl fields from QuikSCAT and the models show local cyclonic and anticyclonic maxima associated with the orographic wind intensification around the capes. The present results are consistent with the hypothesis of a wind-driven mechanism for coastal jet separation and cold water plume and anticyclonic eddy formation in the California Current System south of Cape Blanco.

Continuity of the poleward undercurrent along the eastern boundary of the mid-latitude north Pacific

Several recent data sets improve our view of the poleward undercurrent of the California Current System. As part of a triennial National Marine Fisheries Service (NMFS) survey of Pacific whiting, a series of 105 shipboard acoustic Doppler current profiler (ADCP) velocity sections across the shelf break from 33 to 51°N at about 18 km meridional spacing were collected from July to August 1995. Significant (>0.05 m s −1) subsurface poleward flow occurred in 91% of the sections. A mean cross-shelf section using the entire data set has statistical significance, revealing an undercurrent core >0.1 m s −1 from 200–275 m depth 20–25 km off the shelf break. The mean poleward volume transport in a 125–325 m layer is 0.8±0.2×10 6 m 3 s −1 . We focus particular attention on the Cape Blanco to Cape Mendocino region, and the NMFS results are compared with shipboard ADCP three weeks later from a study of coastal upwelling processes near Cape Blanco. ADCP streamfunction maps are derived and strongly suggest that one portion of flow is continuous over the 440 km meridional extent of the analysis region. Other portions of the flow show evidence of offshore turning, separation, and the formation of anti-cyclonic eddies. We also note that isopycnic potential vorticity from alongslope CTD stations during the NMFS survey appears to be a tracer for the poleward flow.

A laboratory study of the effects of a sloping side boundary on wind-driven circulation in a homogeneous ocean model

A laboratory model is used to investigate the effects of sloping boundaries on homogeneous wind-driven B-plane circulation. The very gentle slopes of real oceanic boundaries raise the possibility that dissipation by lateral diffusion of vorticity to the boundary is largely removed, leaving dissipation only in bottom Ekman layers. The laboratory model is a modification of the rotating ‘sliced-cylinder’ introduced by Pedlosky and Greenspan (1967) and Beardsley (1969) and in which flow is driven by a differentially rotating lid. The vertical wall is replaced with a side wall having a uniform 45” slope around the entire perimeter. This sloping boundary, like a continental slope, tends to steer the flow along the slope. In the geometry chosen for this study it also provides closed potential vorticity contours through every point in the basin, thus removing the blocked contours of the experiments with a vertical wall and the open contours of ocean basins that approach the equator. For cyclonic forcing there is a northward (Sverdrup) flow in the interior superimposed on a zonal flow so that a particle starts out at the southwest, enters the slope region in the northwest, circles cyclonically along a circle of constant radius (and depth) to a point on the southeast where it crosses constant depth contours and rejoins the original point. The direction of flow is reversed for anticyclonic forcing. The main dissipation of vorticity takes place in the southeast where the flow crosses constant depth contours. For cyclonic forcing the flow is stable and steady under all conditions achieved. For anticyclonic forcing the laboratory flow is unsteady under all conditions attainable and unstable to eddy shedding at sufficiently large Rossby or Reynolds numbers. At large Ekman numbers the onset of instability corresponds to shedding of cyclonic eddies in the region where the boundary current enters the interior, whereas at small Ekman numbers it corresponds to periodic breakup of an anticyclonic gyre in the ‘northwest’ and the formation of anticyclonic eddies. Eddies of both sign are shed when the forcing is sufficiently supercritical and the Ekman number small. A simple, qualitative argument explains why the cyclonic flow is ‘stable and the anticyclonic flow is unstable when the system is nonlinear.

Observation of the anticyclonic eddy formation off the Black Sea coast

An anticyclonic eddy with a horizontal dimension of ∼ 20 km travelling north-westwards at a speed of 10 cm/s has been found during an experiment carried out off the Black Sea coast. Dynamic instability owing to horizontal velocity shear could be the reason for its origin. The eddy's passage favours the development of shear instability and the generation of short-period internal waves.

Laboratory experiments on fronts

Abstract We describe a laboratory model of an upwelling front in a two-layer stratification. In the model the interface between the two layers slopes upwards toward a vertical boundary (or coastline) and can intersect the free surface to produce a front. Fluid motion in each layer is density driven and, in the undisturbed state, is in quasi-geostrophic balance. The front is observed to be unstable to (ageostrophic) disturbances with an along-front wavelength proportional to the Rossby radius of deformation. At very large amplitudes these unstable waves form closed circulations. However, in contrast to the behaviour of fronts far from vertical boundaries, where cyclone-anticyclone vortex pairs are formed, the presence of the coastline inhibits formation of anticyclonic eddies in the upper layer and enhances cyclonic rings of upper layer fluid which lie above cyclonic eddies in the lower layer. The cyclones move away from the vertical boundary and (as is also the case when no vertical boundary is present) t...

Laboratory experiments on fronts

Abstract We describe a laboratory model of an upwelling front in a two-layer stratification. In the model the interface between the two layers slopes upwards toward a vertical boundary (or coastline) and can intersect the free surface to produce a front. Fluid motion in each layer is density driven and, in the undisturbed state, is in quasi-geostrophic balance. The front is observed to be unstable to (ageostrophic) disturbances with an along-front wavelength proportional to the Rossby radius of deformation. At very large amplitudes these unstable waves form closed circulations. However, in contrast to the behaviour of fronts far from vertical boundaries, where cyclone-anticyclone vortex pairs are formed, the presence of the coastline inhibits formation of anticyclonic eddies in the upper layer and enhances cyclonic rings of upper layer fluid which lie above cyclonic eddies in the lower layer. The cyclones move away from the vertical boundary and (as is also the case when no vertical boundary is present) t...