Current Meanders Research Articles

The Impact of an Antarctic Circumpolar Current Meander on Air‐Sea Interaction and Water Subduction

AbstractStanding meanders along the Antarctic Circumpolar Current (ACC) have been shown to be regions of elevated eddy variability, meridional heat transport, and vertical exchange. In this study, we investigate the influence of a standing meander south of Australia on air‐sea heat fluxes, upper ocean structure, and subduction in the 1/10° ACCESS‐OM2 ocean‐sea ice model forced by the JRA55 atmospheric reanalysis. We track the model's Subantarctic and Polar Fronts based on their jet and water mass structure, and produce composites of thermodynamical and dynamical properties of the meander in relaxed and flexed states. The standing meander induces trough‐to‐crest variations in surface heat flux, mixed layer depth (MLD), wind stress curl, vertical velocity, and subduction. At the crests, the ocean loses heat and the mixed layer is deeper; at the troughs, the ocean gains heat and the mixed layer is shallower. Wind stress curl, vertical velocity, and subduction change sign on entering and exiting crests and troughs. Vertical velocity due to the curvature of the meander is an order of magnitude larger than Ekman vertical velocity. The poleward excursion of Polar Front meander crests extends subduction to Antarctic Intermediate Water density classes. Finally, flexing of the meander enhances both air‐sea exchange and vertical velocity. The results show that standing meanders of the ACC influence the distribution and magnitude of air‐sea fluxes of heat and momentum and exchange between the surface and interior ocean.

Impacts of Agulhas Current meanders on intermediate water masses along the adjacent continental slope and shelf.

Variability in the Agulhas Current system is dominated by meanders, which constitute cyclonic eddies along the inshore edge of the Current on the southeast coast of South Africa. Few studies have investigated the influence of these meanders on hydrographic variability on the adjacent shelf and slope and to date only a handful have been sampled in situ. This study used available in situ data and GLORYS12v1 model output to investigate the impact of meanders on the distribution of Intermediate waters, namely Red Sea Water (RSW) and Antarctic Intermediate Water (AAIW), as well as mechanisms driving these variations. We focussed on four eddies, sampled in situ during July 1998, April 2010, January–February 2017, and July–August 2017. RSW dominated along the inshore edge of the Agulhas Current in the absence of meanders, but larger proportions of AAIW occurred in the presence of cyclonic eddies. During eddy events, the kinematic steering level was raised above the lower boundary of Intermediate waters, increasing cross-frontal mixing of waters at depths of 800–1800 m. Eddy-induced upwelling of Central and Intermediate waters onto the shelf appeared to be inhibited by bands of strong positive relative vorticity (>0.4 × 10−4 s−1), which likely promoted downwelling conditions inshore of the July 1998, April 2010, and July–August 2017 eddies. Weak positive relative vorticity (<0.2 × 10−4 s−1) inshore of the January–February 2017 eddy was associated with enhanced water mass exchange between the shelf and deeper (>1000 m) ocean. GLORYS12v1 was consistently comparable with satellite and in situ data, and simulated the overall distribution of water masses on the continental shelf and slope despite its inability to reflect the influence of river discharge in nearshore regions during austral summer. The model is thus suitable to investigate the influence of Agulhas Current meanders on the hydrography of South Africa's southeast coast.

Characteristics of ocean current meandering and potential efficacy of maximizing power capacity by tracking short-term meanders with hydro sail enabled active mooring

This research studies the characteristics of ocean current meanders from the viewpoint of power generation engineering and the potential of maximizing ocean current power generation capacity by making generator turbines to track the meanders applying a recently proposed Cross-stream Active Mooring concept. The current meanders will hurt capacity factor of marine turbines moored at fixed locations, somewhat like the issue of wind availability for wind farms. The new mooring concept features a hydro sail system that can deploy generator turbines transversely across streams in a marine current. The hydro sail system can further adjust the horizontal position of the turbines, by changing sail angle of attack, to actively track fast streams in an ocean current. This paper first reviews and discusses the characteristics of short-term track variations of the Florida Current and the Kuroshio near Taiwan and then analyzes potential improvement in power generation using measured and simulated current velocity data in the two ocean currents, suggesting a potential increase of 10–40 % power generation achievable by tracking the meanders. Operations of the hydro sail were demonstrated by tests using remote controllable and passive scale models. Control methods are discussed. Approaches to observe, predict and track the meanders are conceptualized.

The acoustic propagation environment surrounding the New England seamounts

The New England Seamounts lie in a complex oceanographic environment that frequently sees both the colder Slope Sea waters and the warmer Sargasso Sea waters. The Gulf Stream, a meandering current that separates these two bodies of water, can be difficult to locate and frequently produces cold-core and warm-core eddies, complicating ocean modelling efforts. Regional temperature and salinity profile measurements gathered by the Argo program enable an investigation of the accuracy of the Hybrid Coordinate Ocean Model (HYCOM), a popular ocean circulation model. Simple range-independent calculations of acoustic propagation conditions provide a means of characterizing the acoustic environment by detailing common propagation paths associated with the New England Seamounts region. Contrasting the range-independent calculations for the Argo measurements with those of the oceanographic model indicates that persistent temperature features absent from the model impact predictions of the acoustic propagation environment.

Reduction of non-tidal oceanographic fluctuations in ocean-bottom pressure records of DONET using principal component analysis to enhance transient tectonic detectability

Ocean bottom pressure-gauge (OBP) records play an essential role in seafloor geodesy. Oceanographic fluctuations in OBP data, however, pose as a significant noise source in seafloor transient crustal deformation observations, including slow slip events (SSEs), making it crucial to evaluate them quantitatively. To extract the significant fluctuation phenomena common to multiple observation networks, including oceanographic fluctuations and tectonic signals, we applied principal component analysis (PCA) to the 3-year Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) OBP time series for 40 stations during 2016–2019. PCA could separate several oceanographic signals based on the characteristics of their spatial distributions, although evident transient tectonic signals could not be confirmed from the observed pressure records during this observed period. The spatial distribution of the first four principal components (PCs) reflected the common component, inclined component along sea depth, longitude component, and parabola-like pattern, respectively. By subtracting each PC (in particular, PC-2 and PC-4) from the time series, we could significantly reduce the sea depth dependence of OBP records, which has been highlighted in several previous studies and is also evident in this region. We interpreted PCs 2–4 as the reflection of the strength and meandering of ocean geostrophic currents based on a comparison with the PC spatial distribution of the numerical oceanographic models. In addition, to evaluate the ability of PCA to separate transient tectonic signal from OBP time series, including oceanographic fluctuations, we conducted a synthetic ramp assuming an SSE by rectangular fault and then applied PCA. The assumed synthetic tectonic signal could be separated from the oceanographic signals and included in the principal component independently depending on its amplitude, suggesting that the spatial distribution of each PC would change if the amplitude of the synthetic signal were sufficiently large. We propose a transient event-detection method based on the spatial distribution difference of a specific PC with or without a tectonic signal. We used the normalized inner product (NIP) between these PCs as the indicator of their similarities. This method can detect transient tectonic signals more significantly than the moment-magnitude scale of 5.9 from OBP records.Graphical

Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders

Abstract The western boundary current system off southeastern Brazil is composed of the poleward-flowing Brazil Current (BC) in the upper 300 m and the equatorward flowing Intermediate Western Boundary Current (IWBC) underneath it, forming a first-baroclinic mode structure in the mean. Between 22° and 23°S, the BC-IWBC jet develops recurrent cyclonic meanders that grow quasi-stationarily via baroclinic instability, though their triggering mechanisms are not yet well understood. Our study, thus, aims to propose a mechanism that could initiate the formation of these mesoscale eddies by adding the submesoscale component to the hydrodynamic scenario. To address this, we perform a regional 1/50° (∼2 km) resolution numerical simulation using CROCO (Coastal and Regional Ocean Community model). Our results indicate that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instability that can trigger the meanders’ formation. Subsequently, this process generates submesoscale cyclones that contribute, along with baroclinic instability, to the meanders’ growth, resulting in a submesoscale-to-mesoscale inverse cascade. Last, as the mesoscale cyclones grow, they interact with the slope, generating inertially and symmetrically unstable anticyclonic submesoscale vortices and filaments. Significance Statement Off southeastern Brazil, the Brazil Current develops recurrent cyclonic meanders. Such meanders enhance the open-ocean primary productivity and are of societal importance as they are located in a region rich in oil and gas where oil-spill accidents have already happened. This study aims to explore the processes responsible for triggering the formation of these mesoscale eddies. We find that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instabilities that eject submesoscale filaments and vortices and can trigger the meanders’ formation. Such results show that topographically generated submesoscale instabilities can play an important role in the dynamics of mesoscale meanders off southeastern Brazil. Moreover, this may indicate that resolving the submesoscale dynamics in operational numerical models may contribute to an increase in the predictability of the regional eddies.

Study of concentrated anchoring, siting, system layout and preliminary cost analysis for a large scale Kuroshio power plant by the cross-stream active mooring

The development of large scale ocean current power generally encounters difficulties associated with water depth, seafloor engineering, storms and current meanders. The Cross-stream Active Mooring (CSAM) is a recently proposed new concept to overcome these issues. It features serial mooring of multiple turbines on a common tether as a linear array, single point anchoring and applying hydro sails to position the linear array across current streams over deep seafloor or seabed not suitable for anchoring construction. The CSAM can change the linear array's horizontal position to track meandering velocity core to increase system power capacity and adjust its depth to avoid storms. This paper proposes and studies a new concentrated anchoring concept of high capacity based on slope enhancement and assembled anchors to resolve issues of anchoring on rocky undersea ridges in many areas along the Kuroshio. Two new gravity anchor designs making use of seafloor landforms are devised and analyzed. Efficiency of assembled anchor blocks is analyzed and tested using models. Anchoring sites, anchor implementation and large scale deployment with an example 1 GW power plant in the Kuroshio southeast of Taiwan are discussed. A preliminary cost analysis of the anchors and of the CSAM-based example power plant is also presented.

The Classification to Stationary Process of Tidal Motion Observed at the Time of Kuroshio’s Meandering

The tide level displays information about the state of the sea current and the tidal motion. The tide level of the southern coast of Japan Island is affected strongly by Kuroshio Current flowing in the western part of North Pacific Ocean. When Kuroshio takes the straight path and flow along the Japan Islands, the tide level increases, and it is calculated from two tide level data observed at Kushimoto and Uragami in the southern part of Kii Peninsula. In contrast, the tide level decreases at the time when Kuroshio leaves from the Japan Islands. In this paper, the hourly tidal data are analyzed using the Autocorrelation Function (ACF) and the Mutual Information (MI) and the phase trajectories at first. We classify the results into 5 types of tidal motion. Each categorized type is investigated and characterized precisely using the mean tide level and the unit root test (ADF test) next. The frequency of the type having unstable tidal motion increases when the Kuroshio Current is non-meandering or in a transition state or the tide level is high, and the type shows a non-stationary process. On the other hand, when the Kuroshio Current meanders, the tidal motion tends to take a periodical and stable state and the motion is a stationary process. Though it is not frequent, we also discover a type of stationary and irregular tidal motion.

Investigating the Effects of Super Typhoon HAGIBIS in the Northwest Pacific Ocean Using Multiple Observational Data

Various multi-source observational platforms have enabled the exploration of ocean dynamics in the Northwest Pacific Ocean (NPO). This study investigated daily oceanic variables in response to the combined effect of the 2019 super typhoon HAGIBIS and the Kuroshio current meander (KCM), which has caused economic, ecological, and climatic changes in the NPO since August 2017. During HAGIBIS, the six-hourly wind speed data estimated a wind stress power (Pw) which strengthened around the right and left semicircles of the typhoon, and an Ekman pumping velocity (EPV) which intensified at the center of the typhoon track. As a result, firstly, the sea temperature (ST) decreased along a boundary with a high EPV and a strong cyclonic eddy area, and the mixed layer depth (MLD) was shallow. Secondly, a low sea salinity (SS) concentration showed another area where heavy rain fell on the left side of the typhoon track. Phytoplankton bloom (PB) occurred with a large concentration of chlorophyll a (0.641 mg/m3) over a wide extent (56,615 km2; above 0.5 mg/m3) after one day of HAGIBIS. An analysis of a favorable environment of the PB’s growth determined the cause of the PB, and a shift of the subsurface chlorophyll maximum layer (SCML; above 0.7 mg/m3) was estimated by comprehensive impact analysis. This study may contribute to understanding different individually-estimated physical and biological mechanisms and predicting the recurrence of ocean anomalies.

Mesoscale eddies in the southwestern tropical Atlantic

The southwestern tropical Atlantic is a region of complex ocean dynamic where originates the strong western boundary current system composed of North Brazil current and North Brazil undercurrent. The region includes a variety of features including the Atoll das Rocas (AR) and Fernando de Noronha (FN) ridge that may favour mesoscale eddy dynamics. However, origin, occurrence and characteristics of mesoscale eddies were still not described in the region. Using satellite altimetry data from 1993 to 2018 off Northeast Brazil (37-25°W; 13-1°S), we reconstruct eddy trajectories and analyse the main eddy surface characteristics (e.g., size, amplitude, polarity) and their spatiotemporal variations. The study reveals two distinct dynamic regions before quantifying mesoscale eddies characteristics. Approximately 2000 mesoscale eddies crossed the region during the study period, among which 76% were generated inside the region, with amplitudes and radii ranging between 1 and 2 cm and 25 and 205 km, respectively. Eddies are preferentially formed between August and September and propagate westward. In the region around the FN Archipelago (36-26°W; 6-1°S), the formation of cyclonic eddies is likely favoured by barotropic instabilities of surface currents and the wind stress curl. On the other hand, in the south of the region (36-26°W; 12-8°S), eddies formation is likely associated with the barotropic instabilities, wind stress curl and the meandering of surface currents. Based on vertical temperature and salinity profiles from Argo floats’ data, we determined that in average, the core of cyclonic eddies is centred at ~ 130 m (140 m) in the northern (southern) region while the core of anticyclonic eddies is centred at ~90 m (125 m) in the northern (southern) region. Moreover, mesoscale eddies formed in the tropical Atlantic do not connect the eastern tropical Atlantic and northeast Brazil.

Microbial dimethylsulfoniopropionate (DMSP) cycling in the ultraoligotrophic eastern Indian Ocean

Dimethylsulfoniopropionate (DMSP) is an important source of dissolved organic matter for the marine food web and its cycling is a key step in ocean-atmosphere fluxes involved in the global sulfur cycle. To date, the abundance and biogeography of the genes encoding bacterial DMSP cycling in the eastern Indian Ocean (EIO) is virtually unknown. Moreover, DMSP measurements from the IO are sparse compared to other major oceans. In May–June 2019, we characterized dissolved DMSP (DMSPd) concentrations and the abundance of representative bacterial DMSP cycling genes along the 110 °E transect line as part of a voyage that contributed to Australia's involvement in the second International Indian Ocean Expedition. During the multidisciplinary voyage, surface water samples were collected from 19 stations spanning temperate to tropical waters of the EIO (39.5 °S to 11.5 °S, 110 °E). Somewhat surprisingly, a trend of greater DMSPd was measured in ultraoligotrophic (<0.02 μmol L−1 of nitrate/nitrite), low latitude waters compared to relatively nutrient-rich high latitudes, which contradicts global DMSPd patterns of high concentrations at high latitudes. Additionally, the average DMSPd concentration in EIO samples (17.2 ± 18.64 nM) was an order of magnitude greater than concentrations previously reported at similar latitudes in the Pacific and Atlantic Oceans, which suggests DMSPd is a readily available food source for microbes in a region that is often considered an ocean desert. The abundances of the bacterial DMSP production gene (dsyB), the DMSP lyase gene (dddP) and phylogenetically diverse DMSP demethylation genes (dmdA subclade A/1, D/all and E/2) were reported for the first time in the EIO region, demonstrating significant shifts in all genes with latitude. The SAR11 dmdA (D/all) gene was the dominant DMSP degradation gene across the transect (3.4 ± 0.94% of bacteria) and was notably positively correlated to DMSPd, demonstrating a tight coupling between the variables across the 30° transect. Our results also showed greater DMSPd and relative abundance of genes encoding both DMSP degradation pathways (dddP, dmdA A/1 and D/all) within a Leeuwin Current meander when compared to adjacent stations outside of the meander, providing evidence that mesoscale perturbations from the Leeuwin Current can greatly influence the EIO sulfur cycle. Overall, our data indicates that reduced sulfur in the form of DMSP is an abundant and readily available food source for some microbial metabolisms within the ultraoligotrophic surface waters of the EIO.

Characterization of water masses around the southern Ryukyu Islands based on isotopic compositions

We investigated the water-mass structure on the Okinawa Trough and Pacific sides of the southern Ryukyu Island Arc (Yonaguni, Iriomote, and Ishigaki subareas) using the Nd isotope composition (143Nd/144Nd ratios; expressed as εNd values) of benthic foraminiferal tests in surface sediments, which reflect bottom-water composition, along with hydrogen and oxygen isotope compositions (δD and δ18O values, respectively) and physical properties (temperature and salinity) of seawater. The Okinawa Trough side has lower εNd values than the Pacific side due to continental/island material inputs characterized by relatively low εNd values. Moreover, within the Okinawa Trough, other processes control the Nd behavior of seawater and primarily affect the Yonaguni and Iriomote subareas, as follows. (1) Surface and subsurface waters are influenced by Taiwanese river discharge combined with temporospatial variations in oceanographic conditions including Kuroshio Current meandering. (2) Intermediate water is characterized by low εNd values (down to − 8.2), possibly attributable to sediment plumes and turbiditic fluxes. (3) The εNd values of bottom water indicate upwelling and vertical mixing, with composition therefore being similar to those of intermediate water. The εNd profiles are better defined on the Pacific side. High εNd values occur in surface and subsurface (< 300 m depth, potential density < 25.0 kg m−3) waters, and low values (down to − 7.0) occur in subsurface–core-intermediate water (400–600 m depth, 26–27 kg m−3). εNd values increase slightly to − 4.0 below 750 m depth and remain constant down to about 2000 m depth, below which deep water shows a slight decrease in εNd values. Intermediate and bottom/deep waters are distinguished from upper layers by their lower δD and δ18O values.

Dynamics of a Standing Meander of the Subantarctic Front Diagnosed from Satellite Altimetry and Along-Stream Anomalies of Temperature and Salinity

Abstract Meanders formed where the Antarctic Circumpolar Current (ACC) interacts with topography have been identified as dynamical hot spots, characterized by enhanced eddy energy, momentum transfer, and cross-front exchange. However, few studies have used observations to diagnose the dynamics of ACC standing meanders. We use a synoptic hydrographic survey and satellite altimetry to explore the momentum and vorticity balance of a Subantarctic Front standing meander, downstream of the Southeast Indian Ridge. Along-stream anomalies of temperature in the upper ocean (150–600 m) show along-stream cooling entering the surface trough and along-stream warming entering the surface crest, while warming is observed from trough to crest in the deeper ocean (600–1500 m). Advection of relative vorticity is balanced by vortex stretching, as found in model studies of meandering currents. Meander curvature is sufficiently large that the flow is in gradient wind balance, resulting in ageostrophic horizontal divergence. This drives downwelling of cooler water along isopycnals entering the surface trough and upwelling of warmer water entering the surface crest, consistent with the observed evolution of temperature anomalies in the upper ocean. Progressive along-stream warming observed between 600 and 1500 m likely reflects cyclogenesis in the deep ocean. Vortex stretching couples the upper and lower water column, producing a low pressure at depth between surface trough and crest and cyclonic flow that carries cold water equatorward in the surface trough and warm water poleward in the surface crest (poleward heat flux). The results highlight gradient–wind balance and cyclogenesis as central to dynamics of standing meanders and their critical role in the ACC momentum and vorticity balance. Significance Statement The Antarctic Circumpolar Current (ACC) in the Southern Ocean is a nearly zonal current that encircles Antarctica. It acts as a barrier between warmer water equatorward and colder water poleward. In a few regions where the current encounters strong topographic changes, the current meanders and opens a pathway for heat to travel across the ACC toward Antarctica. We surveyed a meander in the ACC and examined the along-stream change of temperature. In the upper ocean, temperature changes are caused by a vertical circulation, bringing cool water down when entering the surface trough (the part of the meander closest to the equator), and warm water up when exiting the surface trough and entering the surface crest. At depth, cold water is transported equatorward in the surface trough and warm water poleward in the surface crest, leading to a net transport of heat poleward. This study highlights the importance of the secondary circulation within a meander for generating cross-ACC flows and moving heat toward Antarctica.

Large South Equatorial Current Meander in the Southeastern Tropical Indian Ocean Captured by Surface Drifters Deployed in 2019

AbstractTo investigate the South Equatorial Current (SEC) in the Indian Ocean, 18 drifters were deployed off Sumatra‐Java during boreal spring 2019. These drifter observations revealed a large SEC meander that swung southward about 1,100 km during 40 days along ∼100°E. The southward motion occurred on the eastern edge of the Seychelles‐Chagos thermocline ridge, accompanied by an anticyclonic eddy/Rossby wave from the east, indicated by the geostrophic velocity induced by the zonal pressure gradient. Meanwhile, the ageostrophic velocity induced by the southeasterly wind also contributes to this southward motion. This is verified by a Lagrangian Trajectory Model in which drifter trajectories are well reproduced using Chinese‐French Oceanic SATellite winds products. Further analysis of the historical drifters with similar trajectories reveals that the geostrophic and ageostrophic currents jointly drive drifters to move southward. Ageostrophic component could contribute up to 45% of the southward movement of the SEC meander.

Big Pine Creek Ditch revisited: Planform recovery to channelization and the timescale of river meandering

Channelization has changed the form and dynamics of rivers on a global scale. In the midwestern United States, widespread straightening of meandering headwater streams has been undertaken for the purpose of improving land drainage. Few studies have examined in detail how meandering streams respond to straightening, especially over timescales of nearly a century following straightening. This study uses historical aerial imagery and contemporary lidar data to examine how a small headwater stream that was straightened nearly a century ago, Big Pine Creek Ditch (BPCD) in Indiana, USA, has adjusted to straightening and to relate observed changes to stream power. The evolution of this fluvial system was examined in the late 1970s and this study updates that previous work using additional imagery and GIS-based methods, extending the timeline of analysis from 1932 to 2018. Results reveal that recovery varies spatially along the length of BPCD with some reaches not adjusting at all, some reaches increasing in sinuosity but then being artificially restraightened, and other reaches evolving continuously in response to channelization. For reaches that have evolved continuously, the rate of increase in sinuosity over time is directly related to bankfull stream power per unit length. Rates of increase in sinuosity per logarithmic unit of power per unit length have been linear and should attain the prechannelized relation between sinuosity and stream power over a timescale of about 100 years. Reaches with estimated bankfull stream power per unit area below 25 W m−2 exhibit no recovery of sinuosity, whereas those with power per unit area greater than 50 W m−2 have progressively increased in sinuosity. Between these thresholds, straightened reaches may or may not increase in sinuosity. Not all aspects of channel planform recovery are captured by changes in sinuosity; the prechannelized meandering stream exhibited greater lateral shifts in the position of the meander belt than do current meandering reaches. Overall, the study provides insight into spatial and temporal variability of recovery to channelization, the long-term recovery of meandering streams to straightening, as well as the timescale of meander development in straight channels.

Characteristics of Ocean Current Meandering and Potential Efficacy of Maximizing Power Capacity by Tracking Short-Term Meanders

Characteristics of Ocean Current Meandering and Potential Efficacy of Maximizing Power Capacity by Tracking Short-Term Meanders

Processes Governing the Seasonal Evolution of Mixed Layers in the Red Sea

AbstractThe processes governing the seasonal evolution of the oceanic mixed layer temperature (MLT) and salinity (MLS) in the Red Sea (RS) are analyzed using the outputs of a high‐resolution (1/100°) ocean general circulation model for 2001 to 2015, forced by a high‐resolution (5 km) regional atmospheric reanalysis. We quantify the roles of atmospheric forcing and the advective, diffusive, and entrainment processes in the seasonal variability of mixed layer (ML) properties by analyzing the closed and complete potential temperature and salinity budgets integrated over the ML depth. The seasonal evolution of the ML density is predominantly driven by the MLT, which is dominated by the air–sea heat exchange. The seasonal evolution of MLS is predominantly driven by the advection of fresher waters from the Gulf of Aden, whereas atmospheric forcing governs its gradual increase along the basin. The spatial distribution of strong mesoscale circulation and semipermanent eddies is imprinted on all processes, whereas advective fluxes tend to follow meandering currents around the periphery of mesoscale eddies. Entrainment processes affect the ML density through the reemergence of heat and salt stored below the ML. Entrainment is especially important in the northern parts of the RS, where increased salinity preconditions the upper layers for ML deepening and denser water formation.

Can the Intermediate Western Boundary Current recirculation trigger the Vitória Eddy formation?

South of the Vitória-Trindade Ridge, a seamount chain off East Brazil, the Brazil Current (BC) meanders cyclonically within Tubarão Bight, occasionally forming the Vitória Eddy. It was recently found that the Intermediate Western Boundary Current (IWBC), which flows equatorward below the BC, cyclonically recirculate within Tubarão Bight. We present an analysis of AVISO observations that suggest that the Vitória Eddy formation is conditioned by the strength of the BC upstream of Tubarão Bight. A weak BC is prone to local meandering and eddy formation in the bight, while a strong BC suppresses eddy formation in the bight but triggers downstream meander growth. To study the effects of the IWBC recirculation on the BC meandering and the Vitória Eddy formation, we formulate a simple two-layer quasi-geostrophic model. In the model, the BC is represented by a meridional jet in the upper layer and the IWBC recirculation is a steady eddy in the lower layer. The lower-layer eddy effectively acts as a topographic bump, affecting the upper-layer jet via the stretching term \(\psi _{2}/{R_{d}^{2}}\), where ψ2 is the lower-layer streamfunction and Rd is the baroclinic deformation radius. Based on the AVISO sea-surface height data and previous observational studies, we define a stationary eddy and reference jet. We conduct a number of initial-value problem experiments varying the upper-layer jet speed. A weak upper-layer jet slowly meanders and develops a cyclone above the lower-layer eddy. As we increase the jet velocity, the meandering is faster and the cyclone is larger. But a too-strong jet has an opposite effect: the potential vorticity anomalies induced by the lower-layer eddy are quickly swept away, leading to explosive downstream meander growth; no cyclone is formed above the lower-layer eddy. In all cases, the initial meandering trigger is a linear process (the steering of the upper-layer jet by the lower-layer eddy). But even when the upper-layer jet is weak, nonlinearity quickly becomes important, dominating the dynamics after 10 days of simulation. The downstream meander growth is fully nonlinear. Our idealized QG model confirms that the IWBC recirculation can trigger the Vitória Eddy formation and elucidates the mechanisms involved in this process.

Spatial and temporal variability of the Agulhas Retroflection: Observations from a new objective detection method

This study presents an improved method to identify the Location of the Agulhas Current's Core and Edges (LACCE), throughout the greater Agulhas system. In this study the method is applied to daily altimetric fields of absolute dynamic topography and geostrophic velocities. LACCE compares favourably with previous algorithms used to track the position of the Agulhas Current. The ability of the LACCE to correctly identify the core and edges of the Agulhas Current is also established through comparisons with along-track altimetry datasets and in situ observations of current speed and direction across both the Agulhas Current and the Agulhas Return Current. Our results show that LACCE can successfully be applied to a merged altimetry dataset to monitor changes in the Agulhas Current's position caused by mesoscale processes such as early retroflection events or transient features like Natal Pulses. To demonstrate the use of the LACCE, the Agulhas Current position outputs from the algorithm computed over a 26-year period (1993 to 2018) are then used to describe the spatial and temporal variability of the Retroflection. Our results show that the Agulhas Retroflection generally occurs in the region from 15.01–20.03°E and 40.47–38.44°S. The retroflection exhibits a trimodal distribution, indicating three preferential longitudes (15, 18 and 19.5°E). Despite not being statistically significant, the results suggest some seasonal variation in the modal position of the Retroflection, with it frequenting western longitudes more during the austral summer, in agreement with previous studies. Early retroflections were found to occur more often during the austral spring and summer, with few taking place in winter and almost none in autumn. The observations from this study agree with previous studies that found the triggering mechanism for early retroflections (east of 22.54°E) to be the interaction between large Agulhas Current meanders (Natal Pulses) with northward extensions of Agulhas Return Current meanders.

Sub-surface current meanders along the Namibian shelf

The influence of long, sub-inertial waves travelling poleward on spatiotemporal fluctuations in the hydrography of coastal upwelling areas has widely been discussed theoretically in the literature since the late 1970s. In this paper, in contrast, measured unexpected wave-like changes in the intermediate density field along the Namibian continental slope are presented and suggested to be the result of a geostrophically adjusted southward current with significant on-offshore meanders. Hydrographic measurements had been carried out on board of r/v ‘METEOR’ along the 200-m isobath above the Namibian continental shelf (19–23 °S) during seven days in October/November 2000. Measured water properties rhythmically change in space between oceanic (on-shore meander, well oxygenated, dominant Eastern South Atlantic Central Water, ESACW) and coastal waters (off-shore meander, poorly oxygenated, dominant South Atlantic Central Water, SACW). The first case is accompanied by downward displacements of the permanent pycnocline, while the second one corresponds to the opposite situation. The distance between these rather periodic along-section changes in stratification is roughly constant and estimated to be about 60 km. At two hydrographic station positions, duplicate measurements enable an estimation of the associated southward propagation speed of the signal. The obtained value of about 0.16 m/s roughly peaks at the pressure level of about 70 dbar, and the corresponding time scale is estimated to be a few days.