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.
•LACCE algorithm identifies core and edges of Agulhas Current system daily.•New algorithm is able to identify early retroflection events and Natal Pulses.•LACCE used to investigate spatial and temporal variability of Agulhas Retroflection.•Agulhas Retroflection generally observed between 15.01‐20.03°E and 40.47–38.44°S.•Early retroflections mostly occur in spring and summer, with fewer in winter.
Slowing of Caribbean through-flow Jury, Mark R.
Deep-sea research. Part II, Topical studies in oceanography,
October 2020, 2020-10-00, Letnik:
180
Journal Article
Recenzirano
Trends in near-surface currents passing through the southeastern Caribbean are described using ocean reanalysis and a coupled model projection. The decline of Caribbean through-flow is linked with ...tropical and polar forcing. In the upstream zone, the North Brazil Current shows increasing retroflection as the southern Hadley Cell expands across the equatorial Atlantic. This inhibits northwestward transport next to the Amazon River plume. In the downstream zone, the Labrador Current feeds low saline Arctic waters into the Gulf Stream, inducing an equatorward shift and reduced flow.
Evidence is growing for spin-down of the North Atlantic gyre and its feeder currents, that inhibit poleward heat exchange and contribute to climatic shifts. Eastward trends of winds and currents in the equatorial Atlantic is projected to bring a drier climate to the lower Amazon basin and southeastern Caribbean, hence greater salinity with knock-on effects for regional sea temperatures.
Abstract
We proposed and designed a non-Hermitian metasurface mirror consisting of two-layers acoustic labyrinthine metamaterials with a full phase control and inherent loss. It yields strongly ...asymmetric acoustic retroflection (>35 dB) for two opposite incident angles: free retroflection for positive incidence (PI) and strong absorption for negative incidence (NI). It is due to the different diffraction orders related to the period grating for two opposite incident angles: the value of 0 (without multiple reflections) for PI and the value of 2 (with multiple reflections) for NI. Our design may have prospective applications in acoustic sensing, acoustic antennas and noise control.
The generation of extreme waves in the Agulhas Current Retroflection as the result of strong wave-current interaction is examined. High-resolution simulations with two third-generation wave models ...are performed considering ocean surface currents for two years and six months. The study is made with high-resolution SWAN (Simulating Waves Nearshore) nested grids which use the boundary conditions from the outer WAM (Wave Advanced Modeling) grid. Surface current velocities of mesoscale eddies influence surface wave properties and wave energy. The effects of the current speed on the wave spectral shapes are explored inside the eddies and in their vicinity. The numerical results show that in the interior of the eddies the extreme wave parameters can reach high values as well as the probability of occurrence of abnormal waves.
•The generation of extreme waves in strong wave-current interaction is studied.•High-resolution simulations with two third-generation wave models are performed for two years and six months.•Surface current velocities of mesoscale eddies influence surface wave properties and wave energy.•The effects of the current speed on the wave spectral shapes are explored inside the eddies and in their vicinity.•The numerical results show that in the interior of the eddies the extreme wave parameters can reach high values.
The western tropical Atlantic is strongly influenced by the Amazon, receiving the full discharge from the largest river basin of the world. In order to ascertain the coastal-oceanic gradient in ...abundance and composition of planktonic decapod communities along the Amazon River Plume (ARP) and its retroflection, 33 plankton samples were obtained along three sampling transects: Coastal and oceanic area influenced by the ARP (Coastal IARP and Ocean IARP, respectively), and oceanic waters without ARP influence (Ocean). A total of 13,117 specimens, belonging to 33 taxa, were analyzed. The distribution of planktonic decapod communities was strongly influenced by the Amazon discharge, showing the following patterns: (1) The coastal-oceanic gradients in decapod abundance are similar to other shelf areas of eastern South America, with higher abundance in the area under ARP influence, (2) meroplanktonic decapods dominate in coastal and some oceanic areas due to the offshore transport of coastal organisms along the ARP retroflection, and (3) the ARP is clearly responsible for the observed differences in community structure between the three transects. Furthermore, this study highlights the importance of coastal and shelf environments as main sources of planktonic decapods for pelagic ecosystems in the tropical Atlantic during the period of strong North Brazil Current retroflection.
•A total of 13,117 specimens, belonging to 33 taxa, were analyzed.•Planktonic decapod distributions were strongly influenced by the Amazon discharge.•Higher abundance of decapods in the area under plume influence•Larval forms dominate in coastal and oceanic plume areas.•Coastal areas are sources of planktonic decapods for offshore pelagic ecosystems.
The freshwater transport (FWT) by the Labrador Current (LC) around the Grand Banks of Newfoundland (GBN) is diagnosed with the 26-year Global Ocean Physical Reanalysis 1/12° data (GLORYS12v1) during ...1993 - 2018. The time-mean FWT of the LC above the 1027.25 kg/m
3
isopycnal surface is 83.6 mSv (1 mSv = 10
3
m
3
/s) southward through the Flemish Pass. Among this 83.6 mSv, 42% (35.2 mSv) is exported into the interior of the North Atlantic along the whole pathway of the LC from the Flemish Pass to the Tail of the GBN, with 25.5 mSv by the mean advection and 7.2 mSv by the mesoscale eddy transport. The seasonal and inter-annual variations of the FWT in the east of the GBN are mainly caused by the variation of the horizontal velocity of the LC, and the variation of salinity makes a nontrivial contribution to the variation of the FWT to the north of 45°N. Around the Tail of the GBN, the mesoscale eddies make significant contributions to the time-mean FWT and the seasonal and inter-annual variations of the FWT.
Lagrangian subsurface isopycnal eddy diffusivities are calculated from numerical floats released in several regions of the Antarctic Circumpolar Current (ACC) of the 0.1° Parallel Ocean Program. ...Lagrangian diffusivities are horizontally highly variable with no consistent latitudinal dependence. Elevated values are found in some areas in the core of the ACC, near topographic features, and close to the Brazil‐Malvinas Confluence Zone and Agulhas Retroflection. Cross‐stream eddy diffusivities are depth invariant in the model ACC. An increase of Lagrangian eddy length scales with depth is masked by the strong decrease with depth of eddy velocities. The cross‐stream diffusivities average 750 ± 250 m2 s−1 around the Polar Frontal Zone. The results imply that parameterizations that (only) use eddy kinetic energy to parameterize the diffusivities are incomplete. We suggest that dominant correlations of Lagrangian eddy diffusivities with eddy kinetic energy found in previous studies may have been due to the use of too short time lags in the integration of the velocity autocovariance used to infer the diffusivities. We find evidence that strong mean flow inhibits cross‐stream mixing within the ACC, but there are also areas where cross‐stream diffusivities are large in spite of strong mean flows, for example, in regions close to topographic obstacles such as the Kerguelen Plateau.
Near the southern tip of the African continent, the Agulhas Current retroflects eastward to feed the Agulhas Return Current, giving rise to the Agulhas Retroflection (AR). The AR is dynamically ...unstable and radiates eddies and rings that constitute a portion of the Agulhas leakage. In this study, by analyzing 26 years (1993–2018) of updated satellite altimetry data, the spatial and temporal characteristic of the surface AR point are revisited. By adopting a refined identification algorithm of the AR point, we show that the AR longitude (XAR) fluctuates widely between 7° and 27°E on timescales ranging from days to months, without a stable seasonal cycle. The XAR variability also shows evident temporal asymmetry with shorter residence time at western positions. The retreat of the AR from the western to eastern positions tends to be swift, with 46% of the events completed in one day. In comparison, the intrusion from the eastern to western positions is much slower, with 32% of the events taking more than one month. The energy transfer from the mean flow to mesoscale eddies is likely the primary regime underlying the AR variability. Further analysis suggests that the AR variability causes sea surface temperature anomalies of ∼0.5°C through anomalous advection and thereby gives rise to turbulent heat flux anomalies of ∼10 W m−2, exerting potential impacts on the local climate.
Plain Language Summary
The Agulhas Retroflection (AR) is a key feature of the Agulhas Current system, and the choke point for the inter‐basin exchange between the Indian and Atlantic Oceans. The AR also serves as the origin of the Agulhas leakage that is constituted by northwestward moving eddies and rings radiated from the AR. Although there has been literature exploring the dynamics of the Agulhas Current and Agulhas leakage, our understanding of the AR variability is still incomplete. Using 26 years of updated satellite observation data of 1993–2018, we show that the mean longitude of the surface AR point is 17.5°E, and its fluctuation can be as wide as 7°−27°E. Owing to its strong variability and irregular behaviors, the AR does not possess a robust seasonal cycle. The AR behaviors are asymmetric: its residence time in the west tends to be much shorter than in the east. The interaction between the mean flow and mesoscale eddies is likely an essential regime controlling the AR variability. We also show that the AR variability can cause strong anomalies in regional sea surface temperature and air‐sea heat fluxes, which has potential influence on the local climate.
Key Points
The mean longitude of the AR is 17.5°E with a standard deviation of 2.6°E and shows temporal asymmetry in westward and eastward phases
The energy transfer from mean flow to eddies is likely the primary mechanism regulating the AR variability
The AR variability causes local SST anomalies of ∼0.5°C and surface heat flux anomalies of ∼10 W m−2
Background
Through-the-scope (TTS) endoscopic clipping devices are widely used. No benchtop testing or direct comparisons of these endoclips have been performed to show their rotational ability and ...inherent mechanical strengths during closure and after deployment. This study aimed to provide benchtop data that can be used to guide clinical applications and to promote future device research and development.
Methods
Benchtop testing and comparisons were performed for three groups of TTS clips: QuickClip2 long, resolution, and instinct clips. The main outcome measurements were device-in-endoscope retroflection angles (DIERA), opening strength, “snapping” force of acute clip closure, and neoprene pulling strength.
Results
The achievable gastroscope DIERA was 10° for QuickClip2, 3° for the resolution clip, and 10° for the instinct clip. The QuickClip and the Instinct clip rotated almost equally well under all endoscope configurations, including endoscopic retrograde cholangiopancreatography (ERCP). With or without a sheath, the resolution clip lacked the ability to rotate. During clip opening force testing (the amount of force required to force open the jaws of a deployed clip by 3.2 mm; 3.2 mm was chosen due to the standard dimension of the gauge used for the measurement), the Instinct clips were the strongest. For the Instinct clips, an opening force of 404 ± 124 g was needed to open the closed clip, and an additional 386 ± 133 g was required to open the clip jaws to 3.2 mm. In terms of snapping force during acute closure and neoprene pulling strength, the instinct and resolution clips performed almost equally. The limitations of the study were the benchtop testing and the finite sample size for closing and pulling strength comparisons.
Conclusions
The QuickClip2 and the Instinct clip rotate equally well under different endoscope configurations. The resolution clips lack rotational ability. The instinct clips are stronger mechanically than the other two TTS clips. Stronger clips are perhaps associated with higher therapeutic efficacy and retention rates.
The east Greenland current (EGC) and the smaller east Greenland coastal current (EGCC) provide the major conduit for cold fresh polar water to enter the lower latitudes of the North Atlantic. They ...flow equatorward through the western Irminger Basin and around Cape Farewell into the Labrador Sea. The surface circulation and transport of the Cape Farewell boundary current region in summer 2005 is described. The EGCC merges with Arctic waters of the EGC to the south of Cape Farewell, forming the west Greenland current. The EGC transport decreases from 15.5 Sv south of Cape Farewell to 11.7 Sv in the eastern Labrador Sea (where the water becomes known as Irminger Sea Water). The decrease in EGC transport is balanced by the retroflection of a substantial proportion of the boundary current (5.1 Sv) into the central Irminger Basin; a new pathway for fresh water into the interior of the subpolar gyre.
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