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This study reinvestigates the work of Lutjeharms et al. (2001, 2003) who documented the properties of a Natal Pulse using isopycnal Lagrangian floats. We combined Lagrangian analyses and Eulerian maps derived from objective analysis to better describe the evolution of a Natal Pulse along three density surfaces referred to as the surface (satellite‐observed), shallow (isopycnal 1026.8 kg m−3), and deep (isopycnal 1027.2 kg m−3) layer. Our observations show that this Natal Pulse extended to a depth of 1000 m and was associated with cyclonic relative vorticity values of about 6.5–8.5 × 10−5s−1in the surface and shallow layer and 4 × 10−5s−1in the deep layer. This Natal Pulse contributed to cross‐shelf exchange through the offshore advection of Eastern Agulhas Bank water near the surface, onshore advection of South Indian Central Water and/or Indian Equatorial Water in the shallow layer, and Antarctic Intermediate Water in the deep layer. Sea surface temperature maps showed that the downstream progression of the Natal Pulse along the 3000 m isobath was related to a readjustment of its rotation axis. This readjustment advected Eastern Agulhas Bank water into the Natal Pulse eddy and triggered a SST cooling of about 3°C in the cyclonic area. The importance of a warm recirculating Agulhas plume originating from the Natal Pulse was highlighted. This warm water plume extended to a depth of 700 m and was associated with onshore velocities exceeding those experienced within the Natal Pulse eddy by a factor of 2. Our observations indicate that the June/July 1998 Natal Pulse and its associated plumes enhanced cross‐shelf exchanges. Reanalysis of RAFOS float from Kapex experiment trapped into the Natal PulseAnalysis of Eulerian temperature, pressure, and vertical velocity fieldVorticity and structure of the cyclonic eddy within the Natal Pulse

A 3‐km resolution regional ocean model is used to investigate the role of wind‐driven coastal circulation and mesoscale variability on the inter‐annual variability of transport success in the southern Benguela between Cape Point (34°S) and St Helena Bay (32°S) from 1992 to 2011. Lagrangian particles are released within the top 100 m of the water column along an across‐shore transect off Cape Point. Transport success is given by the ratio of the number of particles that reach St Helena Bay over the total number of particles released. The analysis of transport success anomalies and their relationship with the local circulation and wind forcing reveal that there is no single driver of the inter‐annual variability. The transport success variability of particles released on the shelf (depths <300 m) mainly depends on their capacity to remain embedded within the coastal Benguela Jet. Nevertheless, peaks in offshore Ekman transport and episodic occurrence of a poleward inner‐shelf counter‐current contribute to negative anomalies. For particles released on the outer shelf edge (depths >500 m), across‐shore transports induced by mesoscale eddies are the main contributors to transport success variability. Rare passage of Agulhas rings near the shelf edge can induce strong offshore advection of particles into the open ocean. In contrast, shelf‐edge cyclonic eddies favor the onshore transport of particles originating from the outer shelf edge and thus contribute to increasing transport success. This study investigates the inter‐annual variation of Lagrangian transport in the southern Benguela Current upwelling system using a high‐resolution regional ocean model and particle tracking experiments. Transport of fish eggs and larvae by upper ocean currents is crucial for the marine ecosystem in this highly productive region since the spawning and nursery areas used by anchovies are separated by large distances (∼400 km). The alongshore connectivity between the Cape Peninsula and St Helena Bay from 1992 to 2011 is analyzed and linked to the regional ocean circulation and wind‐forcing on an inter‐annual time scale. We find that transport success is influenced by several drivers including the Benguela Jet, Ekman transport, the coastal inner‐shelf poleward counter‐current, and occasional interactions with eddies such as Agulhas rings and shelf‐edge cyclonic eddies. These findings provide a valuable information for future studies on the role of the physical drivers that impact the transport of larvae and eggs in the southern Benguela, underlining that no single driver can account solely for extreme positive or negative events in transport success. There are multiple physical drivers of the interannual variability of along‐shore transport success in the southern Benguela upwellingPositive anomalies of transport success are associated with a strong Benguela Jet, reduced offshore and southward transportNegative anomalies are associated with enhanced offshore and southward particles transport, and anticyclonic eddies There are multiple physical drivers of the interannual variability of along‐shore transport success in the southern Benguela upwelling Positive anomalies of transport success are associated with a strong Benguela Jet, reduced offshore and southward transport Negative anomalies are associated with enhanced offshore and southward particles transport, and anticyclonic eddies

The Weddell Sea is of global importance in the formation of dense bottom waters associated with sea ice formation and ocean‐ice sheet interaction occurring on the shelf areas. In this context, the Weddell Sea boundary current system (BCS) presents a major conduit for transporting relatively warm water to the Weddell Sea ice shelves and for exporting some modified form of Wedell Sea deep and bottom waters into the open ocean. This study investigates the downstream evolution of the structure and the seasonality of the BCS along the Weddell Sea continental slope, combining ocean data collected for the past two decades at three study locations. The interannual‐mean geostrophic flow, which follows planetary potential vorticity contours, shifts from being surface intensified to bottom intensified along stream. The shift occurs due to the densification of water masses and the decreasing surface stress that occurs westward, toward the Antarctic Peninsula. A coherent along‐slope seasonal acceleration of the barotropic flow exists, with maximum speed in austral autumn and minimum speed in austral summer. The barotropic flow significantly contributes to the seasonal variability in bottom velocity along the tip of the Antarctic Peninsula. Our analysis suggests that the winds on the eastern/northeastern side of the gyre determines the seasonal acceleration of the barotropic flow. In turn, they might control the export of Weddell Sea Bottom Water on seasonal time scales. The processes controlling the baroclinic seasonality of the flow need further investigation. In the Weddell Sea, large amounts of seawater are cooled to become dense and sink, carrying signals of human‐induced changes such as atmospheric carbon into the abyss of the ocean. Understanding the variability of the ocean currents at the Antarctic continental margin is critical because it controls both the export of the dense water formed in these areas and the access of warm water that may melt the Antarctic ice sheet. This study investigates the structure and the seasonality of the flow at the continental margin in the Atlantic sector of the Southern Ocean, using in situ observations upstream and downstream of the dense water formation regions. Following the bathymetry, ocean currents flow from East to West along the continental shelf edge. As water densifies along this path, the flow speed changes from being maximum at the ocean surface to be maximum at the bottom. The depth‐averaged current varies with a synchronized seasonality along the continental shelf break, reaching a maximum in austral autumn. Our analysis suggests that the winds on the eastern/northeastern margin drives the seasonality of the depth‐averaged flow along the shelf break, significantly contributing to changes in bottom velocity near the export region. The coherent seasonal acceleration of the barotropic flow significantly contributes to the seasonal variability in dense water outflowThe seasonal flow strength is in phase with the intensification of the surface stress upstream of the dense water formation regionOur results suggest a teleconnection to exist between the eastern/northeastern Weddell Sea winds and the barotropic flow

The land‐sea breeze is resonant with the inertial response of the ocean at the critical latitude of 30°N/S. 1‐D vertical numerical experiments were undertaken to study the key drivers of enhanced diapycnal mixing in coastal upwelling systems driven by diurnal‐inertial resonance near the critical latitude. The effect of the land boundary was implicitly included in the model through the “Craig approximation” for first‐order cross‐shore surface elevation gradient response. The model indicates that for shallow water depths (<∼100 m), bottom shear stresses must be accounted for in the formulation of the “Craig approximation,” as they serve to enhance the cross‐shore surface elevation gradient response, while reducing shear and mixing at the thermocline. The model was able to predict the observed temperature and current features during an upwelling/mixing event in 60 m water depth in St Helena Bay (∼32.5°S, southern Benguela), indicating that the locally forced response to the land‐sea breeze is a key driver of diapycnal mixing over the event. Alignment of the subinertial Ekman transport with the surface inertial oscillation produces shear spikes at the diurnal‐inertial frequency; however their impact on mixing is secondary when compared with the diurnal‐inertial resonance phenomenon. The amplitude of the diurnal anticyclonic rotary component of the wind stress represents a good diagnostic for the prediction of diapycnal mixing due to diurnal‐inertial resonance. The local enhancement of this quantity over St Helena Bay provides strong evidence for the importance of the land‐sea breeze in contributing to primary production in this region through nutrient enrichment of the surface layer. Winds near the coast often have a daily cycle known as the land‐sea breeze. Near latitudes of 30°N/S ubiquitous rotating ocean currents also have a daily frequency and therefore become enhanced by daily winds at these latitudes. The ocean currents result in vertical mixing of subsurface and surface water layers, bringing subsurface nutrients to the surface where they stimulate phytoplankton growth. In this study we use a simple model of the ocean (composed of the vertical dimension only) to study the key drivers of vertical mixing due to the land‐sea breeze. We show how vertical mixing is reduced in shallow water (<∼100 m) near the coast, where currents are slowed down by friction at the seabed. We find that vertical mixing can be predicted by a parameter computed from wind speed and direction over time. This parameter is shown to be enhanced over St Helena Bay on the west coast of South Africa, where phytoplankton blooms are known to be particularly prevalent. The results suggest that the land‐sea breeze is likely to be an important contributor to phytoplankton bloom development in this region. Similar processes are likely to be at play in other coastal regions. Land‐sea breeze driven vertical mixing is studied using a 1‐D model including the land boundary effectThe land boundary effect dampens vertical mixing, particularly when bottom friction is nonnegligibleThe diurnal anticyclonic rotary component of the wind stress provides a diagnostic for diapycnal mixing