Alongshore Currents Research Articles

Legacy groundwater pollution as a source of mercury enrichment in marine food web, Haifa Bay, Israel

Haifa Bay (HB), located along the northern Mediterranean shore of Israel was polluted with Hg from a chlor-alkali plant (ECI) and from the Qishon River industries, for decades. From the mid-1980s industrial Hg loads into HB decreased dramatically until their complete cessation in 2000. Consequently, concentrations in marine biota and sediments decreased almost to reference levels. However, during 2006–2014, an unexpected increase of total Hg (THg) concentrations was observed in three commercial fish species collected at northern HB (N.HB). To determine the cause of this increase, THg and methyl Hg (MeHg) were measured in seawater, coastal groundwater, suspended particulate matter, plankton, macroalgae, benthic fauna, and in marine and beach sediments. THg in groundwater and sediments from the vicinity of ECI were extremely high (up to 251 μg L−1 and 2200 ng g−1, respectively). MeHg concentrations in groundwater were low and constituted <0.1% of THg, except in the surf zone opposite the ECI, where MeHg constituted 0.2% of the THg. THg and MeHg concentrations were consistently higher in benthic biota and plankton from N.HB and northwards, compared to corresponding samples from southern HB (S.HB) and the reference site (RS). MeHg in bivalves and sponges from N.HB and SZ was higher than from S.HB and RS, despite having similar THg concentrations, which suggests a stronger source of MeHg in N.HB. Our findings suggest that the discharge into N.HB of Hg polluted groundwater under the ECI increased during the period 2006–2014. The Hg was assimilated by plankton or adsorbed onto inorganic particles, which were further ingested by benthic and pelagic consumers, as well as transported northward with the alongshore current. These findings demonstrate for the first time the potential of relic pollution in groundwater to increase heavy metal burdens in local marine food webs.

Wave dynamics near Barataria Bay tidal inlets during spring–summer time

The wave characteristics and wave–current interaction in the six tidal inlets connecting the Barataria Basin and the northern Gulf of Mexico are investigated during spring–summer time period when both cold front passage and persistent southeast wind events exist. A fully-coupled unstructured-grid, three-dimensional, FVCOM-SWAVE model is employed and validated against observational data during April–June 2010. Model simulation results indicate that waves in the shelf side of the inlets are mainly wind seas and swells from the Gulf of Mexico that are modified by ocean currents in the Louisiana Bight. These currents include Mississippi River plume, buoyancy- and wind-driven alongshore current, as well as the tidal jet in the Barataria Pass. The bay side of the inlets, similar to the interior of the Barataria Basin, is dominated by locally generated waves. In the middle of the inlets, both wind seas and swells are presented at times, though different inlets behave differently in terms of contribution from each component. The contribution of local wind seas enhances during cold front passage. Contrastingly, the influence of swells from continental shelf is more pronounced during persistent southeast wind event. FVCOM-SWAVE predicts a decrease in significant wave height inside the Louisiana Bight due to current-induced refraction and wave stretching effect caused by clockwise gyre from Mississippi plume. The modulation of significant wave height at Barataria Pass due to partial wave blocking for the opposing current, and wave stretching for the following current can be up to 20%. The effect of depth-induced breaking and bottom friction overshadows the one from wave–current interaction at other shallower inlets.

Studying the present-day dynamics of Lena delta by space images

The dynamics of Lena delta coastline since the late XX century up to the present time have been studied using Landsat satellite data. The comparison of different-time images of morphologically different segments of the delta coastline has shown the major portion of the delta to be stable. Minimal changes have been recorded near the mouths of the branches carrying a minor portion of the Lena runoffthe Olenekskaya and Tumatskie. In the eastern part of the delta, near the mouths of copious branches Trofimovskaya and Bykovskaya, no changes have taken place, as well as in its western part, which is represented by a part of abrasion marine terrace embraced by the delta. In the southwestern part, in the Olenek delta, the delta coastline has been eroded with the erosion material transported eastward by along-shore current. Near the mouths of the Tumatskaya branch, the forms of marine accumulation are unstable, and the marine terrace in the northeastern part of the delta shows a minor local erosion. The periods of erosion in different delta areas are not synchronous and the eroded zones are not large in size. No increment in the land, nor shore progradation have been seen.

Alongshore Variability in Crescentic Sandbar Patterns at a Strongly Curved Coast

Sandbars, submerged ridges of sand parallel to the shoreline, tend to develop crescentic patterns while migrating onshore. At straight coasts, these patterns form preferably under near‐normal waves through the generation of circulation cells in the flow field, whereas they decay under energetic oblique waves with associated intense alongshore currents. Recently, observations at a man‐made convex curved coast showed an alongshore variability in patterning that seems related to a spatiotemporal variability of the local wave angle (Sand Engine). Here, we aim to systematically explore how coastline curvature contributes to alongshore variability in crescentic pattern formation, by introducing local differences in wave angle and the resulting flow field. A nonlinear morphodynamic model was used to simulate the patterns in an initially alongshore uniform sandbar that migrates onshore along the imposed curved coast. The model was forced by a time‐invariant and time‐varying offshore wave angle. Simulations show that the presence of patterns and their growth rate relate to the local breaker angle, depending on the schematization of the offshore angle and the local coastline orientation. Growth rates decrease with increasing obliquity as both refraction‐induced reductions of the wave height as well as alongshore currents increase. Furthermore, simulations of variations in coastline curvature show that patterns may develop faster at strongly curved coasts if this curvature leads to an increase in near‐normal angles. This implies that beaches where the coastline orientation changes substantially, for example, due to km‐scale nourishments, become potentially more dangerous to swimmers due to strong currents that develop with pronounced bar patterns.

ПРЕДВАРИТЕЛЬНЫЕ РЕЗУЛЬТАТЫ ЭКСПЕРИМЕНТАЛЬНОГО ИССЛЕДОВАНИЯ ДИНАМИКИ ПРИДОННОГО СЛОЯ НА ШЕЛЬФЕ–КОНТИНЕНТАЛЬНОМ СКЛОНЕ В ЧЕРНОМ МОРЕ

The preliminary results of an experimental study of a mechanism of the Black Sea anoxic layer ventilation related with the descent of oxygen-containing water down the bottom slope in Ekman boundary layer are presented. To study this mechanism, several automatic measuring stations were installed at the bottom of the shelf-continental slope zone in the depth range from 80 to 243 m, on the cross-section abeam of the Gelendzhik Bay. The observations that lasted for 1.5–2 months were fulfilled during two periods in the beginning and the end of 2018. The stations were registering hydrophysical (temperature, salinity, pressure and current velocity) and hydrochemical (dissolved oxygen concentration) parameters at a 0.5–2.5 m distance from the bottom. The acquired data are suitable for estimation of spatio-temporal parameters of water transfer in the bottom layer up and down the slope, depending on direction and intensity of alongshore current. Preliminary analysis of the first installation data confirmed the presence of bottom water transfer along the slope perpendicular to the shore. Also, in case of intense alongshore north-western current, a descent of the bottom layer water was observed. Such manner of water transfer conforms to both geostrophic adjustment and dynamics of the bottom Ekman boundary layer. However, variability of water density in the bottom layer, caused by vertical water transport, had about the same range as density fluctuations in the water column on the same depths. This fact disputes the effectiveness of Ekman transfer in the bottom layer as a water ventilation mechanism for the upper part of continental slope of the Black Sea (Zatsepin et al., 2007; Elkin, Zatsepin, 2017).

On the Wind Contribution to the Variability of Ocean Currents Over Wide Continental Shelves: A Case Study on the Northern Argentine Continental Shelf

AbstractThe southwestern Atlantic Ocean has one of the largest and most productive continental shelves of the southern hemisphere. Despite its relevance, its circulation patterns have been largely inferred from hydrographic observations and numerical models. Here we describe the variability of the shelf circulation based on the analysis of 11 months of multilevel currents measured by two bottom‐mounted acoustic Doppler current profilers deployed over the continental shelf at 39°S. The record‐length mean is 12 and 13 cm/s in the upper layer and decreases to 6 and 8 cm/s near the bottom, at the deployment nearer and further from the coast, respectively. The mean flow direction is toward the NE, following the orientation of the isobaths. Measurements at both sites show that the alongshore barotropic component accounts for 83% of the variability observed and are well correlated (0.86), suggesting a relatively uniform flow, which is presumably driven by large‐scale forcing. Indeed, large‐scale wind stress patterns dominate the temporal variability of the in situ currents and the passage of atmospheric fronts induces significant changes in the observed currents at all depths. We found that for 12% of the measurements the currents reverse the direction to the SW in response to these atmospheric patterns. Furthermore, the analysis of sea surface height reconstructed from bottom pressure measurements at both sites and from a coastal tide gauge reveals that the variability of the alongshore currents is driven by the cross‐shore pressure gradients generated by the alongshore wind stress.

Studying the Present-Day Dynamics of Lena Delta by Space Images

The dynamics of Lena delta coastline since the late XX century up to the present time have been studied using Landsat satellite data. The comparison of different-time images of morphologically different segments of the delta coastline has shown the major portion of the delta to be stable. Minimal changes have been recorded near the mouths of the branches carrying a minor portion of the Lena runoff—the Olenekskaya and Tumatskie. In the eastern part of the delta, near the mouths of copious branches Trofimovskaya and Bykovskaya, no changes have taken place, as well as in its western part, which is represented by a part of abrasion marine terrace embraced by the delta. In the southwestern part, in the Olenek delta, the delta coastline has been eroded with the erosion material transported eastward by along-shore current. Near the mouths of the Tumatskaya branch, the forms of marine accumulation are unstable, and the marine terrace in the northeastern part of the delta shows a minor local erosion. The periods of erosion in different delta areas are not synchronous and the eroded zones are not large in size. No increment in the land, nor shore progradation have been seen.

Oceanographic habitat and the coral microbiomes of urban-impacted reefs.

Coral reefs are in decline worldwide. In response to this habitat loss, there are efforts to grow, outplant, and restore corals in many regions. The physical oceanographic habitat of corals—such as sea temperature, waves, ocean currents, and available light—is spatially heterogeneous. We therefore hypothesize that outplant location may affect microbiomes, and ultimately, coral health and restoration success. We evaluated the influence of the physical oceanographic habitat on microbes in wild Porites astreoides and Siderastrea siderea. Tissue samples were collected at four Florida reefs in March, June, and September of 2015. We estimated oceanographic conditions from moored instruments, diver observations, remote sensing data, and numerical models. We analyzed microbiomes using amplicon 16S rRNA high-throughput sequencing data. We found microbial alpha-diversity negatively correlated with in situ sea temperature (which represented both the annual cycle and upwelling), as well as modeled alongshore currents, in situ sea-level, and modeled tide. Microbial beta-diversity correlated positively with significant wave height and alongshore currents from models, remotely-sensed relative turbidity, and in situ temperature. We found that archaea from the order Marine Group II decrease with increases in significant wave height, suggesting that this taxon may be influenced by waves. Also, during times of high wave activity, the relative abundance of bacteria from the order Flavobacteriales increases, which may be due to resuspension and cross-shelf transport of sediments. We also found that bacteria from the order SAR86 increase in relative abundance with increased temperature, which suggests that this taxon may play a role in the coral microbiome during periods of higher temperature. Overall, we find that physical oceanographic variability correlates with the structure of these coral microbiomes in ways that could be significant to coral health.

An Analytical Solution for Nearshore Circulation Driven by Focused/Defocused Waves

An offshore shoal or bar refracts ocean surface waves and causes wave focusing/defocusing on the adjacent beach. Wave focal patterns characterized by alongshore variations in wave height, wave angle, and breaking location induce alongshore non-uniformities of wave setup and nearshore circulation, e.g., rip currents and alongshore currents, in the surfzone. A simplified analytic model for nearshore circulation generated by focused/defocused waves on a planar beach is developed and theoretical solutions are obtained using transport stream function and perturbations in alongshore distributions of wave height and wave angle at the breaker line. The analytic model suggests that alongshore currents are strongly affected by a pair of counter-rotating vortices generated shoreward of the wave focal zone. The vortices are persistent, and their strengths depend on the amplitudes of alongshore variations in wave height and wave angle. The alongshore gradient in wave height tends to intensify the vortices while the convergence of wave angle tends to weaken the vortices. Divergent flows associated with the vortices in the surfzone are intense, strengthening alongshore currents in the downstream of the wave focal zone and weakening alongshore currents or causing flows reversal in the upstream. Alongshore currents are modulated by rip currents associated with the wave focusing/defocusing patterns.

Structure of alongshore mobilization of sediments, eastern part of gulf of Danzig

Systematization of analyzed schemes of alongshore mobilization of sediments off the east coast of the Gulf of Danzig was conducted. Significant differences in the structure and localization of the countercurrent flows in convergence zones were detected. The results of near-bottom steady-state measurements of currents in the vicinity of the Baltic Canal in the depth range 6–16 m demonstrated that &gt;6-m/s winds cause the alongshore currents to be reoriented in the direction of the alongshore wind component. The morphodynamic accumulative criterion (the filling of the re-entrant angle and abrasion in the obstacle-shadow zone) and “abrasive” criterion (the orientation of abrasion kettle holes at the ends of breakwalls) were examined. The abrasive criterion is apparently preferred over the accumulative one during transportation of sediments along segments of the coast where waves approach the coast at normal or near-normal angles. Peculiarities of the mechanism of the northward transportation of sediments, bypassing Baltiysk backwalls and changing from abrasive to accumulative criteria for different segments of the coast, are demonstrated. Two schema representing the opposite Vistula and Sambiysky alongshore flows of sediments are proposed: prior to construction of the backwalls facing Baltic Canal incoming waters with the vast area of migration at the north end of the Vistula bar, and the post-construction backwalls with the resulting narrowing of the area and displacement of its south boundary on the north side to the level of these backwalls.

Intra-annual variability of the North West Shelf of Australia and its impact on the Holloway Current: Excitement and propagation of coastally trapped waves

Shelf circulation on the North West Shelf (NWS) of Australia is dominated by seasonal variations of the alongshore Holloway Current forced by the monsoonal winds. However, the intra-annual variability of the current remains poorly understood. In this study, a combination of model outputs and two years of mooring time series data were used to analyse the intra-annual variability of the Holloway Current. The dominant intra-annual variability of the alongshore current on the NWS were at intraseasonal and semiannual frequencies, as revealed by sea level data. Whereas the interannual variability in the region is mostly forced remotely by tropical Pacific processes, the intraseasonal and semiannual signals are mostly driven by variations of regional winds off the northern coast of Australia. Phase analysis of sea level anomalies indicated that the excitation and propagation of Coastally Trapped Waves (CTWs) are forced by Madden-Julian Oscillations (MJO; intraseasonal) and semiannual wind anomalies. Intraseasonal CTWs are excited over the eastern parts of the Kimberley shelf, whereas semiannual waves are generated further to the east, over the northern Australian coast. The phase speed of intraseasonal CTWs is ~5.5 ms−1, characteristic of the first CTW mode, and 2.5 ms−1for semiannual CTWs, characteristic of the second CTW mode. The fluctuations in the velocity field over the NWS translates into significant alongshore current transport anomalies, ranging on average from ~0.4 Sv to ~0.6 Sv, for intraseasonal and semiannual frequency, respectively. Southwestward (northeastward) intraseasonal transport anomalies peak during Phase 1 (Phase 5) of the MJO cycle (out of the 8 phases of MJO) on the Kimberley shelf, whereas the corresponding transport anomalies on the Pilbara shelf peak during Phase 3 (Phase 7). Semiannual southwestward (northeastward) transport anomalies on the Kimberley shelf peak in April and October (January and July), preceding those on the Pilbara shelf by one month.

Shallow‐water deltaic clinoforms and process regime

AbstractUtilizing two outcrop data sets with dip direction exposures of shallow‐water (tens of meters) deltaic clinoforms, this paper quantifies sedimentary facies proportions and clinoform lengths and gradients, and links process regimes to delta clinoform dimensions. Both data sets are from foreland basins, the Cretaceous Chimney Rock Sandstone of the Rock Springs Formation from the US Western Interior, and the Eocene Brogniartfjellet Clinoform Complex 8 of the Battfjellet Formation from the Central Basin of Spitsbergen. Sedimentary facies indicate presence of both river‐ and wave‐dominated clinothems in each data set. Facies characteristics and distribution implies that river‐dominated clinothem progradation was primarily driven by deposition from weak hyperpycnal flow turbidity currents across the clinoforms, and minor slumps. Wave‐dominated clinothems were constructed by wave processes rather than alongshore currents, and are also progradational subaerial clinoforms, with one exception, where the formation of a compound subaqueous clinoform set indicates erosion and sediment bypass above the wave base. Sediment distribution and lithological heterogeneity in the river‐dominated clinothems is controlled by individual hyperpycnal flow events or mouth‐bar collapse events, and thus by self‐organization and minimal reworking that results in a heterogeneity that is difficult to predict (high entropy). The efficient reworking of river‐derived sediments in wave‐dominated clinothems results in predictable lithological sediment partitioning (low entropy). Clinoform dimension analyses show that although of similar sediment caliber, river‐dominated clinoforms in both data sets are on average 3–4 times steeper and 3–4 times shorter than the wave‐dominated clinoforms, with mean gradients of ca 4 degrees and ca 1 degree, respectively, and mean lengths of 150–230 m and 640–760 m. These results require corroboration from additional data sets, but do suggest that river‐ and wave‐dominated delta clinoforms are likely to have distinct downdip extents (lengths) and gradients for given clinoform heights. Clinoform shape can thus be a method for differentiating ancient river‐ vs. wave‐dominated deltaic clinoforms, in addition to their sedimentary facies, biogenic features and sandstone maturity, and helpful when incorporated into reservoir models.

Observations of Diurnal Coastal-Trapped Waves with a Thermocline-Intensified Velocity Field

AbstractUsing 18 days of field observations, we investigate the diurnal (D1) frequency wave dynamics on the Tasmanian eastern continental shelf. At this latitude, the D1 frequency is subinertial and separable from the highly energetic near-inertial motion. We use a linear coastal-trapped wave (CTW) solution with the observed background current, stratification, and shelf bathymetry to determine the modal structure of the first three resonant CTWs. We associate the observed D1 velocity with a superimposed mode-zero and mode-one CTW, with mode one dominating mode zero. Both the observed and mode-one D1 velocity was intensified near the thermocline, with stronger velocities occurring when the thermocline stratification was stronger and/or the thermocline was deeper (up to the shelfbreak depth). The CTW modal structure and amplitude varied with the background stratification and alongshore current, with no spring–neap relationship evident for the observed 18 days. Within the surface and bottom Ekman layers on the shelf, the observed velocity phase changed in the cross-shelf and/or vertical directions, inconsistent with an alongshore propagating CTW. In the near-surface and near-bottom regions, the linear CTW solution also did not match the observed velocity, particularly within the bottom Ekman layer. Boundary layer processes were likely causing this observed inconsistency with linear CTW theory. As linear CTW solutions have an idealized representation of boundary dynamics, they should be cautiously applied on the shelf.

The Structure of the Net Alongshore Sediment Transport in the Eastern Gulf of Gdansk

The published schemes of the net alongshore sediment transport near the eastern coast of Gdansk Bay have been systematized. Considerable discrepancies have been identified in the structure and localization of the convergence zones of opposite currents. The results of station-based measurements of bottom currents near the Strait of Baltiysk at depths of 6–16 m were used to show that, at winds >6 m/s, the alongshore currents run along the alongshore wind component. The morphodynamic accumulation (filling of the entry corner and abrasion in the shadow zone of the obstacle) and abrasion (orientation of the abrasion depression at the extreme end of the harbor moles) criteria are discussed. The abrasion criterion is shown to work better than the accumulation criterion in determining the net sediment transport in the coastal reaches with a normal or near-normal approach angle of the wave energy flow toward the shore. The specific features of the mechanism of sediment transport from south to north, by-passing Baltiisk harbor moles, from the abrasion to accumulation shore segments are given. Two schemes of the opposite Vistula and Sambian alongshore sediment flows are proposed: before the construction of the entry harbor moles in the Baltic Strait with a vast migration zone near the northern end of the Vistula Spit and after the construction of these moles, which has narrowed this zone and shifted its southern boundary northward toward the moles.

Influence of plume advection and particle settling on wastewater dispersion and distribution

Oceanographic processes are examined and used to assess their effect on the movement and dilution of wastewater discharged from Sydney's three deepwater ocean outfalls. Such processes link wastewater discharges with observed changes in biological communities and the accumulation of contaminants in the sediments. The East Australian Current, coastal trapped waves and local winds contribute 40%, 20% and 10% respectively to the total spectral energy in the along-shore currents off Sydney. Median dilutions at the edge of the initial dilution zone range between 185:1 and 347:1. Plumes remain submerged 95% of the time, surfacing only in the middle of winter and during large storms. Negatively buoyant particles sink from the plumes and first reach the ocean floor within 10 km of the discharge location. Resuspension of the sediments due to currents occurs 2.5% of the time and due to waves 1%–4% of the time.

Sand‐Mud Tidal Flat Morphodynamics Influenced by Alongshore Tidal Currents

AbstractTidal flats, where significant land‐ocean interactions take place, are often abstracted as a cross‐shore bed profile with sediment zonation from the lower sand flat to the upper mud flat. However, in addition to cross‐shore tidal currents, the impact of the alongshore components on cross‐shore sediment transport, morphological evolution, and sediment grain‐size change remains unclear. Here we investigated sand‐mud tidal flat morphodynamics in the absence of waves by combining approaches of field observations, numerical modeling, and analytical interpretations, based on the example of the tidal flat on the central Jiangsu coast, China. The results show that the morphodynamic processes are complicated by the interactions of cross‐shore and alongshore tidal currents, cross‐shore flat morphology, and sediment zonation. With amplified phase lags of the alongshore tidal level at the boundaries, alongshore currents become dominant over cross‐shore tidal currents on the lower flat, while the upper flat is always dominated by the cross‐shore currents. Therefore, bed profiles and mud content on the upper flat were independent of the alongshore tidal current magnitude, being convex‐up and consisting of mud. In contrast, the strong alongshore currents can erode mud on the lower flat and promote landward sand transport from the subtidal area to the lower flat, forming a sand flat. The maximum tidal bed shear stress is almost spatially uniform across the muddy area but pronouncedly elevated where the bed sediment coarsens on the lower flat. The contributions of the alongshore tidal currents and sand‐mud sorting processes should be appropriately addressed in similar coastal environments.

Coastal currents from Jason-2 and SARAL/AltiKa in the Indian region

ABSTRACT In this study, data from space-based altimeters (Jason-2 and Satellite for ARgos and AltiKa [SARAL/AltiKa]) have been used to compute alongshore geostrophic currents in the coastal regions of the Indian mainland. These derived currents are compared with high-frequency (HF) radar observations. Beyond 30–40 km away from the coastline, altimeter-derived currents match fairly well with the HF radar data. Root mean square error (RMSE) of Jason-2-derived currents ranges between 0.3 and 0.6 m s−1 while the same in the case of SARAL/AltiKa lies between 0.3 and 0.7 m s−1. Satellite-derived across-track geostrophic current components (alongshore current) were also used to study the spatiotemporal variations of the east India coastal current (EICC). The coastal trapping of the EICC, its annual and intra-seasonal peaks are clearly observed in the power spectrum of time-series of Jason-2 and SARAL/AltiKa derived currents.

Widespread shifts in the coastal biota of northern California during the 2014\u20132016 marine heatwaves

During 2014–2016, severe marine heatwaves in the northeast Pacific triggered well-documented disturbances including mass mortalities, harmful algal blooms, and declines in subtidal kelp beds. However, less attention has been directed towards understanding how changes in sea surface temperature (SST) and alongshore currents during this period influenced the geographic distribution of coastal taxa. Here, we examine these effects in northern California, USA, with a focus on the region between Point Reyes and Point Arena. This region represents an important biogeographic transition zone that lies <150 km north of Monterey Bay, California, where numerous southern species have historically reached their northern (poleward) range limits. We report substantial changes in geographic distributions and/or abundances across a diverse suite of 67 southern species, including an unprecedented number of poleward range extensions (37) and striking increases in the recruitment of owl limpets (Lottia gigantea) and volcano barnacles (Tetraclita rubescens). These ecological responses likely arose through the combined effects of extreme SST, periods of anomalous poleward flow, and the unusually long duration of heatwave events. Prolonged marine heatwaves and enhanced poleward dispersal may play an important role in longer-term shifts in the composition of coastal communities in northern California and other biogeographic transition zones.

Coastal Trapped Wave Propagation along the Southwest African Shelf as Revealed by Moored Observations

AbstractCoastal trapped waves (CTWs) that propagate poleward along the southwest African shelf potentially leak energy from lower latitudes into the Benguela Upwelling System (BUS). Thus, in addition to local winds, these waves provide an important remote forcing mechanism for the upwelling region. The present study aims at elucidating the nature of CTWs in the northern BUS. To this end, we make use of multisite velocity observations from the Namibian shelf (18°, 20°, 23°S) and examine the alongshore velocity signal for signatures of CTWs by means of wavelet methods. We found that a substantial amount of energy is concentrated within a submonthly to subseasonal frequency band (10–50 days). Based on the coherence and phase spectra of the alongshelf currents, we provide evidence for a predominantly southward phase propagation and establish typical time and length scales of CTWs in the region. It turns out that their properties differ significantly within a few hundred kilometers along the coast. A comparison of the results with theoretical dispersion curves shows that this difference may be explained by variations in the bottom topography. Finally, we investigate the coupling of the alongshore currents with the coastal and equatorial wind stress and highlight regions of potential wave generation.

Ocean circulation along the southern Chile transition region (38°–46°S): Mean, seasonal and interannual variability, with a focus on 2014–2016

Satellite and atmospheric model fields are used to describe the wind forcing, surface ocean circulation, temperature and chlorophyll-a pigment concentrations along the coast of southern Chile in the transition region between 38° and 46°S. Located inshore of the bifurcation of the eastward South Pacific Current into the equatorward Humboldt and the poleward Cape Horn Currents, the region also includes the Chiloé Inner Sea and the northern extent of the complex system of fjords, islands and canals that stretch south from near 42°S. The high resolution satellite data reveal that equatorward currents next to the coast extend as far south as 48°–51°S in spring-summer. They also display detailed distributions of forcing from wind stress and wind stress curl near the coast and within the Inner Sea. Between 38°–46°S, both winds and surface currents during 1993–2016 change directions seasonally from equatorward during summer upwelling to poleward during winter downwelling, with cooler SST and greater surface chlorophyll-a concentrations next to the coast during upwelling, opposite conditions during downwelling. Over interannual time scales during 1993–2016, there is a strong correlation between equatorial El Niño events and sea level and a moderate correlation with alongshore currents. Looking more closely at the 2014–2016 period, we find a marginal El Niño during 2014 and a strong El Niño during 2015 that connect the region to the tropics through the oceanic pathway, with some atmospheric connections through the phenomenon of atmospheric blocking (as noted by others). The period also includes a Harmful Algal Bloom of the dinoflagellate Alexandrium catenella during early-2016 that occurred during a sequence of physical conditions (winds, currents and temperatures) that would favor such a bloom. The most anomalous physical condition during this specific bloom is an extreme case of atmospheric blocking that creates a long period of calm in austral autumn after strong upwelling in austral summer. The blocking is related to the 2015–2016 El Niño and an unusual coincident positive phase of the Southern Annular Mode.