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This study analyzes fluctuations in the atmospheric boundary layer height (aBLH) over a Caribbean island using hourly measured and model-interpolated data from the 2019–2023 period. Our focus is the mean structure, diurnal cycle, and aBLH correlation with meteorological parameters on the leeward coast at Mayaguez (18.2 N, 67.1 W). The mean diurnal cycle of the aBLH increases from 300 m near sunrise (07:00) to 1200 m by 13:00 because of turbulent heating. Summer-time thermal circulations lead to a 3 °C increase in near-surface dewpoint temperature (Td) that propagates upward to 3000 m by 16:00. A case study demonstrates how mid-day trade winds turn onshore and generate significant rainfall and river discharge across the island. The context for this study is provided by a 24 yr cluster analysis that identifies rainfall over the island's northwest interior driven by upstream heating. Analysis of linear trends from 1979 to 2023 shows that Td declined by −0.02 °C/yr above 1500 m because of large-scale subsidence. However, cool interior forests transpire humidity and instill contrasting trends that may amplify climate extremes. A better understanding of entrainment at the top of the atmospheric boundary layer could be critical for managing future water resources in Caribbean islands. [ABSTRACT FROM AUTHOR]

This study analyzes fluctuations in the atmospheric boundary layer height (aBLH) over a Caribbean island using hourly measured and model-interpolated data from the 2019–2023 period. Our focus is the mean structure, diurnal cycle, and aBLH correlation with meteorological parameters on the leeward coast at Mayaguez (18.2 N, 67.1 W). The mean diurnal cycle of the aBLH increases from 300 m near sunrise (07:00) to 1200 m by 13:00 because of turbulent heating. Summer-time thermal circulations lead to a 3 °C increase in near-surface dewpoint temperature (Td) that propagates upward to 3000 m by 16:00. A case study demonstrates how mid-day trade winds turn onshore and generate significant rainfall and river discharge across the island. The context for this study is provided by a 24 yr cluster analysis that identifies rainfall over the island’s northwest interior driven by upstream heating. Analysis of linear trends from 1979 to 2023 shows that Td declined by −0.02 °C/yr above 1500 m because of large-scale subsidence. However, cool interior forests transpire humidity and instill contrasting trends that may amplify climate extremes. A better understanding of entrainment at the top of the atmospheric boundary layer could be critical for managing future water resources in Caribbean islands.

Over 2 years, underwater gliders measured currents, temperature, and salinity along lines 40 km up‐ and downstream of the abrupt topography of Palau to examine topographic effects on the incident flow. The mean flow is westward and is associated with the North Equatorial Current. Salinity on isopycnals is a passive tracer. During westward flow, this tracer variance is elevated downstream in the strongest flow north of the topography at scales of 36 km (evaluated from 12 to 60 km) consistent with a wake eddy at the scale of the topography. During eastward flow, variance is elevated downstream at all measured scales. This result suggests eddies are trapped at the topography because westward eddy propagation is opposed by the incident eastward flow. This time‐mean tracer and current structure up‐ and downstream are used to estimate terms in the advection‐diffusion equation. The mean isopycnal diffusivity is about 17 times greater than background values at 36‐km scales (and 40 times greater within 20 km of the topography), while the mean diapycnal diffusivity is 100 times greater than background values for this latitude. The contrast between up‐and downstream salinity on isopycnals can reach 0.2 psu over 5–10 km, which is seen in the glider measurements and one ship‐based survey. This contrast arises due to eddies detaching from the North Equatorial Countercurrent or its recirculations. These episodes are associated with westward‐propagating, intraseasonal Rossby waves. Plain Language Summary: Two years of underwater glider measurements were repeated along lines up‐ and downstream of Palau to examine topographic effects on the incident flow. For over half of the time, in the upper 200 m, the flow was westward in the North Equatorial Current. Previous work showed an eddy of about 40 km extent formed in the wake of the topography during westward flow. A passive tracer displays elevated variance at these scales as the westward flow passes the north point of Palau. The tracer is salinity on a density surface; density differences lead to currents and are active, but salinity differences at a given density have no effect on currents and instead are stirred by the currents. Diffusion is estimated from the difference in the mean tracer structure between up‐ and downstream. Diffusion along isopycnals is about 17 times greater than background at the scale of the wake eddies. Diffusion across isopycnals is 100 times greater than background values and is often associated with current shear or internal waves. Episodes of tracer contrast between up‐ and downstream come from either recirculation of North Equatorial Countercurrent or eddies detaching from it. These episodes appear linked to westward‐propagating, intraseasonal waves on the thermocline. Key Points: Thermohaline variability on isopycnals is elevated at scales of 12–60 km in westward mean flow past topography consistent with wake eddiesUp‐/downstream thermohaline differences on isopycnals coincide with westward‐moving eddies, which seem related to intraseasonal Rossby wavesMean isopycnal (diapycnal) diffusivities are 17 (100) times greater than background values within 40 km of the topography [ABSTRACT FROM AUTHOR]

Upwelling and downwelling at two near‐equatorial islands in the western Pacific, Hatohobei Island and Helen Reef, Palau, are investigated using time series from a temperature array in the upper 57 m from 2014 to 2019 and a ship survey from 3 to 5 July 2013. During the survey, on the upstream side, upwelling/downwelling is found above/below an eastward subsurface current maximum due to an unusual mesoscale eddy. This eddy detached from a meander in the eastward North Equatorial Countercurrent in May 2013 and translated southwestward to the islands over 2 months bringing among the strongest currents seen at these islands in altimetry from 1993 to 2020. Following a streamline, the flow satisfied the Bernoulli equation outside of the wake. In both the cyclonic and anticyclonic wakes from the south and north points of Helen Reef, elevated estimated diffusivities from a shear‐based parameterization were similar at 10−5 m2 s−1 and a factor of 10 above background values. In the time series, downwelling in the upper 57 m is associated with a broader, surface‐intensified NECC converging at topography. Also, the shallower thermocline in 2014 and during the El Niños of 2015–2016 and 2018–2019 led to increased stratification at this depth and permitted greater internal wave activity. Peaked semidiurnal internal waves with peak‐to‐peak amplitudes of 4°C were equal to interannual changes. The net downwelling/warming of 0.5°C at the temperature array when smoothed over 2 months is consistent with both current‐driven downwelling and internal‐wave driven mixing of warmer waters downwards. Plain Language Summary: Spatial and temporal variability of upwelling and downwelling are investigated using time series from a bottom‐mounted temperature array in the upper 57 m and a ship survey at two near‐equatorial islands in the western Pacific, Hatohobei Island and Helen Reef, Palau. When the strong, eastward North Equatorial Countercurrent encounters the upstream side of an island, it can produce upwelling or downwelling depending on conditions, such as the depth of the maximum current, changes in current speed or direction with depth, and the depth of the stratified thermocline. With a subsurface/surface current maximum, downwelling/upwelling is found in the upper ocean. El Niños produce a surface current maximum at the islands and a shallow thermocline throughout the western Pacific, which permits semidiurnal internal gravity wave activity to reach the upper ocean. Temperature fluctuations are 4°C peak‐to‐peak, which equal the interannual changes from El Niño. When averaged over 2 months, these waves contribute to a net warming/downwelling of 0.5°C in the upper ocean. In the wake of the eastern island in both clockwise and anticlockwise wake eddies, mixing estimated indirectly from ship‐based current measurements is similar and elevated above background by a factor of about 10. Key Points: Interannual shoaling of the thermocline permits internal wave activity in the upper ocean and may mix warmer water downwardsPeak‐to‐peak temperature changes from internal waves and El Niño are each 4°C, which may be a common at other Pacific islandsOutside of the wake, the Bernoulli equation for steady, inviscid flow is satisfied [ABSTRACT FROM AUTHOR]

Using current, hydrographic and satellite observations collected off Northeast Brazil around the Fernando de Noronha Archipelago and Rocas Atoll during two oceanographic cruises (spring 2015 and fall 2017), we investigated the general oceanic circulation and its modifications induced by the islands. In spring 2015, the area was characterized by lower SST (26.6°C) and deep mixed-layer (∼90 m). At this depth, a strong current shear was observed between the central branch of the eastward flowing near-surface South Equatorial Current and the westward flowing South Equatorial Undercurrent. In contrast, in fall 2017, SST was higher (∼28.8°C) and the mixed-layer shallower (∼50 m). The shear between the central South Equatorial Current and the South Equatorial Undercurrent was weaker during this period. Interestingly, no oxygen-rich water from the south (retroflection of the North Brazil undercurrent) was observed in the region in fall 2017. In contrast, we revealed the presence of an oxygen-rich water entrained by the South Equatorial Undercurrent reaching Rocas Atoll in spring 2015. Beside these global patterns, island wake effects were noted. The presence of islands, in particular Fernando de Noronha, strongly perturbs central South Equatorial Current and South Equatorial Undercurrent features, with an upstream core splitting and a reorganization of single current core structures downstream of the islands. Near islands, flow disturbances impact the thermohaline structure and biogeochemistry, with a negative anomaly in temperature (−1.3°C) and salinity (−0.15) between 200 and 400 m depth in the southeast side of Fernando Noronha (station 5), where the fluorescence peak (>1.0 mg m–3) was shallower than at other stations located around Fernando de Noronha, reinforcing the influence of flow-topography. Satellite maps of sea-surface temperature and chlorophyll-a confirmed the presence of several submesoscale features in the study region. Altimetry data suggested the presence of a cyclonic mesoscale eddy around Rocas Atoll in spring 2015. A cyclonic vortex (radius of 28 km) was actually observed in subsurface (150–350 m depth) southeast of Rocas Atoll. This vortex was associated with topographically induced South Equatorial Undercurrent flow separation. These features are likely key processes providing an enrichment from the subsurface to the euphotic layer near islands, supplying local productivity.

The Kuroshio current is well known for generating cold wakes behind islands over Izu Ridge in Northwestern Pacific. Observational data from the geostationary Himawari-8 satellite for 2015–2017 revealed the occurrence of cold waters during the period when the Kuroshio current flows away from the islands. With a focus on tidal currents, this study presents an investigation of dynamical processes responsible for the formation of areas with low sea surface temperature (SST) through the adoption of a high-resolution numerical ocean model for an event that happened in July 2017. Areas with cold water emerged only when tidal currents are included in the numerical model. The model results indicate the cold surface waters are formed in the vicinity of the islands because of upwelling and vertical mixing. Qualitative features of the cold water formation for each island are found to depend on its size, topography, and ambient currents. Near Kozu Island, the tidal excursion is large enough to cause eddy shedding. These shed eddies are stirred by tidal currents to extend the surface cooling effect to wider areas. Near Hachijo Island, a persistent wake is formed by the ambient northward current. Inclusion of tidal currents destabilizes the wake, and consequently leads to the formation of a low SST area, although no clear eddy shedding is detected. The flow patterns around the islands are classified using an additional non-dimensional parameter, defined as the ratio between tidal excursion and island diameter. [ABSTRACT FROM AUTHOR]

Abstract When the Kuroshio passes Green Island off Taiwan, well-organized wakes are formed by the interaction between the Kuroshio and the island. The vertical mixing in the wake produces cold water, which entrains from the mixed layer below the surface and results in relatively colder and saltier waters with a higher chlorophyll-a concentration on the sea surface to the lee of the island. The meander of the Kuroshio maximum velocity axis has a major influence on the island wake. The shift of the Kuroshio in front of Green Island not only weakens the flow velocity but also affects the development of the wake. In the Green Island wake, the density overturns with a Thorpe scale between 2.9 m and 20.5 m and the turbulent kinetic energy dissipation rate is 0.2 × 10−6–8.5 × 10−5 W kg−1, which corresponds to an eddy diffusivity in the range of 0.01–0.23 m2 s−1. The spatial distribution patterns of the sea surface temperature (SST) in the island wake are classified into four distinct types, as obtained from moderate-resolution imaging spectroradiometer (MODIS) SST images. The most frequently occurring type is of the wake alone, which accounts for 86.7% of the island wake patterns. The other three types are a wake with a tail stretching downstream (4.0%), a wake with a small cyclonic cold core but no tail stretching downstream (6.8%) and an S-shaped meandering wake (2.5%). Highlights • The vertical structure of Kuroshio-induced Green Island wake is presented. • Four distinct types of surface pattern of the island wake are identified. • Wind force has an obvious impact on the spatial size of the island coldwake. [ABSTRACT FROM AUTHOR]

Dynamics of ocean current-induced island wake has been an important issue in global oceanography. Green Island, a small island located off southeast of Taiwan on the Kuroshio path was selected as the study area to more understand the spatial structure and temporal variation of well-organized vortices formed by the interaction between the Kuroshio and the island. Sea surface temperature (SST) and chlorophyll-a (Chl-a) concentration data derived from the Himawari-8 satellite and the second generation global imager (SGLI) of global change observation mission (GCOM-C) were used in this study. The spatial SST and Chl-a variations in designed observation lines and the cooling zone transitions on the left and right sides of the vortices were investigated using 250 m spatial resolution GCOM-C data. The Massachusetts Institute of Technology general circulation model (MITgcm) simulation confirmed that the positive and negative vortices were sequentially detached from each other in a few hours. In addition, totals of 101 vortexes from July 2015 to December 2019 were calculated from the 1-h temporal resolution Himawari-8 imagery. The average vortex propagation speed was 0.95 m/s. Totals of 38 cases of two continuous vortices suggested that the average vortex shedding period is 14.8 h with 1.15 m/s of the average incoming surface current speed of Green Island, and the results agreed to the ideal Strouhal-Reynolds number fitting curve relation. Combined with the satellite observation and numerical model simulation, this study demonstrates the structure of the wake area could change quickly, and the water may mix in different vorticity states for each observation station.

Island wakes in shallow water are investigated using the Regional Ocean Modeling System (ROMS). In contrast to deep water where bottom stress can be neglected in island wakes, shallow water implies that inhomogeneity in the bottom stress plays an important role in the wake vorticity generation. A series of numerical experiments are conducted to investigate wake formation and evolution in shallow water. It is found that the vertical structures of shallow-water and deep-water wakes are significantly different because of the presence of a density frontal jet, which results from the interaction between stratification and bottom topography. The frontal jet reaches its maximum within the bottom boundary layer over the shelf, giving rise to vorticity. The potential vorticity (PV) balance analysis reveals that frictional and diapycnal processes play different roles in the PV anomalies. With the absence of lateral stress (i.e., a sea mountain case), the surface vorticity becomes much weaker than that in the presence of an island. [ABSTRACT FROM AUTHOR]

As an important marine renewable energy resource, tidal stream energy has increasingly attracted public attention. However, the detailed hydrodynamic properties are often poorly understood at potential tidal stream energy sites, where local topographical features usually induce complicated current structures, such as eddies. In the present study a three-dimensional hydrodynamic model is employed to reproduce the flow field at a promising coastal site for tidal current energy extraction, where there is an island located near a headland. Comparisons between the model predictions and the survey data show good agreement. Two large eddies are found near the island-headland system in each tidal cycle. The eddies form during the flood and ebb tides, and the rotation does not cease until slack water. When the eddies move across the island-headland system, fluctuations are detected both in current speed and direction. The eddies induced velocity fluctuations cause a significant change in the energy density, with its magnitude being about half the maximum, whilst the water elevation change is limited. Therefore, the impact of eddies should be taken into account seriously in practical tidal stream energy explorations. [ABSTRACT FROM AUTHOR]

Vortex shedding downstream of Green Island due to passing of the Kuroshio is numerically studied. The numerical model is based on depth-averaged shallow-water equations with the space-time least-squares finite-element method. Vortex characteristics, such as spatial scales (aspect ratio and dimensionless width), temporal scale (Strouhal number), and propagation speed of the vortices are quantified. It is found that aspect ratio = 2.4, dimensionless width = 1.1, and Strouhal number = 0.157 for Reynolds number 100. The computed spatial scales and temporal scale agree reasonably with available satellite images and in situ measurements, as well as other simulation results. The magnitude of the propagation speed of the vortices is found to be of the same order as that of the Kuroshio, implying that the nonlinearity of vortices as well as the interactions between vortices and the Kuroshio are strong. [ABSTRACT FROM AUTHOR]

Idealized studies of island wakes often use a cylinder-like island to generate the wake, whereas most realistic studies use a close representation of the oceanic bathymetry immersed in a complex representation of the 'ambient' geophysical flows. Here, a system of multiple islands was placed into numerical and experimental channels, in order to focus on the complexity of the archipelago wake, including (a) the influence of small neighboring islands and (b) the role of the island-shelf. The numerical geostrophic and stratified channel was built using a three-dimensional primitive equation model, considering a realistic representation of the Madeira archipelago bathymetry, with prescribed initial and boundary conditions. Results from the simulations show that the neighboring islands alter the near-field wake. Small eddies generated by the neighboring islands lead to destabilization of the shear layers of the larger island. Laboratory experiments carried out in the Coriolis rotating tank corroborated this near-field disruptive mechanism. The neighboring island perturbation effect was present whatever the direction of the incoming flow, but under different regimes. North-south wakes produced geostrophic eddies (≥ R), whereas west-east wakes produced (exclusively) ageostrophic submesoscale eddies (< < R) which traveled offshore with wave-like motion. The archipelago shelf contributed to the asymmetric vertical migration of oceanic vorticity. Cyclonic vorticity dominated the surface dynamics, whereas anticyclonic circulation prevailed at the bottom part of the linearly stratified upper layer. This study identifies several likely wake scenarios induced by the Madeira archipelago, and may serve as guide for future multiscale numerical studies and in situ campaigns. [ABSTRACT FROM AUTHOR]

Abstract: An idealized numerical study of the influence of a tidal flow around an island has been undertaken with ROMS. The study focusses on coastal island wakes which are mainly controlled by elliptical tidal current flows on shallow shelves. This model is typical of some isolated continental shelf islands. The model is forced by a semi-diurnal barotropic inertia gravity wave imposed on the four open boundaries of a rectangular domain and its propagation results in an elliptical tidal flow within the domain in which the circular island lies. The influence of the surrounding island bathymetry and of the ellipse shape has been studied both in two and three dimensions. In the island vicinity, the residual circulation patterns over a tidal period show alongshore flow divergence along the major axis and convergence along the minor axis. A thin tidal ellipse (i.e. with a large ratio between major and minor axes) leads to strong eddy activity periods in the lee of the island during the flood and ebb phases, with eddy dissipation phases in between. By contrast, an almost round ellipse (axis ratio nearly 1) leads to vorticity filaments which continuously progress around the island without eddy shedding. The presence of a topographic slope in the vicinity of the island strengthens the eddy activity. This study suggests that the tidal current rotation favors the development of the eddy rotating in the same direction and weakens the development of the second eddy. In three dimensions with a surrounding bathymetry, an intense upwelling occurs in a large area in the lee of the island and the vertical velocities are stronger with thinner ellipses. With a flat bottom the vertical motions are almost fully generated by convergence and divergence of the secondary flow. With a varying bottom topography, the vertical motions come from a combination of this mechanism with convergence and divergence of the depth averaged flow. [Copyright &y& Elsevier]

Abstract: A series of Synthetic Aperture Radar (SAR) images acquired during the summer of 2006 revealed island wakes in the lee of the Izu Islands, south of Japan. The wakes were formed of not only meandering disturbances but also a series of eddies with diameters on the order of those of the islands. The Kuroshio flowed near these islands in the summer of 2006, indicating that the island wakes were induced by a Kuroshio-island interaction. Satellite sea-surface temperature (SST) and Chlorophyll-a (Chl-a) images observed during the summer of 2006 revealed low-SST and high Chl-a wakes, some of which included low-SST eddy trains. It is thus inferred that the phenomenon entailed upwelling and mixing processes and biological productivity known as the “island-mass effect.” High-spatial-resolution SST derived by combining a LANDSAT infrared channel and AVHRR SST clearly revealed a well-defined SST front associated with the island wakes and a 1 km-scale low-SST wake pattern. A numerical simulation was performed to investigate the formation mechanism. The simulation qualitatively reproduced the cold-eddy pattern, with eddy-driven mixing developing a mixed layer down to 200 m, causing low-SST island wakes. The shedding frequency and spacing of the model-produced eddies were roughly close to those of the Kármán vortex theory, suggesting that Kármán-type cold-eddy trains are commonly formed behind the islands when the Kuroshio strong flow impinges on them. [Copyright &y& Elsevier]

Abstract: Hydrodynamic forcing plays an important role in shaping the dynamics of marine organisms, in particular of plankton. In this work we study the planktonic biological activity in the wake of an island which is close to an upwelling region. Our research is based on numerical analysis of a kinematic flow mimicking the hydrodynamics in the wake, coupled to a three-component plankton model. We use parameter values relevant for the Canary wake, and the main results for a realistic range of parameters in this area are as follows: (a) Primary production is enhanced in the region of the wake opposite to the upwelling zone. (b) There is a strong dependence of the productivity on the inflow conditions of biological material entering the wake transported by the main current. Finally (c) we show that under certain conditions the interplay between wake structures and biological growth leads to plankton blooms inside mesoscale hydrodynamic vortices that act as incubators of primary production. [Copyright &y& Elsevier]

Abstract: This paper reports on the case study of Rattray Island (Great Barrier Reef, northeast Australia), lying perpendicular to tidal flow in shallow waters. At ebb and flood, attached (stable) eddies develop in the wake where swirls of turbidity suggest that sediment-laden waters are brought to the surface as a result of vertical transport. Both eddy and tip upwellings are encountered in the tidal flow around Rattray Island but there is currently no clear-cut answer as to which secondary flow generates upwelling with the largest intensity. This paper addresses this specific issue through idealized and realistic high-resolution numerical experiments. The analysis is supported by physical arguments based on the theory of flow separation. Given Rattray''s geometry and surrounding bathymetry, the mechanism of flow separation in shallow waters helps explain the asymmetry in size of the eddies and their intensity. The results of idealized numerical experiments also suggest that eddy and tip upwellings may be of similar intensity at Rattray Island. [Copyright &y& Elsevier]

Abstract: A three-dimensional finite element model is used to investigate the formation of shallow-water eddies in the wake of Rattray Island (Great Barrier Reef, Australia). Field measurements and visual observations show that stable eddies develop in the lee of the island at rising and falling tides. The water turbidity downstream of the island suggests the existence of strong upwelling that would be responsible for carrying bed sediments up to the sea surface. We first propose to look at the upwelling velocity and then use the theory of the age to diagnose vertical transport. The water age is defined as the time elapsed since particles of water left the sea bottom, where the age is prescribed to be zero. Two versions of this diagnosis are considered. Although the model predicts upwelling within the eddies, it is not sufficiently intense to account for vertical transport throughout the water column during the life span of the eddies. As mesh resolution increases, this upwelling does not intensify. However, strong upwelling is then resolved off the island''s tips, which is confirmed by the results obtained with the age. This study also shows that the finite element method, together with unstructured meshes, performs well for representing three-dimensional flow past an island. [Copyright &y& Elsevier]

Correlations between fine-scale oceanographic features and aggregations of marine mammals are frequently reported, but the physical forces shaping these relationships are rarely explored. We conducted a series of oceanographic observations and remote sensing surveys of an oceanographic feature near Grand Manan Island known to attract marine mammals on flood tides. We tracked drift drogues from cliff-top with a theodolite and conducted box-type surveys with an acoustic Doppler current profiler (ADCP) to assess flow patterns within the oceanographic feature. The feature was also visualized with Synthetic Aperture Radar (SAR) scenes. Drift drogues were advected towards a shear line originating near the northern tip of the island and entrained in one or more eddies downstream. ADCP surveys confirmed the presence of the shear line between rapid easterly flow and slower return flow. As the tide progressed, the shear line extended and manifested a single anti-cyclonic eddy at its distal end. As the flood tide progressed, northerly flow along the eastern shore of the island intensified and deflected the shear line northwards, shedding the eddy at slack high water. SAR images confirmed the presence of the shearline and eddy system, illustrating the evolution of a wake behind the island on flood tides. Profiles of flow direction and acoustic backscatter revealed secondary flows within the wake consistent with models and observations of other wakes. Oceanographic and remote sensing observations confirm that an island wake is generated by tidal flow past Grand Manan Island and provide an ecological context for the predictable aggregations of odontocete and mysticete cetaceans observed foraging within this region. [ABSTRACT FROM AUTHOR]

Abstract: During the summer, a northward, wind-driven current dominates the Rottnest Island region in southwestern Australia. Field studies have shown that the interaction between Rottnest Island and the northward current generates upwelling at the western end of the island, which is advected downstream, resulting in isotherm doming in the wake region. Flow curvature-induced secondary circulation has been proposed as the dominant mechanism responsible for this upwelling. Here, a three-dimensional numerical model, together with field observations, was used to undertake a detailed investigation of the three-dimensional flow structure in the wake region. Comparison of the observed upwelling pattern and the simulated flows revealed the island''s dominant role in generating upwelling. This result was confirmed with the use of idealized numerical experiments. The modeling results confirmed the presence of secondary circulation, generated as a result of flow curvature at the western end of the island, which caused strong upwelling and extended downstream. [Copyright &y& Elsevier]

On 26 June 2003 (approximately 1030 local time) the Helios ultralight aircraft broke apart off the west coast of Kauai Island, Hawaii as it was climbing out of the Kauai wind shadow. Following the aircraft mishap, a study was carried out to understand the conditions on the day of the crash and to better characterize the wind in the lee of Kauai. As part of this effort, both aircraft measurements and numerical modelling studies were carried out. Measurements and models showed the trade wind flow was enhanced around the island creating a region of wind shear surrounding the leeside calm zone. This wind shear region was found to be vertically oriented along the south side but tilted northward with height along the northern side of the calm zone. Several other factors on the day of the crash were investigated including water vapour gradients, diurnal Island heating, and gravity waves but their possible influences on the crash could not be confirmed. While the numerical model captured the general features of the Kauai leeside winds, the orientation of the calm zone was north of the observed one. [ABSTRACT FROM AUTHOR]

Beamer Rock, a 50-m-wide island in the Firth of Forth, produces a distinctive von Kármán vortex street wake, the characteristics of which depend on the speed and direction of the tidal flow. ADCP (acoustic Doppler current profiler), CTD (conductivity-temperature-depth) and aerial photograph data were collected from the region during flood and ebb tidal flow under neap and spring conditions. Good agreement was found between the observations and the results from a fixed-grid depth-averaged numerical tidal model. The island wake parameter correctly predicted the unsteady nature of the wake, and the Strouhal number (defined in terms of flow past a circular cylinder) was found to give excellent predictions of the wake wavelength when scaled on the island width. Contrary to published results, the study shows that it is possible to accurately simulate an unsteady island wake using a relatively coarse fixed-grid numerical model. [ABSTRACT FROM AUTHOR]

Field data and a numerical model have been used to investigate the von Kármán vortex street generated by a 50 m wide island in the Firth of Forth for which the characteristics differ between flood and ebb. During the flood, water depth in the lee of the island is shallow, hence little vertical structure develops in the velocity due to bottom-induced mixing. In contrast, the ebbing water depth is deeper in the lee of the island, allowing vertical structure to develop in the velocity field. However, the ebb wake is complicated by the presence of a lateral flow due to a bathymetric restriction. The effect of this lateral flow is to inhibit development of the ebb wake during spring tide conditions; whereas the effect is minimal during neap (lower freestream velocity) conditions. This change in wake development was clearly visible in the in situ horizontal and vertical velocity fields. While the observed flood wake varied considerably between neap and spring, the characteristics of the ebb wake were similar for neap and spring because of this overbank flow. The overbank flow has an important role in determining the varying magnitude of the vorticity of eddies generated either side of the island. [Copyright &y& Elsevier]

The aim of the present study is to apply the three-dimensional Princeton Ocean Model to study the wind effects on Kuroshio-induced island wake in the lee of Green Island, Taiwan. Numerical results indicate that the effect of NE winds squeezes the Kuroshio-induced island vortex street close to the coast and the SW winds tend to push the island vortex street farther away from the coast. The simulated vortex streets are analyzed by the dimensionless spatial lengths to quantify the prescribed feature. By comparing the three-dimensional results with different wind conditions, the Ekman transports are observed and the influence depths of wind effects are studied. Additionally, some cold eddies are found in temperature fields resulting from numerical simulations. These results are in qualitative agreement with field measurements and satellite images. [ABSTRACT FROM AUTHOR]

Tidal flow can generate unsteady wakes, large eddies, and recirculation zones in the lee or around complex natural and artificial obstructions, such as islands, headlands, or harbours. It is essential to understand the flow patterns around such structures given the potential impacts they can have on sedimentation, the marine environment, ecology, and anthropogenic activities. In this paper, the wake around an island in a macro-tidal environment has been studied using a widely used hydro-environmental model, Telemac-2D. Current data collected using moored acoustic Doppler current profilers (ADCPs) were used to validate and refine the Telemac-2D model. Four different turbulence models and several different solver options for the k- ε model were tested in this study to assess which representation could best replicate the hydrodynamics. The classic k- ε model with the solver of conjugate residual was the most suitable method to simulate the wake in the lee of the island. The model results showed good correlation with measured data. The island wake parameter used to predict the wake behaviour and its predictions matched the model results for different tidal conditions, suggesting that the island wake parameter could be used to predict the wake behind obstacles in macro-tidal environments. The model predictions showed the development of a wake is similar between ebb and flood tides in the neap tide while showing more difference in spring tide. With the increase of velocity in the neap tide, two side-by-side vortices will appear and then changing to stable Karman Vortex Street. During the ebb phase of spring tide, the wake will develop from a stable vortex to an unstable Karman Vortex Street, while the wake remained stable with two vortices during an flood tide. [ABSTRACT FROM AUTHOR]

Dynamics of ocean current-induced island wake has been an important issue in global oceanography. Green Island, a small island located off southeast of Taiwan on the Kuroshio path was selected as the study area to more understand the spatial structure and temporal variation of well-organized vortices formed by the interaction between the Kuroshio and the island. Sea surface temperature (SST) and chlorophyll-a (Chl-a) concentration data derived from the Himawari-8 satellite and the second generation global imager (SGLI) of global change observation mission (GCOM-C) were used in this study. The spatial SST and Chl-a variations in designed observation lines and the cooling zone transitions on the left and right sides of the vortices were investigated using 250 m spatial resolution GCOM-C data. The Massachusetts Institute of Technology general circulation model (MITgcm) simulation confirmed that the positive and negative vortices were sequentially detached from each other in a few hours. In addition, totals of 101 vortexes from July 2015 to December 2019 were calculated from the 1-h temporal resolution Himawari-8 imagery. The average vortex propagation speed was 0.95 m/s. Totals of 38 cases of two continuous vortices suggested that the average vortex shedding period is 14.8 h with 1.15 m/s of the average incoming surface current speed of Green Island, and the results agreed to the ideal Strouhal-Reynolds number fitting curve relation. Combined with the satellite observation and numerical model simulation, this study demonstrates the structure of the wake area could change quickly, and the water may mix in different vorticity states for each observation station. [ABSTRACT FROM AUTHOR]

Eddies caused by island wakes often appear downstream of an island where upwelling currents are coupled with nutrient transport. Several island wake eddies downstream of the Yongle atoll (YA), Huaguang Reef (HR) and Yuzhuo Reef (YR) in the Xisha Archipelago (XA) are found based on SAR and GF-1 satellite images taken on April 14, 2009 and May 30, 2014, respectively. The present study aims to explore the features and mechanisms of spring island wakes based on a high-resolution numerical model as well as satellite data. Our results are: 1) The cyclonic wake eddies (CWEs) are surface-intensified, and introduce a 20 – 40 m uplift of the isopycnal for the most intense eddies; While the anticyclonic wake eddies (AWEs) reach a subsurface maximum at a depth of 25 – 50 m, which leads to the weak isopycnal downwelling. The CWEs are generally more abundant than the AWEs in different radius classes under different incoming flows, which is due to the inertial-centrifugal instability (ICI) of the AWEs. 2) In the XA, lower SST features observed in the downstream side of the reefs demonstrate the presence of current-induced island wakes. In the near wakes of YA, HR and YR, the periodic vorticity generation is mainly caused by shear flow instabilities arising from the side boundaries of the reefs. Anticyclonic vorticity generation is slightly larger than that of the cyclonic for these three reefs. 3) For the submesoscale wake dynamics, the barotropic instability is more significant than the baroclinic in the XA. Strong toroidal vortices of alternating sign, which strongly perturb the density, were observed as the existence of the ICI in the transects of an unstable intense AWE. But these could hardly be found in a weak AWE. The unstable AWEs may have a larger decay of the velocity at their eddy edges and large persistent vorticities at their cores, which lead to an increase of the ratio between the core vorticity and the Rossby number Γ. While stable AWEs have longer lifetimes with nearly constant Γ values that are similar to those of the stable CWEs. • Island wakes were found in the Xisha Archipelago based on SAR and GF-1 satellite images. • Cyclones lead to a strong isopycnal uplift while anticyclones get a weak isopycnal downwelling in the reefs' wakes. • Strong toroidal vortices of alternating sign, which strongly perturb the density, were observed in intense anticyclones. • Periodic vorticity generation is mainly caused by shear flow instabilities arising from the side boundaries of the reefs. • The barotropic instability is more significant than the baroclinic in the Xisha Archipelago area. [ABSTRACT FROM AUTHOR]