Horizontal Current Velocity Research Articles

Mesoscale Eddies and Near-Inertial Internal Waves Modulate Seasonal Variations of Vertical Shear Variance in the Northern South China Sea

Abstract Diapycnal mixing in the South China Sea (SCS) is commonly attributed to the vertical shear variance S2 of horizontal ocean current velocity, but the seasonal modulation of S2 is still poorly understood due to the scarcity of long-term velocity observations. Here, this issue is explored in detail based on nearly 10-yr-long acoustic Doppler current profiler (ADCP) velocity data from a mooring in the northern SCS. We find that S2 in the northern SCS exhibits significant seasonal variations at both the near-surface (90–180 m) and subsurface (180–400 m) layers, but their seasonal cycles and modulation mechanisms are quite different. For the near-surface layer, S2 is stronger in late summer, autumn, and winter but weaker in spring and early summer, while in the subsurface layer, it is much stronger in winter than in other seasons. Further analysis suggests that in the near-surface layer, the stronger S2 in autumn and winter is primarily caused by typhoon-induced near-inertial internal waves (NIWs) and the large subinertial (SI) velocity shear of the baroclinic mesoscale eddies, respectively. With respect to the subsurface layer, the enhanced wintertime S2 is primarily associated with the “inertial chimney” effect of anticyclonic eddies, trapping wind-forced downward-propagating NIWs and significantly increasing the near-inertial shear at the critical layer. The findings in this study highlight the potentially important roles of mesoscale eddies and NIWs in modulating the seasonality of upper-ocean mixing in the northern SCS. This modulation is attributed not only to the strong shear of these features but also to their interactions. Significance Statement Vertical shear variance of velocity S2 significantly modulates turbulent mixing in the thermocline, but its climatologically seasonal variations and the associated mechanisms are still obscure due to the scarcity of long-term in situ velocity data. By analyzing nearly a decade of velocity data, we reveal significant seasonal variations in S2 at different ocean layers in the northern SCS and uncover different seasonal cycles and modulation mechanisms. The study sheds light on the pivotal roles of mesoscale eddies and near-inertial internal waves in modulating seasonality of S2 in the upper ocean. These findings have important implications for improving mixing parameterizations in numerical models.

A New Rotating Axes Method for Processing High-Resolution Horizontal Velocity Measurements on EM-APEX floats

Abstract A new rotating axes method (RAM) is developed to improve the vertical resolution of the horizontal current velocity measurements u at EM-APEX floats. Unlike the traditional harmonic fitting method (HFM), which yields u averaged in 50-s intervals, RAM decodes and interprets 1-Hz measurements of horizontal seawater velocity , and averages in 12-s windows for removing wind waves with a typical peak frequency ∼ 0.12 Hz. Estimates of u from RAM agree with those from HFM but with a higher vertical resolution of ∼1.5 m, 4 times better than HFM. Note that extracting float signals due to seawater motion needs to assume slow-varying voltage offset ΔΦoffset. The typical variations of estimated ΔΦoffset do not affect the results of u significantly. Estimates of u are excluded when ΔΦoffset fluctuates strongly in time and scatter significantly. RAM is applied to float measurements taken near Mien-Hua Canyon, Taiwan. Composite vertical shear spectra Ψ computed using u from RAM exhibit a spectral slope of −1, as expected for the saturated internal waves in the vertical fine-scale range. The RAM provides EM-APEX float’s horizontal velocity measurements into fine vertical scales and will help improve our understanding of energy cascade from internal wave breaking and shear instability into turbulence mixing.

Frequency spectra of horizontal current velocities on the northeastern shelf of the Black Sea

The paper presents a technique for restoring gaps in the original data, which made it possible to study the variability of fluctuations in the currents speed over significant time intervals. Analysis of the reconstructed continuous series of observations showed that high-frequency fluctuations in current speed predominate in the coastal zone, the possible source of which is the submesoscale eddies present here most of the time. On the continental slope, i.e. in the deep-water part of the sea, larger-scale processes dominate, associated with the dynamics of mesoscale eddies, meandering of the main Black Sea current, as well as global (on the scale of the entire sea) wind variability.

Diapycnal and isopycnal mixing along the continental rise in the Australian–Antarctic Basin

Shipboard measurements of temperature, salinity and horizontal current velocity were used to estimate diapycnal and isopycnal diffusivities in the Australian sector of the Southern Ocean between 120∘E and 150∘E near 64∘S, along the climatological positions of the Southern Antarctic Circumpolar Current Front and the Southern Boundary. The diapycnal diffusivities estimated by the finescale parameterization on the neutral density surfaces of 1028.05, 1028.2, and 1028.3 kg m−3 are (9.5±6.8)×10−5, (10.7±2.6)×10−5, and (13.8±3.7)×10−5 m2s−1, respectively (mean ± one standard deviation). At these densities, salinity and temperature decreased with depth but salt fingering was unlikely. Using the water mass transformation framework and a box inverse model, the isopycnal diffusivities were estimated from the diapycnal diffusivities. Climatological estimates of the isopycnal diffusivities in the density ranges of Upper Circumpolar Deep Water, Lower Circumpolar Deep Water, and Antarctic Bottom Water were −30±116, 16±319, and −30±83 m2s−1, respectively. These diffusivities were likely underestimates by 10 to 20 m2s−1 because eddy transport could not be considered in the climatological data. The weak isopycnal diffusivities were not inconsistent with diffusivity suppressed by the frontal jets. The large uncertainties were caused by the uncertainty in the box inverse model and the fact that the size of the box was comparable to the advection speed times the diffusion time scale.

Reconstructing the Parameters of Internal Tidal Bores in the Banda Sea from Satellite Remote Sensing Data

This article presents an algorithm for estimating the vertical displacements of isopycn and horizontal current velocities caused by internal tidal borings from their manifestations in the roughness field on satellite images of the Banda Sea. The calculation algorithm is formulated within the framework of the Korteweg-de Vries equation (KdV), augmented with a term taking into account the cylindrical divergence. The coefficients of the KdV equation — quadratic nonlinearity, dispersion and tidal linear internal wave velocity — were calculated using the mean — climatic density profile in the Banda Sea. The leading soliton velocity and wavelength in the rear zone of the internal tidal borings were determined from satellite images. Using the proposed methodology, estimates of the maximum vertical displacement of the pycnocline/maximum amplitude of the leading soliton and the maximum velocity of surface currents induced by the internal tidal borings were obtained. It follows from the analysis of satellite images that the internal tidal borings in the Banda Sea are transformed under the action not only of nonlinearity and dispersion, but also of cylindrical divergence. It is shown in the framework of the knoidal model of internal tidal borings that in the process of evolution the energy of its leading and subsequent waves is attenuated due to the cylindrical divergence. This attenuation is estimated when the leading soliton travels a distance equal to twice the tidal period.

A Mesoscale Eddy With Submesoscale Spiral Bands Observed From Seismic Reflection Sections in the Northwind Basin, Arctic Ocean

AbstractThe Arctic Ocean is well‐known for its large number of mesoscale eddies. We observed a lens‐shaped mesoscale eddy by using two roughly perpendicular multichannel reflection seismic sections collected in the Northwind Basin, Arctic Ocean in late September 2011. This is the first seismic imaging of the water column in the Arctic Ocean. The two lines were acquired about seven days apart. The eddy had a horizontal scale of ∼56 km and extended from near‐surface to 300 m. The thickness of the eddy was ∼250 m. The eddy core, characterized by weak reflections, has a radius of ∼13 km. The volume of core water was ∼89 km3. The horizontal current velocity collected by acoustic Doppler current profiler simultaneously with the seismic data shows that the eddy had a rigid core with a rotation period of ∼26 hr, and max azimuthal velocities of 0.59 m s−1. The velocity outside the core decreased in a quadratic inverse curve with the radius. During the observation, the eddy moved westward with an advection velocity of ∼1 cm s−1, and the net transport of the core water was about 0.05 Sv. There were multi‐arm structures at the boundaries of the eddy, forming submesoscale spiral bands. The steep spiral arms (dip angle can be up to ∼4°) may be caused by eddy stirring. Seismic observations provide important insights into our understanding of the vertical structure of ocean eddies, the horizontal and vertical transport of matter, and fluxes of heat that can cause sea ice melting in the Arctic Ocean.

Nonlinear dispersion in wave-current interactions

<p style='text-indent:20px;'>Via a sequence of approximations of the Lagrangian in Hamilton's principle for dispersive nonlinear gravity waves we derive a hierarchy of Hamiltonian models for describing wave-current interaction (WCI) in nonlinear dispersive wave dynamics on free surfaces. A subclass of these WCI Hamiltonians admits <i>emergent singular solutions</i> for certain initial conditions. These singular solutions are identified with a singular momentum map for left action of the diffeomorphisms on a semidirect-product Lie algebra. This semidirect-product Lie algebra comprises vector fields representing horizontal current velocity acting on scalar functions representing wave elevation. We use computational simulations to demonstrate the dynamical interactions of the emergent wavefront trains which are admitted by this special subclass of Hamiltonians for a variety of initial conditions.</p><p style='text-indent:20px;'>In this paper, we investigate:</p><p style='text-indent:20px;'>(1) A variety of localised initial current configurations in still water whose subsequent propagation generates surface-elevation dynamics on an initially flat surface; and</p><p style='text-indent:20px;'>(2) The release of initially confined configurations of surface elevation in still water that generate dynamically interacting fronts of localised currents and wave trains. The results of these simulations show intricate wave-current interaction patterns whose structures are similar to those seen, for example, in Synthetic Aperture Radar (SAR) images taken from the space shuttle.</p>

Observing Surface Wave Directional Spectra under Typhoon Megi (2010) Using Subsurface EM-APEX Floats

AbstractEM-APEX floats as autonomous vehicles have been used for profiling temperature, salinity, and current velocity for more than a decade. In the traditional method for processing horizontal current velocity from float measurements, signals of surface wave motion are removed as residuals. Here, a new data processing method is proposed for deriving the horizontal velocity of surface waves at the floats. Combined with the vertical acceleration measurements of waves, surface wave directional spectra E(f, θ) can be computed. This method is applied to the float measurements on the right of Typhoon Megi’s 2010 track. At 0.6 days before the passage of Megi’s eye to the floats, the fast-propagating swell may affect wind waves forced by the local storm wind. When the storm moves closer to the floats, the increasing wind speed and decreasing angle between wind and dominant wave direction may enhance the wind forcing and form a monomodal spectrum E(f). The peak frequency fp ~ 0.08 Hz and significant wave height > 10 m are found near the eyewall. After the passage of the eye to the floats, fp increases to >0.1 Hz. Although E(f) still has a single spectral peak at the rear-right quadrant of Megi, E(f, θ) at frequencies from 0.08 to 0.12 Hz has waves propagating in three different directions as a trimodal spectrum, partially due to the swell propagating from the rear-left quadrant. Enhancing the capability of EM-APEX floats to observe wave spectra is critical for exploring the roles of surface waves in the upper ocean dynamics in the future.

SINS/DVL Integrated System With Current and Misalignment Estimation for Midwater Navigation

Motivated by the problem that water-track Doppler Velocity Log (DVL) cannot effectively suppress the error accumulation of strap-down inertial navigation system (SINS), this paper proposes a novel SINS/DVL integrated navigation algorithm for deep and long cruising range Human Occupied Vehicle (HOV). Such algorithm decomposes the navigation process into two tightly coupled working modes: alignment mode and navigation mode. In the alignment mode, HOV is controlled to perform horizontal circular motion for several minutes to realize Self-Aided SINS. Combining the precise navigation solutions provided by Self-Aided SINS and measurements from DVL with water track, recursive least square (RLS) algorithm is adopted to estimate heading misalignment angle between SINS and DVL and horizontal ocean current velocity. In the navigation mode, both horizontal ocean current velocity obtained in the alignment mode and DVL measurements are utilized to assist SINS, thus enabling SINS/DVL/Current integrated navigation. A square trajectory with a navigation-grade inertial measurement unit (IMU) is simulated to evaluate the proposed SINS/DVL integrated navigation algorithm. Simulation results show that the proposed SINS/DVL integrated navigation is capable of suppressing SINS error divergence effectively and efficiently. In addition, the feasibility and effectiveness of Self-Aided SINS based on horizontal circular motion is also verified by field test with real IMU data.

Diel vertical migration promotes zooplankton horizontal patchiness

Spatial patchiness of plankton enhances fishery production and carbon export in the ocean. While diel vertical migration (DVM) has been identified as an important factor contributing to vertical patchiness, its effect on horizontal patchiness has never been investigated. We use a simple individual-based zooplankton model to examine the effect of DVM on the horizontal patchiness of four zooplankton groups with differing DVM patterns in a two-dimensional ocean circulation model. We find that zooplankton horizontal patchiness can be induced by two mechanisms: (1) in stratified waters, DVM can synchronize zooplankton vertical positions with the horizontal current velocities that drive them, resulting in horizontal patchiness; and (2) migrating zooplankton tend to aggregate in deep waters when they encounter sea bottom. Due to these mechanisms, zooplankton horizontal patchiness may be ubiquitous in the ocean, enhancing secondary production and fisheries.

Internal tide energy flux over a ridge measured by a co-located ocean glider and moored acoustic Doppler current profiler

Abstract. Internal tide energy flux is an important diagnostic for the study of energy pathways in the ocean, from large-scale input by the surface tide to small-scale dissipation by turbulent mixing. Accurate calculation of energy flux requires repeated full-depth measurements of both potential density (ρ) and horizontal current velocity (u) over at least a tidal cycle and over several weeks to resolve the internal spring–neap cycle. Typically, these observations are made using full-depth oceanographic moorings that are vulnerable to being “fished out” by commercial trawlers when deployed on continental shelves and slopes. Here we test an alternative approach to minimize these risks, with u measured by a low-frequency acoustic Doppler current profiler (ADCP) moored near the seabed and ρ measured by an autonomous ocean glider holding station by the ADCP. The method is used to measure the semidiurnal internal tide radiating from the Wyville Thomson Ridge in the North Atlantic. The observed energy flux (4.2±0.2 kW m−1) compares favourably with historic observations and a previous numerical model study. Error in the energy flux calculation due to imperfect co-location of the glider and ADCP is estimated by subsampling potential density in an idealized internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (<10 %) if all the glider data are contained within a “watch circle” with a diameter smaller than 1∕8 the mode-1 horizontal wavelength of the internal tide. Energy flux is biased low because the glider samples density with a broad range of phase shifts, resulting in underestimation of vertical isopycnal displacement and available potential energy. The negative bias increases with increasing watch circle diameter. If watch circle diameter is larger than 1∕8 the mode-1 horizontal wavelength, the negative bias is more than 3 % and all realizations within the 95 % confidence interval are underestimates. Over the Wyville Thomson Ridge, where the semidiurnal mode-1 horizontal wavelength is ≈100 km and all the glider dives are within a 5 km diameter watch circle, the observed energy flux is estimated to have a negative bias of only 0.4 % and an error of less than 3 % at the 95 % confidence limit. With typical glider performance, we expect energy flux error due to imperfect co-location to be <10 % in most mid-latitude shelf slope regions.

Experimental Study on the Performance of an Innovative Tide-Induced Device for Artificial Downwelling

Hypoxia has been increasingly observed in estuaries and coastal marine ecosystems around the world. In this paper, a tide-powered artificial downwelling device is proposed to potentially alleviate hypoxia in bottom waters. The downwelling device mainly consists of a vertical square tube, a 90° bend sitting on the top of the tube, two symmetrical-guide plates which installed alongside the vertical tube, a static mixer, and an artificial reef. Scale model experiments are performed with respect to different density difference heads, horizontal current velocities, and tube geometries. The results show that the downwelling flow rate is dependent on horizontal current velocity, tube geometry parameters, and the density profile of ambient water. In addition, increasing the equivalent diameter and bend radius of the device can decrease the total loss coefficient in the tube, which in turns enhance the downwelling efficiency. The two symmetrical-guide plates also generate obvious downwelling of surface water which further improves the whole performance of the device. Further work will need to determine the influence of the other parts of the device, such as the static mixer and artificial reef, on the downwelling efficiency.

Experimental and numerical study of current-induced artificial upwelling

The aim of the current paper is to investigate hydrodynamic characteristics of the artificial upwelling induced by ocean currents. Experiments were performed in a flume at different density difference heads, horizontal current velocities and upwelling pipe diameters. A three-dimensional computational fluid dynamics (CFD) model was employed on wider range of parameters for further analysis. The performance of the numerical model has been confirmed by the experimental findings. The present results show that the volume flow rate of current-induced artificial upwelling is influenced by geometrical parameters and inclination angle of the pipe, the horizontal current velocity and vertical distribution of water density. In ideal two-layer density stratified water, the critical current velocity to generate upwelling linearly increases with the increase of the density difference, and the maximum rising height for upwelling is inversely proportional to the density difference. Feasibility analysis was taken by using current and density profiles of the East China Sea near Dongji Islands, which provides an useful reference for engineering practice.

Underwater docking of an under-actuated autonomous underwater vehicle: system design and control implementation

Underwater docking greatly facilitates and extends operation of an autonomous underwater vehicle (AUV) without the support of a surface vessel. Robust and accurate control is critically important for docking an AUV into a small underwater funnel-type dock station. In this paper, a docking system with an under-actuated AUV is presented, with special attention paid to control algorithm design and implementation. For an under-actuated AUV, the cross-track error can be controlled only via vehicle heading modulation, so both the cross-track error and heading error have to be constrained to achieve successful docking operations, while the control problem can be even more complicated in practical scenarios with the presence of unknown ocean currents. To cope with the above issues, a control scheme of a three-hierarchy structure of control loops is developed, which has been embedded with online current estimator/compensator and effective control parameter tuning. The current estimator can evaluate both horizontal and vertical current velocity components, based only on the measurement of AUV’s velocity relative to the ground; in contrast, most existing methods use the measurements of both AUV’s velocities respectively relative to the ground and the water column. In addition to numerical simulation, the proposed docking scheme is fully implemented in a prototype AUV using MOOS-IvP architecture. Simulation results show that the current estimator/compensator works well even in the presence of lateral current disturbance. Finally, a series of sea trials are conducted to validate the current estimator/compensator and the whole docking system. The sea trial results show that our control methods can drive the AUV into the dock station effectively and robustly.

A near-inertial current event in the homogeneous deep layer of the northern Sea of Japan during winter

Under strong surface wind forcing during winter, direct current observations in the northern Sea of Japan show the existence of strong near-inertial currents in the deep water that is characterized by the extremely homogeneous vertical structures of temperature and salinity. However, the mechanism generating internal waves in the deep water of the northern Sea of Japan has not been well understood. In this study, to clarify the dynamical link between the surface wind forcing and near-inertial currents in the deep water of the northern Sea of Japan, we drive a general circulation model taking into account realistic wind stress, ocean bottom and land topography. In the northern Sea of Japan, the numerical results show that vertically coherent horizontal currents with a speed of ~ 0.05 m s−1 are excited throughout the homogeneous deep water. A two-layer model successfully reproduces the pattern of the horizontal current velocities shown by the general circulation model, indicating that internal waves emanate westward from the northwestern coast of Japan through coastal adjustment to the strong wind forcing event and, while propagating into the ocean interior, they excite evanescent near-inertial response throughout the lower layer below the interface.

Eddies Observed by Argo Floats. Part II: Form Stress and Streamline Length in the Southern Ocean

AbstractArgo floats measure horizontal current velocities at the parking depth and vertical profiles of temperature and salinity. These data were used to study the roles that eddies play in the dynamics of the Antarctic Circumpolar Current (ACC) in the Southern Ocean. A zonal momentum budget was quantified in a box spanning the latitudes of the Drake Passage and bounded by the sea surface and the 1000-dbar depth. The input of eastward zonal momentum from the wind (17.1 × 1011 N) was approximately twice the downward transfer of eastward momentum across the isopycnal whose mean depth was 1000 dbar, which was mediated via form stress carried by eddies [(8.1 ± 1.9) × 1011 N]. The zonal momentum budget was closed to within uncertainty, meaning that the momentum not accounted for by eddies was explained by the Coriolis term associated with meridional transport. The form stress was spatially concentrated near meridional ridges, particularly on their eastern flanks. The localization was extreme: 7% of the total area contributed about 90% of the form stress. Lengths of streamlines were stretched around steady standing meanders. Seven major meanders were found at large topographic barriers along the ACC, with cyclonic meander collocated with the peaks of the topographic barriers. Eddies were found to lengthen the streamlines mostly on the eastern flanks of the meridional ridges, where the eddy transport was southward. Poleward eddy transport on the eastern flanks of meridional ridges is thus highlighted in the ACC dynamics in transferring eastward zonal momentum downward and in adjusting to wind changes by stretching streamlines.

Vertical turbulent exchange in the Black Sea pycnocline and its relation to water dynamics

Estimates of vertical turbulent diffusion coefficient (K t ) in the Black Sea pycnohalocline have been obtained from data of simultaneous observations of seawater temperature, salinity, density, and horizontal current velocity, obtained in the northeastern part of the Black Sea during 2013–2014 with a moored Aqualog profiler. A Munk and Andersson (1948) type parameterization, adapted for the Black Sea environment, is proposed for calculating K t . Strong short-period (several days) variability of turbulent exchange is revealed, induced by vertical shear variations of the current velocity.

Eddies Observed by Argo Floats. Part I: Eddy Transport in the Upper 1000 dbar

AbstractArgo floats measure horizontal current velocities at the parking depth and vertical profiles of temperature and salinity. The data are sufficient for simultaneous estimates of velocities and vertical displacements of isopycnal surfaces. More than 980 000 pairs of observations of current velocity and water column stratification were used to calculate eddy transport above 1000 dbar and its uncertainty based on the temporal-residual-mean framework. Eddy transports larger than 1.0 m2 s−1 were found in the North Atlantic, western North Pacific, and Southern Oceans. The eddy transport T had components perpendicular and parallel to the density contours at 1000 dbar. In the midlatitude oceans, eddy transport was weaker (<0.5 m2 s−1), mostly perpendicular to the density contours, and equatorward. A large area of northward was found in the south Indian Ocean; analysis of velocity and thickness perturbations suggested that this transport was a northward intrusion of Antarctic Intermediate Water. In the midlatitude oceans and in most of the southern part of the Antarctic Circumpolar Current (ACC), was generally upgradient in density on 1000 dbar. Downgradient was found along the North Atlantic Current and Kuroshio Extension as well as in the northern part of the ACC. Zonally integrated meridional transport was poleward at latitudes higher than approximately 40° and equatorward at lower latitudes. The quasi-Stokes or Gent–McWilliams diffusivity coefficient was on the order of 1000 m2 s−1 but was associated with such large uncertainty that it was statistically indistinguishable from zero, except at midlatitudes in the Southern Hemisphere.

Observations of pockmark flow structure in Belfast Bay, Maine, Part 3: implications for sediment transport

Current observations and sediment characteristics acquired within and along the rim of two pockmarks in Belfast Bay, Maine, were used to characterize periods of sediment transport and to investigate conditions favorable to the settling of suspended sediment. Hourly averaged Shields parameters determined from horizontal current velocity profiles within the center of each pockmark never exceed the critical value (approximated with the theoretical model of Dade et al. 1992). However, Shields parameters estimated at the pockmark rims periodically exceed the critical value, consistent with conditions that support the onset of sediment transport and suspension. Below the rim in the near-center of each pockmark, depth-averaged vertical velocities were less than zero (downward) 60% and 55% of the time in the northern and southern pockmarks, and were often comparable to depth-averaged horizontal velocities. Along the rim, depth-averaged vertical velocities over the lower 8 m of the water column were primarily downward but much less than depth-averaged horizontal velocities indicating that suspended sediment may be moved to distant locations. Maximum grain sizes capable of remaining in suspension under terminal settling flow conditions (ranging 10–170 μm) were typically much greater than the observed median grain diameter (about 7 μm) at the bed. During upwelling flow within the pockmarks, and in the absence of flocculation, suspended sediment would not settle. The greater frequency of predicted periods of sediment transport along the rim of the southern pockmark is consistent with pockmark morphology in Belfast Bay, which transitions from more spherical to more elongated toward the south, suggesting near-bed sediment transport may contribute to post-formation pockmark evolution during typical conditions in Belfast Bay.

Effect of vorticity on the generation of rogue waves due to dispersive focusing

The kinematic and dynamic of steep two-dimensional focusing wave trains on a shearing flow in deep water are investigated analytically and numerically. In the absence of waves, the vorticity due to the vertical gradient of the horizontal current velocity is assumed constant. A linear kinematic model based on the spatio-temporal evolution of the frequency is derived, predicting the focusing distance and time of a chirped wave packet in the presence of constant vorticity. Furthermore, a linear model, based on a Fourier integral, is used to describe the evolution of the free surface on shearing current. To compute the fully nonlinear evolution of the wave group in the presence of vorticity, a new numerical model, based on a BIEM approach, is developed. On the basis of these different approaches, the role of constant vorticity on rogue wave occurrence is analysed. Two main results are obtained: (1) the linear behaviour expected in the presence of constant vorticity is significantly different from what is commonly expected in the presence of constant current and (2) the nonlinear effects are found to be of significant influence in the case at hand.