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Although the zonal propagation of mesoscale eddies is intrinsically westward in oceans worldwide, eddies occasionally propagate eastward. Westward propagating eddies (WPEs) and eastward propagating eddies (EPEs) have distinctively different characteristics. In this article, the propagation features of WPEs and EPEs were studied using a combination of current, wind, and topographical data. We found that, except for in the North Atlantic, the energetic areas for WPEs and EPEs did not overlap. Furthermore, the propagation channels of WPEs and EPEs were located at different depths. The magnitude of change in current and wind stress simultaneously affected both the spread direction and velocity of eddies. The average translation speeds of eddies increased as subsurface current velocity increased, regardless of the angle between the direction of eddy propagation and flow. Compared with WPEs, increases in wind stress had little effect on the propagation speeds of EPEs. The directions of EPEs were more sensitive to changes in flow and wind speed. Even small changes in bathymetry slowed the propagation speeds of eddies. This topographical inhibition was biggest when the topographic gradients were around 0.02 and 0.04 for WPEs and EPEs, respectively. The propagation directions of EPEs were also sensitive to the surrounding terrain and tended to move along seamounts and ridges. These findings may refine eddy predictions and aid eddy observations in the future.

The North Equatorial Subsurface Current (NESC) is a subthermocline ocean current uncovered recently in the tropical Pacific Ocean, flowing westward below the North Equatorial Countercurrent. In this study, the dynamics of the seasonal cycle of this current are studied using historical shipboard acoustic Doppler current profiler measurements and Argo absolute geostrophic currents. Both data show a westward current at the depths of 200–1000 m between 4° and 6°N, with a typical core speed of about 5 and 2 cm s−1, respectively. The subsurface current originates in the eastern Pacific, with its core descending to deeper isopycnal surfaces and moving to the equator as it flows westward. The zonal velocity of the NESC shows pronounced seasonal variability, with the annual-cycle harmonics of vertical isothermal displacement and zonal velocity presenting characters of vertically propagating baroclinic Rossby waves. A simple analytical Rossby wave model is employed to simulate the propagation of the seasonal variations of the westward zonal currents successfully, which is the basis for exploring the wind forcing dynamics. The results suggest that the wind curl forcing in the central-eastern basin between 170° and 140°W associated with the meridional movement of the intertropical convergence zone dominates the NESC seasonal variability in the western Pacific, with the winds west of 170°W and east of 140°W playing a minor role in the forcing.

Strait of Istanbul is one of the most difficult waterways in the world. It’s curved structure, sharp bends which require 12 major course alterations form a unique marine environment for passing vessels. Currents also form a natural difficulty for navigation, especially at areas at the North of Kandilli point. There are three types of currents prevail in the Strait of Istanbul: the first is the surface currents, which finds it’s dynamics at the difference of sea levels between Sea of Marmara and Black Sea. Due to this difference in levels, which is around 40 centimeters on average and fed by the rivers flowing into Black Sea, waters run down to Sea of Marmara and to outer seas. Second, is the counter currents and eddies inside the bays. Third is the subsurface currents, formed by the difference of salinity between Marmara and Black Sea, starts from 10 meters to 40 deep and in counter direction with the surface currents. To make this currents system more complicated, in the case of strong SW winds when lasted more than 2-3 days, the whole currents system changes when the surface currents reversed by the wind and above-mentioned difference in sea levels exchange positions to the favor of Sea of Marmara. Due to this unique system of currents, Strait of Istanbul used to have a unique navigational system, too. Starting from ancient times, ships navigating in the Strait of Istanbul used to navigate on the Port side, especially when navigating from Sea of Marmara to Black Sea direction. Thus, ships could get better protection against current. This practice was inked on the paper in 1933 and by the Istanbul Port Regulation through which it became compulsory for ships navigating in the Strait of Istanbul. But as the international regulations on the preventing collision at sea emerged and it appeared that right-hand navigation was established for narrow channels and altering the course to Starboard in the head-on situations has become a rule, the Left-hand side navigation in the Strait of Istanbul started to lead to confusion. There have been three major accidents in the Strait of Istanbul which could be attributed to confusion created by this national rule against the international rule; in 1960, 1966 and 1979, finally Left-hand side navigation in the Strait of Istanbul was abolished in 1982. This article is about these long-term proceedings of Left-hand side navigation in the Strait of Istanbul, concentrating on and analysing three major accidents.

The upper layer of the ocean participates directly in the exchange of momentum, heat and moisture with the atmosphere. We consider three examples of upper-ocean flow and heat transfer in the present contribution. These examples range from the canonical problem of a stratified shear layer to the surface boundary layer driven by wind and a diurnally varying heat flux to deep cycle turbulence in the Equatorial UnderCurrents (EUC). These problems illustrate stratified shear flow turbulence, wind-driven entrainment in a stratified, rotating fluid, and the communication of surface forcing to subsurface currents in the upper ocean. We discuss the three cases by including new simulations as well as some of our previous work. Direct numerical simulation (DNS) is our tool for the canonical shear layer and, for the other problems, our tool is large eddy simulation (LES) which is increasingly being used to examine turbulent transport and mixing in the ocean. We discuss how buoyancy and rotation affects the spatial structure and temporal evolution of turbulent fluxes, and thereby the distribution of surface inputs of momentum and heat in the upper ocean.

A hindcast modelling study conducted with the PM3D hydrodynamic model revealed some new effects. The study involved determination of seasonal characteristics of surface and subsurface currents. In addition, long-term relationships between the Baltic Sea currents and the North Atlantic Oscillation were explored. Analysis of the Baltic Sea current data spanning five decades has shown new, previously unknown, current characteristics, including higher stability (0.4–0.7) of subsurface current eddies relative to the stability of surface currents (0.2–0.5). Circulation anomaly patterns relative to the annual mean in spring and autumn present opposite flow directions, whereas winter and summer circulation patterns were similar in this respect. The spring circulation anomalies were dominated by the low sea-level outflow from the Baltic Sea, whereas the autumn circulation showed a strong inflow and infilling of the sea. In the analysed half-century (1958–2007), velocities of surface and bottom currents showed an increasing trend (0.9 and 0.1 cm s−1 per 50 years, respectively) and were related to the dominance of the positive NAO index phase. Current velocity fluctuations correlated positively with the NAO and BSI indices (correlation coefficients of 0.66 and 0.80, respectively).

Coupling between the surface and near‐bottom currents in the Gulf of Mexico (GoM) has been reported in many case studies. However, geographical variations of this coupling need more examination. In this study, surface geostrophic currents derived from satellite‐observed sea surface height and subsurface currents from a collection of deep ocean moorings are used to examine the surface and bottom coupling in different parts of the GoM. The short‐period (30–90 days) fluctuations generated by the Loop Current (LC) and the LC eddies (LCEs) have a more vertically coherent structure and stronger deep ocean expressions than the long‐period fluctuations (>90 days). In addition, the strength of the coupling is modulated by the long‐period variations of the LC and LCE sheddings. Moreover, the surface and bottom coupling varies geographically. In the LC region, the surface fluctuations along the eastern side of the LC are important in causing the bottom current fluctuations through baroclinic instability under the LC and through traveling topographic Rossby waves (TRWs) north of the LC. In the central deep GoM, the bottom currents are affected by the upper fluctuations of the northern LC through both local baroclinic instability and remote TRW propagation. In the northwestern GoM, the bottom current fluctuations are largely related to the remote surface variability from the west side of the LC by TRWs propagating northwestward. This study will help us better understand mechanisms of the bottom current fluctuations that are important for the dispersal of deep ocean materials and properties., Key Points Surface and bottom currents in the Gulf of Mexico can be coupled locally by baroclinic instability and remotely by topographic Rossby wavesFluctuations generated along the Loop Current have stronger deep ocean expressions at the period of 30–90 days than at longer periodsSurface and bottom coupling in the Gulf of Mexico varies geographically and is modulated by the Loop Current and the Loop Current eddies

In this paper, we analyse the results from a numerical model at high resolution. We focus on the formation and maintenance of subsurface equatorial currents in the Gulf of Guinea and we base our analysis on the evolution of potential vorticity (PV). We highlight the link between submesoscale processes (involving mixing, friction and filamentation), mesoscale vortices and the mean currents in the area. In the simulation, eastward currents, the South and North Equatorial Undercurrents (SEUC and NEUC respectively) and the Guinea Undercurrent (GUC), are shown to be linked to the westward currents located equatorward. We show that east of 20&deg, W, both westward and eastward currents are associated with the spreading of PV tongues by mesoscale vortices. The Equatorial Undercurrent (EUC) brings salty waters into the Gulf of Guinea. Mixing diffuses the salty anomaly downward. Meridional advection, mixing and friction are involved in the formation of fluid parcels with PV anomalies in the lower part and below the pycnocline, north and south of the EUC, in the Gulf of Guinea. These parcels gradually merge and vertically align, forming nonlinear anticyclonic vortices that propagate westward, spreading and horizontally mixing their PV content by stirring filamentation and diffusion, up to 20&deg, W. When averaged over time, this creates regions of nearly homogeneous PV within zonal bands between 1.5&deg, and 5&deg, S or N. This mean PV field is associated with westward and eastward zonal jets flanking the EUC with the homogeneous PV tongues corresponding to the westward currents, and the strong PV gradient regions at their edges corresponding to the eastward currents. Mesoscale vortices strongly modulate the mean fields explaining the high spatial and temporal variability of the jets.

The North Equatorial Subsurface Current (NESC) has recently been found to flow westward below the North Equatorial Countercurrent in the subsurface layer across the Pacific Ocean. The structure, water mass properties, and the dynamics of the NESC are studied using Argo profiles and geostrophic currents, combined with moored current meter observations. The mean westward geostrophic currents of the NESC has been validated with moored current meter measurements at 4.7° N, 142° E in the far western tropical Pacific Ocean. Sizable seasonal-to-interannual variability of the NESC is indicated by the observations, with strong transports in boreal summer and during La Niña events, whereas weak transports in boreal winter and during El Niño events. The water masses of the NESC appear to be the mixture of the North and South Pacific intermediate waters, with the waters immediately below the thermocline closer to the North than to the South Pacific waters. A simulation using a linear continuously stratified model of ocean circulation suggests that the mean NESC is forced by wind curl through low baroclinic mode responses of the ocean.

A comprehensive investigation and review of literature has been made concerning the nature and characteristics of the wave-generated currents which cause alongshore sediment transport (littoral drift).An historical summary of early research up to about 1969 revealed many of the qualitative characteristics of wave-generated currents and associated nearshore circulation systems, but also showed that no consistent theoretical basis nor any adequate empirical data were then available for the quantitative prediction of current velocities. The "radiation stress" theory of Longuet-Higgins and Stewart now provides a unified theory for wave-generated currents and associated phenomena such as wave set-up. A comprehensive review of this theory is presented, together with its application both to the computation of uniform alongshore currents and to mathematical models of nearshore circulation systems.The author has made an experimental laboratory investigation of an idealized nonuniform alongshore current system generated on a beach behind an offshore breakwater. In this study the beach plan form was arranged so that the breaking wave crests were parallel to the beach at all times, and the resulting alongshore current was driven by the alongshore gradient of wave set-up created by the alongshore gradient of the breaking wave height resulting from diffraction behind the offshore breakwater. The results of experiments with various wave heights and periods show that the wave height is the dominant factor in determining the resultant alongshore current, and that the wave period is of secondary importance only. Wave-current interaction effects are significant.The assumption of shallow water conditions has enabled the author to derive simple formulae for computing the wave set-up of both plunging and spilling breakers on both steep and flat beaches. These formulae are used in conjunction with the momentum principle including radiation stress terms to obtain an expression for computing the velocity of the nonuniform alongshore current.Further analysis permits the velocity distribution to be computed neglecting lateral mixing effects. Bottom friction is allowed for using the nonlinear expressions of either Jonsson, Skovgaard and Jacobsen or Bijker and Swart. These expressions are also applied to uniform alongshore current data from various sources and the results compared with those obtained using linearized theory. The author's expressions for computing the velocity of a nonuniform alongshore current are applied to three actual situations.Finally the influence of wave-generated currents on the alongshore sediment transport in the vicinity of engineering structures and/or natural coastal features is considered for a number of specific cases. Some of the general principles involved in the development of the alignment and morphology of sedimentary coasts are outlined, together with a discussion of rip current systems and their interaction with beach and bottom topography.

Due to the high cost of ocean observation system, the scientific design of observation network becomes much important. The current network of the high frequency radar system in the Gulf of Thailand has been studied using a three-dimensional coastal ocean model. At first, the observations from current radars have been assimilated into this coastal model and the forecast results have improved due to the data assimilation. But the results also show that further optimization of the observing network is necessary. And then, a series of experiments were carried out to assess the performance of the existing high frequency ground wave radar surface current observation system. The simulated surface current data in three regions were assimilated sequentially using an efficient ensemble Kalman filter data assimilation scheme. The experimental results showed that the coastal surface current observation system plays a positive role in improving the numerical simulation of the currents. Compared with the control experiment without assimilation, the simulation precision of surface and subsurface current had been improved after assimilated the surface currents observed at current networks. However, the improvement for three observing regions was quite different and current observing network in the Gulf of Thailand is not effective and a further optimization is required. Based on these evaluations, a manual scheme has been designed by discarding the redundant and inefficient locations and adding new stations where the performance after data assimilation is still low. For comparison, an objective scheme based on the idea of data assimilation has been obtained. Results show that all the two schemes of observing network perform better than the original network and optimal scheme-based data assimilation is much superior to the manual scheme that based on the evaluation of original observing network in the Gulf of Thailand. The distributions of the optimal network of radars could be a useful guidance for future design of observing system in this region.

This work examines the response of the upper ocean during the passage of cyclone Thane (December 25–30, 2011) over the Bay of Bengal. Observations from a moored buoy are employed to assess the changes in the upper 100 m depth of the ocean. Observations revealed strong temperature inversion prevailing in this region. The temperature inversion resulted in varying response in the subsurface, with warming in the upper 40 m depth and cooling in the depths of 40–80 m at the moored buoy location. Analysis of subsurface temperature and salinity revealed the signatures of vertical mixing induced by Thane. The subsurface currents are observed as strongly responding to the cyclone winds. Analysis revealed that the influence of Thane was generally confined to within 100 m depth. Intense biological response is revealed along the track of the cyclone. However, the bloom is localized in two distinct regions: one in the central Bay and other off the south-western coastal region. This is attributed to the intense Ekman pumping velocity, slow translation speed and the availability of nitrate concentration in the shallow coastal waters.

We report on the development and implementation of a novel physics-based algorithm that includes the effects of subsurface current on buoyant particles in a Lagrangian particle tracking model, specifically the Larval TRANSport Lagrangian (LTRANS) model. In the upper ocean, the direction and the magnitude of horizontal current change with water depth due to local effects such as the Ekman balance and large-scale influences such as the thermal wind balance. The subsurface current alters both the advection and the dispersion of weakly buoyant particles that could be suspended in the ocean surface boundary layer by turbulence. The transport and fate of riverborne plastics from the Mississippi and Atchafalaya Rivers were examined using a hindcast model solution for the Louisiana and Texas Continental Shelf (LATEX shelf). The model results show that after leaving the river outlets, plastic particles travel on average in a southwestward direction over the LATEX shelf for up to four months, transiting from a few hundred kilometers to more than a thousand kilometers. The pathways and the fate of particles change as a function of the particle size, with smaller particles traveling slower. Also, less buoyant particles transit longer distances over more offshore regions and remain longer on the LATEX shelf. Particle pathways also display a strong seasonal cycle associate with the seasonal cycle of circulation on the LATEX shelf. The model results further reveal that more than 50% of the plastic particles delivered by the Mississippi and Atchafalaya Rivers reach the land boundary of the LATEX shelf, and more than 18% of those reach the land boundary west of Louisiana including Texas and portions of Mexico. This modeling framework can be used to study the transport of other positively buoyant particulate materials in the ocean, such as spilled oil and plankton.

An extensive record of current velocities at all levels in the water column is an indispensable requirement for a tidal resource assessment and is fully necessary for accurate determination of available energy throughout the water column as well as estimating likely energy capture for any particular device. Traditional tidal prediction using the least squares method requires a large number of harmonic parameters calculated from lengthy acoustic Doppler current profiler (ADCP) measurements, while long-term in situ ADCPs have the advantage of measuring the real current but are logistically expensive. This study aims to show how these issues can be overcome with the use of a neural network to predict current velocities throughout the water column, using surface currents measured by a high-frequency radar. Various structured neural networks were trained with the aim of finding the network which could best simulate unseen subsurface current velocities, compared to ADCP data. This study shows that a recurrent neural network, trained by the Bayesian regularisation algorithm, produces current velocities highly correlated with measured values: r2 (0.98), mean absolute error (0.05 ms−1), and the Nash–Sutcliffe efficiency (0.98). The method demonstrates its high prediction ability using only 2 weeks of training data to predict subsurface currents up to 6 months in the future, whilst a constant surface current input is available. The resulting current predictions can be used to calculate flow power, with only a 0.4% mean error. The method is shown to be as accurate as harmonic analysis whilst requiring comparatively few input data and outperforms harmonics by identifying non-celestial influences; however, the model remains site specific.

The kinetic energy (KE) fluxes into subsurface currents (EFc) is an important boundary condition for ocean circulation models. Traditionally, numerical models assume the KE flux from wind (EFair) is identical to EFc, that is, no net KE is gained (or lost) by surface waves. This assumption, however, is invalid when the surface wave field is not fully developed, and acquires KE when it grows in space or time. In this study, numerical experiments are performed to investigate the KE flux budget across the air-sea interface under both uniform and idealized tropical cyclone (TC) winds. The wave fields are simulated using the WAVEWATCH III model under different wind forcing. The difference between EFair and EFc is estimated using an air-sea KE budget model. To address the uncertainty of these estimates resides in the variation of source functions, two source function packages are used for this study: the ST4 source package (Ardhuin et al, 2010), and the ST6 source package (Babanin, 2011). The modeled EFc is significantly reduced relative to EFair under growing seas for both the uniform and TC experiments. The reduction can be as large as 20%, and the variation of this ratio is highly dependent on the choice of source function for the wave model. Normalized EFc are found to be consistent with analytical expressions by Hwang and Sletten (2008) and Hwang and Walsh (2016) and field observations by Terray et al. (1996) and Drennan et al. (1996), while the scatters are more widely in the TC cases due to the complexity of the associated wave field. The waves may even give up KE to subsurface currents in the left rear quadrant of fast moving storms. Our results also suggest that the normalized KE fluxes may depend on both wave age and friction velocity (u*).

In inductive electromagnetics, the magnetic field measured in the air at any instant can be considered to be a potential field. As such, we can invert measured magnetic fields (at a fixed time or frequency) for the causative subsurface current system. These currents can be approximated with a 3D subsurface grid of 3D magnetic (closed-loop current) or electric (line current) dipoles whose location and orientation can be solved for using a potential-field-style smooth-model inversion. Because the problem is linear, both inversions can be solved quickly even for large subsurface volumes; and both can be run on a single data set for complementary information. Synthetic studies suggest that for discrete induction dominated targets, the magnetic and electric dipole inversions can be used to determine the center and top edge of the target, respectively. Furthermore, the orientation of plate targets can be estimated from visual examination of the orientations of the 3D vector dipoles and/or using the interpreted location of the center and top edge of the target. In the first field example, ground data from a deep massive sulfide body (mineral exploration target) was inverted and the results were consistent with the conclusions drawn from the synthetic examples and with the existing interpretation of the body (shallow dipping conductor at a depth of approximately 400 m). A second example over a near-surface mine tailing (a near-surface environmental/engineering study) highlighted the strength of being able to invert data using either magnetic or electric dipoles. Although both models were able to fit the data, the electric dipole model was considerably simpler and revealed a southwest−northeast-trending conductive zone. This fast approximate 3D inversion can be used as a starting point for more rigorous interpretation and/or, in some cases, as a stand-alone interpretation tool.

This study incorporates observations from Array of Real-time Geostrophic Oceanography (ARGO) floats and surface drifters to identify seasonal circulation patterns at the surface, 1000 m, 1500 m, and 2000 m in the northwest Indian Ocean, and quantify velocities associated with them. A skill comparison of the Simple Ocean Data Assimilation (SODA) reanalysis output was also performed to contribute to the understanding of the circulation dynamics in this region. Subsurface currents were quantified and validated using the ARGO float data. Surface currents were identified using surface drifter data and compared to the subsurface observations to enhance our previous understanding of surface circulations. Quantified Southwest Monsoon surface currents include the Somali Current (v max = 179.5 cm/s), the East Arabian Current (v max = 52.3 cm/s), and the Southwest Monsoon Current (v max = 51.2 cm/s). Northeastward flow along the Somali coast is also observed at 1000 m (v max = 26.1 cm/s) and 1500 m (v max = 12.7 cm/s). Currents associated with the Great Whirl are observed at the surface (v max = 161.4 cm/s) and at 1000 m (v max = 16.2 cm/s). In contrast to previous studies, both ARGO and surface drifter data show the Great Whirl can form as early as the boreal Spring intermonsoon, lasting until the boreal Fall intermonsoon. The Arabian Sea exhibits eastward/southeastward flow at the surface, 1000 m, 1500 m, and 2000 m. Quantified Northeast Monsoon surface currents include the Somali Current (v max = 97.3 cm/s), Northeast Monsoon Current (v max = 30.0 cm/s), and the North Equatorial Current (v max = 28.5 cm/s). Southwestward flow along the Somali coast extends as deep as 1500 m. Point-by-point vector and scalar correlations of SODA output to ARGO and surface drifter data showed that surface SODA output and surface drifter data generally produced a strong correlation attributed to surface currents strongly controlled by the monsoons, while subsurface correlations of SODA output and ARGO were mostly insignificant due to variability associated with intermonsoonal transitions. SODA output produced overall smaller velocities than both observational datasets. Assimilating ARGO velocities into the SODA reanalysis could improve subsurface velocity assimilation, especially during the boreal fall and spring when ARGO observations suggest that flow is highly variable.

The variability of subsurface currents in the equatorial Indian Ocean is studied using high resolution Ocean General Circulation Model (OGCM) simulations during 1958–2009. February–March eastward equatorial subsurface current (ESC) shows weak variability whereas strong variability is observed in northern summer and fall ESC. An eastward subsurface current with maximum amplitude in the pycnocline is prominent right from summer to winter during strong Indian Ocean Dipole (IOD) years when air-sea coupling is significant. On the other hand during weak IOD years, both the air-sea coupling and the ESC are weak. This strongly suggests the role of ESC on the strength of IOD. The extension of the ESC to the summer months during the strong IOD years strengthens the oceanic response and supports intensification and maintenance of IODs through modulation of air sea coupling. Although the ESC is triggered by equatorial winds, the coupled air-sea interaction associated with IODs strengthens the ESC to persist for several seasons thereby establishing a positive feedback cycle with the surface. This suggests that the ESC plays a significant role in the coupled processes associated with the evolution and intensification of IOD events by cooling the eastern basin and strengthening thermocline-SST (sea surface temperature) interaction. As the impact of IOD events on Indian summer monsoon is significant only during strong IOD years, understanding and monitoring the evolution of ESC during these years is important for summer monsoon forecasting purposes. There is a westward phase propagation of anomalous subsurface currents which persists for a year during strong IOD years, whereas such persistence or phase propagation is not seen during weak IOD years, supporting the close association between ESC and strength of air sea coupling during strong IOD years. In this study we report the processes which strengthen the IOD events and the air sea coupling associated with IOD. It also unravels the connection between equatorial Indian Ocean circulation and evolution and strengthening of IOD.

We show that heavy fuel leaking out of an old ship wreck 10 km off the Bulgarian Black Sea coast can be identified on spaceborne Synthetic Aperture Radar (SAR) imagery. Sentinel-1A SAR-C and ALOS-2 PALSAR-2 imagery is used to demonstrate that the leakage started more than three years ago, in summer 2015, hence it has been going on for a much longer time than stated by local experts. The resulting oil spills can have lengths exceeding 9 km, and they may be (partly) displaced due to strong subsurface currents.

The current and wind records and the physical parameter structures such as temperature and salinity in the southwestern part of the Caspian Sea adjacent to Anzali Port were investigated from November 2004 up to the end of January 2005. Results show that, despite the existence of relatively weak winds along the coast in the area, the measurements indicate strong long shore currents. However, when heavy wind tension is observed in the area, then strong currents are also present, which - from the perspective of direction – also have good coordination with the wind. The direction of dominated currents was parallel to the coast from the west to the east. In most cases, the flow rate was identical from the surface to the seabed, and in this condition - because the values of the measured temperature points were almost identical-barotropic currents were present. However, in the autumn at some region, a significant difference was observed between the surface currents and subsurface currents due to temperature differences that affected the density and caused the creation of barclinic currents. Due to the high velocity of currents compared to wind velocity, and the intense slope of the coast and low-frequency movements in the area, we can hypothesize the existence of motions such as Kelvin waves and conclude that the effect of the wind compared to the other factors of the coastal current in the area was weaker.

Direction-finding SeaSonde (4.463 MHz; 5.2625 MHz) and phased-array WEllen RAdar WERA (9.33 MHz; 13.5 MHz) High-frequency radar (HFR) systems are routinely operated in Australia for scientific research, operational modeling, coastal monitoring, fisheries, and other applications. Coverage of WERA and SeaSonde HFRs in Western Australia overlap. Comparisons with subsurface currents show that both HFR types agree well with current meter records. Correlation (R), root-mean-squares differences (RMSDs), and mean bias (bias) for hourly-averaged radial currents range between R = (−0.03, 0.78), RMSD = (9.2, 30.3) cm/s, and bias = (−5.2, 5.2) cm/s for WERAs; and R = (0.1, 0.76), RMSD = (17.4, 33.6) cm/s, bias = (0.03, 0.36) cm/s for SeaSonde HFRs. Pointing errors (θ) are in the range θ = (1°, 21°) for SeaSonde HFRs, and θ = (3°, 8°) for WERA HFRs. For WERA HFR current components, comparison metrics are RU = (−0.12, 0.86), RMSDU = (12.3, 15.7) cm/s, biasU = (−5.1, −0.5) cm/s; and, RV = (0.61, 0.86), RMSDV = (15.4, 21.1) cm/s, and biasV = (−0.5, 9.6) cm/s for the zonal (u) and the meridional (v) components. Magnitude and phase angle for the vector correlation are ρ = (0.58, 0.86), φ = (−10°, 28°). Good match was found in a direct comparison of SeaSonde and WERA HFR currents in their overlap (ρ = (0.19, 0.59), φ = (−4°, +54°)). Comparison metrics at the mooring slightly decrease when SeaSonde HFR radials are combined with WERA HFR: scalar (vector) correlations for RU, V, (ρ) are in the range RU = (−0.20, 0.83), RV = (0.39, 0.79), ρ = (0.47, 0.72). When directly compared over the same grid, however, vectors from WERA HFR radials and vectors from merged SeaSonde⁻WERA show RU (RV) exceeding 0.9 (0.7) within the HFR grid. Despite the intrinsic differences between the two types of radars used here, findings show that different HFR genres can be successfully merged, thus increasing current mapping capability of the existing HFR networks, and minimising operational downtime, however at a likely cost of slightly decreased data quality.

The National Centers for Environmental Prediction (NCEP) and the Indian National Centre for Ocean Information Services (INCOIS) produces global ocean analysis based on the Global Ocean Data Assimilation System (GODAS). This study shows how upgrades to the forward model simulations from MOM4p0d to MOM4p1 impact ocean analyses over the tropical Indian Ocean in GODAS. Three experiments were performed with same atmospheric forcing fields: (i) using MOM4p0d (GODAS_p0), (ii) using MOM4p1 (GODAS_p1), both using observed temperature and synthetic salinity, and (iii) using MOM4p1 (GODAS_p1S) assimilating both observed temperature and observed salinity. Validation with independent observations shows significant improvement of subsurface temperature and salinity in the new analysis using MOM4p1 versus MOM4p0d. There is also improvement in the upper ocean current of the equatorial Indian Ocean. The impact of observed salinity on the upper ocean surface current is marginal, but there is significant improvement in the subsurface current. The seasonal and inter-annual variability of the Wyrtki jet and the equatorial undercurrent is improved in GODAS_p1 versus GODAS_p0. All analyses reproduced the Indian Ocean dipole, with the GODAS_p1S simulated sea surface temperature (SST) the most accurate. The temperature inversion over the north Bay of Bengal (BoB) is reproduced only in GODAS_p1S. The mean sea level over BoB and equatorial Indian Ocean improved in GODAS_p1S as compared with AVISO observation. The combined model upgrade and assimilation of observed salinity led to reduced root-mean-square deviation and higher correlation coefficient values in the sea level anomaly (SLA) when compared with satellite observations.

The oceanic circulation in the upper layers of the north of the Gulf of Guinea is analyzed, as inferred from in situ observations and numerical simulations. This particular region, in spite of the presence of a coastal upwelling and its impact on resources and regional climate, is still poorly documented. Cruises carried out in the framework of different international programs (e.g. EGEE/AMMA, PIRATA) allowed to show the existence of an eastward flowing undercurrent, found under the Guinea Current, named the Guinea UnderCurrent (GUC). Numerical results from high resolution simulation allowed the description of the seasonal variability of this current. It appears that the GUC is stronger in spring and reverses westward in August-September. We also depict the fate and the sources of the GUC based on selected trajectories from numerical particle tracking. The simulated trajectories reveal: i) a preferred route of the GUC along 4°N from Cape Palmas to Cape Three Points and following the coast east of Cape Three Points; ii) strong recirculations in the most eastern part of the Gulf of Guinea and off Cape Palmas including warm and salty waters of the South Equatorial Current; iv) a weak inflow from northern latitudes through a subsurface current flowing southward along the West African coast. In addition, Lagrangian experiments show that the GUC is not an extension of the North Equatorial UnderCurrent and confirm that this current does not penetrate into the Gulf of Guinea. This article is protected by copyright. All rights reserved.

Estimating the erosion potential of seabed sediments and the magnitude of the resulting suspended load in relation to current dynamics near the shelf break is a key issue for better understanding shelf-slope sediment transport. On the outer Mackenzie Shelf (Canadian Beaufort Sea, Arctic Ocean), a thin and discontinuous veneer of recent surficial clays overlie old glaciomarine sediments that further pinch out at the shelf edge. Gas and fluid venting is known to underlie part of sediment instability in the area, but recent mooring-based measurements also indicate that sediments near the shelf break are recurrently remobilized by strong subsurface currents. Here, we relate storms to the development of current surges that resulted in the abrupt resuspension of sediments at two locations along the shelf break. Near-bottom concentrations of suspended sediments were estimated using the acoustic backscatter of high-frequency acoustic Doppler current profilers deployed from September 2011 to September 2013 as part of the Beaufort Regional Environmental Assessment (BREA) program. Near-bottom currents near the shelf edge (140 to 150 m isobaths) were characterized by recurring episodes of elevated velocities (instantaneous speeds up to ~ 40-50 cm s -1 ) that were extensions of current surges (~ 60-80 cm s -1 ) occurring in the core of the shelfbreak jet located at ca. 90-120 m. Sudden peaks in suspended sediments (above 100 g m -3 ) corresponded closely with current surges in the near-bottom boundary layer ( -1 was calculated based on the relationship between suspended sediment concentrations and near-bottom current speeds. Two meteorological scenarios were identified to explain the current surges underlying these erosion events at the shelf edge: (1) Pacific or Arctic-born low pressure systems that propagate into the southern Beaufort Sea and are associated with intense downwelling-favorable westerly winds. These amplify the eastward-setting shelfbreak jet along the upper slope, such as in September 2012 when successive cyclones developed during the record-low sea ice extent. (2) The persistence of a high-pressure system over the Beaufort Sea that causes strong upwelling-favorable easterly winds and a strengthening of anti-cyclonic ice motion. This induces a reversal of the shelfbreak jet to the west and a subsequent eastward-directed relaxation flow when the winds fade, such as during the major ice fracturing events of January to March 2013 that were associated with strong and persistent current speeds.

Algerian eddies are the strongest and largest propagating mesoscale structures in the Western Mediterranean Sea. They have a large influence on the mean circulation, water masses and biological processes. Over 20 years of satellite altimeter data have been analyzed to characterize the propagation of these eddies using automatic detection methods and cross-correlation analysis. We found that, on average, Algerian eddy trajectories form two subbasin scale anticlockwise gyres that coincide with the two Algerian gyres which were described in the literature as the barotropic circulation in the area. This result suggests that altimetry sea surface observations can provide information on subsurface currents and their variability through the study of the propagation of deep mesoscale eddies in semienclosed seas. The analysis of eddy sea level anomalies along the mean pathways reveals three preferred areas of formation. Eddies are usually formed at a specific time of the year in these areas, with a strong interannual variability over the last 20 years.

Time-depth variations of the equatorial currents over the upper 1000 m depth in the western Pacific Ocean were directly measured by acoustic Doppler current profiler moorings at 2 degrees N, 140 degrees E and 4.7 degrees N, 140 degrees E during January-August 2014. Intraseasonal variations of the equatorial currents, with periods of 37-73 days, were observed encompassing the North Equatorial Countercurrent (NECC), northern branch of the South Equatorial Current (SEC), Equatorial Undercurrent (EUC), Equatorial Intermediate Current (EIC), North Intermediate Countercurrent (NICC), and North Equatorial Subsurface Current (NESC). Compared with previous studies based mainly on shipboard synoptic surveys, the 8-month time series of velocity profiles provided direct evidence for the existence of NESC, captured reversals of the EIC in May and the NESC in June from westward to eastward direction, and revealed larger vertical extensions of the SEC and NESC and greater depths of the EIC and NICC than previously thought. According to a global analysis product of ocean surface current, during January-April 2014, the NECC was located around its southernmost position and with its the weakest intensity over the past 20 years. Some of the anomalous characteristics of these flows may be related to the fickle El Nino of 2014.

Expendable bathythermograph (XBT) data were the major component of the ocean temperature profile observations from the late 1960s through the early 2000s, and XBTs still continue to provide critical data to monitor surface and subsurface currents, meridional heat transport, and ocean heat content. Systematic errors have been identified in the XBT data, some of which originate from computing the depth in the profile using a theoretically and experimentally derived fall-rate equation (FRE). After in-depth studies of these biases and discussions held in several workshops dedicated to discussing XBT biases, the XBT science community met at the Fourth XBT Science Workshop and concluded that XBT biases consist of 1) errors in depth values due to the inadequacy of the probe motion description done by standard FRE and 2) independent pure temperature biases. The depth error and temperature bias are temperature dependent and may depend on the data acquisition and recording system. In addition, the depth bias also includes an offset term. Some biases affecting the XBT-derived temperature profiles vary with manufacturer/probe type and have been shown to be time dependent. Best practices for historical XBT data corrections, recommendations for future collection of metadata to accompany XBT data, impact of XBT biases on scientific applications, and challenges encountered are presented in this manuscript. Analysis of XBT data shows that, despite the existence of these biases, historical XBT data without bias corrections are still suitable for many scientific applications, and that bias-corrected data can be used for climate research.

This paper is devoted to the subsurface current dynamics in equatorial regions, where the hallmark of a strong stratification is a sharp interface (thermocline), separating two layers of different density, and whose depth is dependent upon the strength of the winds above the ocean's surface. We give here a few monotonicity results concerning the dynamics of the thermocline in the equatorial region. The most important one asserts that the level of the thermocline decreases as the strength of the wind at ten meters above the ocean surface, denoted |Uw|, increases. Moreover, the strength of the current at the thermocline decreases as |Uw| increases.

A physically-based technique for interpolating external magnetic field disturbances across large spatial areas can be achieved with the Spherical Elementary Current System (SECS) method using data from ground-based magnetic observatories. The SECS method represents complex electrical current systems as a simple set of equivalent currents placed at a specific height in the ionosphere. The magnetic field recorded at observatories can be used to invert for the electrical currents, which can subsequently be employed to interpolate or extrapolate the magnetic field across a large area. We show that, in addition to the ionospheric currents, inverting for induced subsurface current systems can result in strong improvements to the estimate of the interpolated magnetic field. We investigate the application of the SECS method at mid- to high geomagnetic latitudes using a series of observatory networks to test the performance of the external field interpolation over large distances. We demonstrate that relatively few observatories are required to produce an estimate that is better than either assuming no external field change or interpolation using latitudinal weighting of data from two other observatories.

Ocean surface currents estimated from the satellite data consist of two terms: Ekman currents from the wind stress and geostrophic currents from the sea surface height (SSH). But the classical Ekman model does not consider the wave effects. By taking the wave-induced Coriolis-Stokes forcing into account, the impact of waves (primarily the Stokes drift) on ocean surface currents is investigated and the wave-modified currents are formed. The products are validated by comparing with OSCAR currents and Lagrangian drifter velocity. The result shows that our products with the Stokes drift are better adapted to the in situ Lagrangian drifter currents. Especially in the Southern Ocean region (40 degrees S-65 degrees S), 90% (91%) of the zonal (meridional) currents have been improved compared with currents that do not include Stokes drift. The correlation (RMSE) in the Southern Ocean has also increased (decreased) from 0.78 (13) to 0.81 (10.99) for the zonal component and 0.76 (10.87) to 0.79 (10.09) for the meridional component. This finding provides the evidence that waves indeed play an important role in the ocean circulation, and need to be represented in numerical simulations of the global ocean circulation.

With a focus on the Bay of Bengal, models ranging from a 1/12.5 global ocean model to a 1/4 regional Indian Ocean model to a 2 km local high-resolution coupled model are used to simulate salinity exchanges in the Indian Ocean. Global Hybrid Coordinate Ocean Model simulations show a surprisingly large persistent flow of high-salinity water from the equatorial Indian Ocean into the Bay of Bengal during the northeast monsoon, although it is weaker than during the southwest monsoon. On average, salt is transported into the Bay of Bengal between 83E and 95E, and low-salinity water flows southward near the east coast of Sri Lanka and east of 95E. The Regional Ocean Modeling System shows that knowledge of river input of freshwater is essential for modeling surface salinities correctly in the Bay of Bengal. High-resolution coupled model simulations are in agreement with recent observations and show that a strong subsurface current with a speed of about 1 m s -1 intrudes into the Bay of Bengal beneath southward-flowing low-salinity water during the northeast monsoon. The subsurface high-salinity water, which originates in the northern Arabian Sea, spreads northward into the Bay of Bengal and downward along constant density surfaces. North of 10N, the model simulation implies that mixing takes place on density surfaces at depths of 100 -150 m after advection of cold, low-salinity water from the north, and subsequent stirring of the two water masses. Vertical diffusion plays an insignificant role in this mixing.

The California Undercurrent transports warm, salty, nutrient-rich, oxygen-depleted water along the continental slope from the equatorial Pacific to the Aleutian Islands. We use multiyear acoustic Doppler current profiler records collected simultaneously at two mooring sites off Vancouver Island to detail the regional structure of the undercurrent and to show that much of its variability is attributable to the passage of remotely forced, coastal-trapped waves. We also document two subsurface currents missed by earlier current measurements. The undercurrent becomes evident in spring, intensifies through summer and fall, and merges with the wind-driven poleward surface flow in winter. During intensification at the southern mooring site (A1), the undercurrent shoals from 250 ± 50 m in early summer to 150 ± 50 m depth in late fall. At the northern site (BP2), 225 km to the northwest of A1, the current is weaker and maintains a year-round depth of 150 ± 50 m. Temporal variability in the undercurrent velocity attains highest coherence with winds along the southern Oregon-northern California coast, with peak coherence occurring for “synoptic” (10–40 day period) alongshore winds off Cape Blanco in southern Oregon. The undercurrent lag of 3 ± 2 days relative to the Cape Blanco winds at synoptic periods is consistent with low mode, poleward propagating, coastally trapped waves. For periods >40 days, the wind-current coherence remains high for winds off the Oregon-California coast but lags are often negative, indicating possible forcing by alongshore baroclinic pressure gradients. At interannual time scales, the undercurrent variations have links to climate-scale processes in the equatorial Pacific.

The influence of varying horizontal and vertical stratification in the upper layer ( O(10) m) associated with riverine waters and seasonal atmospheric fluxes on coastal near-inertial currents is investigated with remotely sensed and in situ observations of surface and subsurface currents and realistic numerical model outputs off the coast of Oregon. Based on numerical simulations with and without the Columbia River (CR) during summer, the directly wind-forced near-inertial surface currents are enhanced by 30%–60% when the near-surface layer has a stratified condition due to riverine water inputs from the CR. Comparing model results without the CR for summer and winter conditions indicates that the directly wind-forced near-inertial surface current response to a unit wind forcing during summer are 20%–70% stronger than those during winter depending on the cross-shore location, which is in contrast to the seasonal patterns of both mixed-layer depth and amplitudes of near-inertial currents. The model simulations are used to examine aspects of coastal inhibition of near-inertial currents, manifested in their spatial coherence in the cross-shore direction, where the phase propagates upward over the continental shelf, bounces at the coast, and continues increasing upward offshore (toward surface) and then downward offshore at the surface, with magnitudes and length scales in the near-surface layer increasing offshore. This pattern exhibits a particularly well-organized structure during winter. Similarly, the raypaths of clockwise near-inertial internal waves are consistent with the phase propagation of coherence, showing the influence of upper layer stratification and coastal inhibition.

This study explores the characteristics of wind-driven steady-state flows in water bodies of constant density focusing on situations in which the surface and bottom Ekman layer interfere. Under the assumption of constant eddy viscosity in conjunction with a zero-flow bottom boundary condition, such flows can be linearly decomposed into wind-driven and pressure gradient-driven flow components, each affiliated with a frictional boundary layer. The resultant interference patterns, including the creation of undercurrents, are discussed using a one-dimensional water-column model. The second part of this paper employs a three-dimensional hydrodynamic model to study interferences for an idealized large and shallow oceanic bay at low latitudes under the action of a uniform wind stress. Lee effects trigger a surface pressure field that tends to slope against the wind direction. The associated pressure gradient force creates an undercurrent in deeper portions of the bay, while unidirectional flows prevail in shallower water. It is demonstrated that such undercurrents can operate as an effective upwelling mechanism, moving sub-surface water into a bay over large distances (~100 km). Based on estimates of eddy viscosities, it is also shown that Ekman layer dynamics play a central role in the dynamics of most mid-latitude lakes. On the continental shelf of the modern ocean, inferences between the surface and bottom Ekman layers leading to undercurrents do currently only exist in shallow shelf seas at low latitudes, such as the Arafura Sea.

Material transport and dispersion near the mouth of a tidal inlet (New River Inlet, NC) are investigated using GPS-tracked drifters and numerical models. For ebb tide releases, velocities are largest (>1 m s−1) in two approximately 30 m wide channels that bisect the 1–3 m deep ebb shoal. In the channels, drifter and subsurface current meter velocities are similar, consistent with strong vertical mixing and 2-D hydrodynamics. Drifters were preferentially entrained in the channelized jets where drifter cluster lateral spreading rates μin were small ( μin≈0.5 m2 s−1). At the seaward edge of the ebb shoal, jet velocities decrease linearly with distance (to ≤0.2 m s−1, about 1 km from shore), and cluster spreading rates are larger with μout≈3 m2 s−1. Although the models COAWST and NearCom generally reproduce the observed trajectory directions, certain observed drifter properties are poorly modeled. For example, modeled mean drifter velocities are smaller than observed, and upon exiting the inlet, observed drifters turn north more than modeled drifters. The model simulations do reproduce qualitatively the spreading rates observed in the inner inlet, the flow deceleration, and the increase in μout observed in the outer inlet. However, model spreading rates increase only to μout

The Gulf of Naples is a coastal area in the south-eastern Tyrrhenian Sea (western Mediterranean). Zones of great environmental and touristic value coexist in this area with one of the largest seaports in the Mediterranean Sea, industrial settlements and many other pollution sources. In such an environment, water renewal mechanisms are crucial for maintaining the ecological status of the coastal waters. In this paper, we focus on the water exchange between the interior of the Gulf and the neighbouring open Tyrrhenian Sea. The surface dynamics of the Gulf have been investigated based on measurements carried out with a high-frequency (HF) radar system. The vertical component of the current field has been provided by the Regional Ocean Modelling System (ROMS) model of ocean circulation. We present the results of a one-year-long analysis of data and simulation results relative to the year 2009. Inshore/offshore exchanges were assessed by looking at the zonal component of the surface and subsurface current field across a transect representing a sort of threshold between the interior of the Gulf and the open sea. This also allows for the reconstruction of the short-term origin of waters found inside the Gulf in the different forcing and circulation conditions.

Analysis of year-long drifter trajectories and records of simulated surface Lagrangian Coherent Structures (LCSs) have suggested the presence of a resilient Cross-Shelf Transport Barrier (CSTB) on the West Florida Shelf (WFS). The CSTB was conjectured to provide a large degree of isolation, which is consequential for the fueling of red tides on the southern WFS by nutrients possibly released by rivers and canals directly on the region. Here this conjecture is thoroughly tested by identifying LCSs as well as performing tracer advection calculations based on seven-year-long records of surface and subsurface currents produced by a HYbrid-Coordinate Ocean Model (HYCOM) simulation of the Gulf of Mexico (GoM). The identified LCSs suggest that the CSTB extends downward in the water column. The tracer calculations suggest that, while the majority of the nutrients possibly released by rivers and canals directly on the southern WFS are retained within the region for long times, only a small fraction of the nutrients possibly released by rivers outside the WFS reach the southern WFS, mainly accompanying shoreward excursions of the CSTB. These results add importance to the role played by the CSTB in controlling red tide development on the WFS. Implications of the results for the dispersal of pollutants, such as oil, in the GoM are discussed.

This work received support and funding from • The BOHAB project (Biological Oceanography of Harmful Algal Blooms off the west coast of Ireland) through the National Development Plan 2000 – 2006 with the support of the Marine Institute and the Marine RTDI (Research, Technology, Development and Innovation) Measure, Productive Sector Operational Programme, Grant-aid Agreement No. ST/02/01. • the ASIMUTH project (Applied simulations and Integrated modelling for the understanding of toxic and harmful algal blooms) through the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) for Research and Technological Development - EC FP7 Programme, Space Theme, Grant Agreement No. 261860 • the MyOcean 2 project through the EC FP7 Programme, Space Theme, Grant Agreement No. 283367 • the AtlantOS project through the European Union's Horizon 2020 research and innovation programme, Grant Agreement No. 633211 • The PRIMROSE (Predicting the impact of regional scale events on the aquaculture sector) project, co-financed by the European Regional Development Fund through the Interreg Atlantic Area Programme, EAPA_182/2016 • The CoCliME (Co-development of climate services for adaptation to changing marine ecosystems) project is part of the European Research Area for Climate Services (ERA4CS), an ERA-NET initiated by JPI Climate, and funded by EPA (IE), ANR (FR), BMBF (DE), UEFISCDI (RO), RCN (NO) and FORMAS (SE), with co-funding by the European Union (Grant Agreement No. 690462)., This document describes the procedural steps in creating an information product focused on toxic and harmful phytoplankton. The product is an online Harmful Algal Bloom (HAB) bulletin for aquaculturists, who can face serious operational challenges in the days after a HAB event. Data from satellite, numerical hydrodynamic models and In-situ ocean observations are organised and presented into visual information products. These products are enhanced through local expert evaluation and their interpretation is summarised in the bulletin. This document aims to provide both process overviews (the “what” of the Best Practice in producing the bulletins) and detail procedures (the “how” of the Best Practice”) so that the bulletins may be replicated in other geographic regions., European Commission

The subsurface countercurrents in the Philippine Sea and their roles in water mass transportation have been reported in previous studies. Their existence is still controversial, and the underlying dynamics remains unclear. This study investigates the climatological structures and relationships of three subsurface countercurrents, namely the Mindanao Undercurrent (MUC), the Luzon Undercurrent (LUC), and the North Equatorial Undercurrent (NEUC), using recently available hydrographic and satellite altimeter data. The three subsurface currents below and opposite to the surface currents are confirmed by multisections analysis. The MUC, as traced at zonal sections between 6.5 degrees N and 10.5 degrees N, shows two northward velocity cores, both with maximum speed larger than 10 cm s(-1). The LUC exhibits an obscure core with southward velocity larger than 2 cm s(-1) under the Kuroshio at 18 degrees N and 16.25 degrees N sections. The eastward flowing NEUC also has two separated cores at 128.2 degrees E and 130 degrees E sections with velocity larger than 1 cm s(-1). Analyses of -S relationship suggest that the southern part of NEUC is fed by the MUC with the South Pacific water and South/North Pacific water mixture, while the northern NEUC is likely a destiny of the North Pacific water carried by the LUC. Tightly associated with the opposite horizontal gradients between sea surface height (SSH) and the depth of thermocline (DTC), the subsurface countercurrents exist in connected zones where the baroclinic adjustment below the thermocline overcomes the barotropic forcing at the sea surface, which indicates the dynamical linkages among the three subsurface countercurrents.

Information about near-surface currents provided in near-real-time is required in a variety of applications. However, direct measurements of near-surface currents are scarce. In the Gulf of Mexico, the oil industry has been collecting current profiles from deepwater rigs, platforms, and moorings in accordance with BSEE Notices to Lessees (NTLs) since 2005. On the rigs and some platforms, these profiles are usually collected with long-range ADCPs positioned beneath the rig/platform keel, so the measured current profiles typically extend from 70-80m below the surface to as deep as 1000m. Unfortunately, only a few stations are equipped to provide good-quality near surface current data. This study assesses the validity and limitations of some simplistic current profile extrapolation algorithms, including approximation of the near surface current by "slab" flow and profile extrapolation algorithms derived from the shape of the measured subsurface profile or from a generic current profile shape. The analysis is based on data collected by a long-term mooring anchored in approximately 1400m of water in the north central Gulf of Mexico (Green Canyon 782, NDBC ID 42369) and on a year-long data set from a mooring deployed in Keathly Canyon in approximately 1900m of water. The available data include periods of Loop Current Eddies (LCEs), weak cold-core eddies, relatively strong flows characterized by a subsurface current intensification, and high-amplitude inertial oscillations. Velocity profiles were mapped onto fixed depth levels. The subsurface section of the profiles was used to extrapolate mappings from 80-100m to the near-surface. Our analysis shows that, in the winter and in the absence of an LCE, currents in the upper 80m of the water column were quasi-homogeneous. Although current speed near the surface is likely to be higher than current speed at 80m, there is a 90% probability that this difference does not exceed 10cm/s and a slab flow extrapolation is reasonable. When an LCE is present, the accuracy of extrapolation is better if the shape of the near-surface current profile is based on deeper observations. This is also true for the stratified summer water column. The uncertainties of profile extrapolation are quantified for each data set and for each flow regime.

Dedicated to the memory of our colleague Klaus Hochheim, who tragically lost his life in the Arctic expedition in September 2013. A distinct, subsurface density front along the eastern St. Anna Trough in the northern Kara Sea is inferred from hydrographic observations in 1996 and 2008–2010. Direct velocity measurements show a persistent northward subsurface current (~ 18 cm s−1) along the St. Anna Trough eastern flank. This sheared flow, carrying the outflow from the Barents and Kara seas to the Arctic Ocean, is also evident from shipboard observations as well as from geostrophic velocities and numerical model simulations. Although we cannot substantiate our conclusions by direct observation-based estimates of mixing rates in the area, we hypothesize that the enhanced vertical mixing along the St. Anna Trough eastern flank favors the upward heat loss from the intermediate warm Atlantic water layer. Modeling results support this hypothesis. The upward heat flux inferred from hydrographic data and model simulations is of O(30–100) W m−2. The region of lowered sea ice thickness and concentration seen both in sea ice remote sensing observations and model simulations marks the Atlantic water pathway in the St. Anna Trough and adjacent Nansen Basin continental margin. In fact, the sea ice shows a delayed freeze-up onset during fall and a reduction in the sea ice thickness during winter. This is consistent with our results on the enhanced Atlantic water heat loss along the Atlantic water pathway in the St. Anna Trough.

The mixed layer and upper ocean are a region of immense interest to physical oceanographers, meteorologists, and climate scientists because this is the boundary through which energy, momentum, buoyancy, and gasses are exchanged between the ocean and the atmosphere. The upper ocean is also a rich ecosystem for a vast array of ocean organisms and marine wildlife as well as a region of great Naval tactical importance. Given these factors, it is rather surprising that ocean acoustics has paid little attention to this significant region, aside from very high-frequency studies of bubble properties and gas entrainment. From the standpoint of transmission loss, the major work on the problem seems to go back to the Acoustic, Meteorological, Oceanographic Survey (AMOS) of the mid-50s. Here, a review is presented of relevant ocean processes that may be important for acoustic propagation at frequencies ranging from several hundreds of hertz to several kilohertz, where the foundation of the analysis is normal mode and transport theory. Processes of interest are surface gravity waves including subsurface currents, internal waves and tides, wind driven inertial oscillations and Langmuir circulations, eddies and submesoscale processes, turbulence and spice, bubbles, and fishes.

We investigated the impact of assimilating a mapped dataset of subsurface ocean currents into an ocean state estimation. We carried out two global ocean state estimations from 2000 to 2007 using the K7 four-dimensional variational data synthesis system, one of which included an additional map of climatological geostrophic currents estimated from the global set of Argo floats. We assessed the representativeness of the volume transport in the two exercises. The assimilation of Argo ocean current data at only one level, 1000 dbar depth, had subtle impacts on the estimated volume transports, which were strongest in the subtropical North Pacific. The corrections at 10°N, where the impact was most notable, arose through the nearly complete offset of wind stress curl by the data synthesis system in conjunction with the first mode baroclinic Rossby wave adjustment. Our results imply that subsurface current data can be effective for improving the estimation of global oceanic circulation by a data synthesis.

[1] Observations from moored subsurface acoustic Doppler current profilers (ADCPs) have revealed the structure of cross-equatorial meridional currents in the eastern equatorial Indian Ocean. Using observational data longer than 8 years, we present observations of the seasonal and interannual variation in the cross-equatorial currents in the 2000s. In line with earlier observations and numerical simulations, there were significant subsurface currents that displayed seasonal variation. The annual mean condition was dominated by currents during the boreal summer monsoon in which the Ekman transports are southward on both sides of the equator. The vertical structure of the mean meridional current displayed evidence of a shallow “equatorial roll,” consisting of northward surface flow and southward subsurface flow above the base of the isothermal mixed layer. The seasonal variation in upper 120 m meridional transport can be explained by wind-driven cross-equatorial Ekman/Sverdrup transport. Interannual variability was higher in the winter monsoon period than in the summer monsoon. The interannual variation could be related to the Pacific El Nino/Southern Oscillation (ENSO). ENSO-induced variation in the southeasterly trade wind could modulate meridional Sverdrup transport. Implications of these observational results for meridional heat transport are discussed.

A dual-station high-frequency (HF) Wellen Radar (WERA) transmitting at 16 MHz has observed near-real-time surface currents over an approximate range of 100 km across the Florida Straits since July 2004. During a 10-day period in April 2005 (15–25 April), a pair of 12.6-MHz SeaSondes (SS) were deployed south of the WERAs sites by NOAA's Center for Operational Oceanographic Products and Services (CO-OPS). The resulting SS grid overlapped the southern portion of the WERA domain. During the same period of time, a bottom-mounted acoustic Doppler current profiler (ADCP) acquired subsurface current measurements within these HF radar grids starting at 14 m below the surface in water of 86-m depth. The interoperability of beam-forming (WERA) and direction-finding (SS) HF radar technologies was examined. Comparisons of radial and vector currents for an 8-day concurrent time series suggested good agreement in current direction over both domains, where the surface currents' magnitudes were a maximum of 1.2 m s−1. In the core of the radar domains consisting of 108 cells, hourly vector currents were obtained by combining WERA and SS radials. Generally, this can be done in a relatively straightforward manner, considering the geometric dilution of precision (GDOP). A second key issue is downscaling the SS measurements from a 3-km grid to a 1.1-km grid to match the WERA output. This enhanced grid spacing is important along the western flank of the Florida Current, where energetic, small-scale surface features have been observed.