Browse
Abstract
Lateral mesoscale eddy-induced tracer transport is traditionally represented in coarse-resolution models by the flux–gradient relation. In its most complete form, the relation assumes the eddy tracer flux as a product of the large-scale tracer concentration gradient and an eddy transport coefficient tensor. However, several recent studies reported that the tensor has significant spatiotemporal complexity and is not uniquely defined, that is, it is sensitive to the tracer distributions and to the presence of nondivergent (“rotational”) components of the eddy flux. These issues could lead to significant biases in the representation of the eddy-induced transport. Using a high-resolution tracer model of the Gulf Stream region, we examine the diffusive and advective properties of lateral eddy-induced transport of dynamically passive tracers, reevaluate the utility of the flux–gradient relation, and propose an alternative approach based on modeling the local eddy forcing by a combination of diffusion and generalized eddy-induced advection. Mesoscale eddies are defined by a scale-based spatial filtering, which leads to the importance of new eddy-induced terms, including eddy-mean covariances in the eddy fluxes. The results show that the biases in representing these terms are noticeably reduced by the new approach. A series of targeted simulations in the high-resolution model further demonstrates that the approach outperforms the flux–gradient model in reproducing the stirring and dispersing effect of eddies. Our study indicates potential to upgrade the traditional flux–gradient relation for representing the eddy-induced tracer transport.
Abstract
Lateral mesoscale eddy-induced tracer transport is traditionally represented in coarse-resolution models by the flux–gradient relation. In its most complete form, the relation assumes the eddy tracer flux as a product of the large-scale tracer concentration gradient and an eddy transport coefficient tensor. However, several recent studies reported that the tensor has significant spatiotemporal complexity and is not uniquely defined, that is, it is sensitive to the tracer distributions and to the presence of nondivergent (“rotational”) components of the eddy flux. These issues could lead to significant biases in the representation of the eddy-induced transport. Using a high-resolution tracer model of the Gulf Stream region, we examine the diffusive and advective properties of lateral eddy-induced transport of dynamically passive tracers, reevaluate the utility of the flux–gradient relation, and propose an alternative approach based on modeling the local eddy forcing by a combination of diffusion and generalized eddy-induced advection. Mesoscale eddies are defined by a scale-based spatial filtering, which leads to the importance of new eddy-induced terms, including eddy-mean covariances in the eddy fluxes. The results show that the biases in representing these terms are noticeably reduced by the new approach. A series of targeted simulations in the high-resolution model further demonstrates that the approach outperforms the flux–gradient model in reproducing the stirring and dispersing effect of eddies. Our study indicates potential to upgrade the traditional flux–gradient relation for representing the eddy-induced tracer transport.
Abstract
We analyze approximately four years of heat-flux measurements at two levels, profiles of air temperature, and multiple measurements of the water temperature collected at a coastal zone site. Our analysis considers underestimation of the sea surface flux resulting from vertical divergence of the heat flux between the surface and the lowest flux level. We examine simple relationships of the heat flux to the wind speed and stratification and the potential influence of fetch and temperature advection. The fetch ranges from about 4 to near 400 km. For a given wind-direction sector, the transfer coefficient varies only slowly with increasing instability but decreases significantly with increasing stability. The intention here is not to recommend a new parameterization but rather to establish relationships that underlie the bulk formula that could lead to assessments of uncertainty and improvement of the bulk formula.
Significance Statement
The behavior of surface heat fluxes in the coastal zone is normally more complex than over the open ocean but has a large impact on human activity. Our study examines extensive flux measurements on a tower in the Baltic Sea that allows partitioning of the fluxes according to wind direction without seriously depleting the data for a given wind-direction sector. Because some of the normal assumptions for the usual parameterization are not met, our study examines relationships behind the parameterization of the surface fluxes.
Abstract
We analyze approximately four years of heat-flux measurements at two levels, profiles of air temperature, and multiple measurements of the water temperature collected at a coastal zone site. Our analysis considers underestimation of the sea surface flux resulting from vertical divergence of the heat flux between the surface and the lowest flux level. We examine simple relationships of the heat flux to the wind speed and stratification and the potential influence of fetch and temperature advection. The fetch ranges from about 4 to near 400 km. For a given wind-direction sector, the transfer coefficient varies only slowly with increasing instability but decreases significantly with increasing stability. The intention here is not to recommend a new parameterization but rather to establish relationships that underlie the bulk formula that could lead to assessments of uncertainty and improvement of the bulk formula.
Significance Statement
The behavior of surface heat fluxes in the coastal zone is normally more complex than over the open ocean but has a large impact on human activity. Our study examines extensive flux measurements on a tower in the Baltic Sea that allows partitioning of the fluxes according to wind direction without seriously depleting the data for a given wind-direction sector. Because some of the normal assumptions for the usual parameterization are not met, our study examines relationships behind the parameterization of the surface fluxes.
Abstract
Internal waves are predominantly generated by winds, tide–topography interactions, and balanced flow–topography interactions. Observations of vertical shear of horizontal velocity (uz
, υz
) from lowered acoustic Doppler current profilers (LADCP) profiles conducted during GO-SHIP hydrographic surveys, as well as vessel-mounted sonars, are used to interpret these signals. Vertical directionality of intermediate-wavenumber [
Abstract
Internal waves are predominantly generated by winds, tide–topography interactions, and balanced flow–topography interactions. Observations of vertical shear of horizontal velocity (uz
, υz
) from lowered acoustic Doppler current profilers (LADCP) profiles conducted during GO-SHIP hydrographic surveys, as well as vessel-mounted sonars, are used to interpret these signals. Vertical directionality of intermediate-wavenumber [
Abstract
Tide-induced near-inertial internal waves (NIWs) are generated by tide–topography interaction and are energized by internal tides through triadic resonant interaction of internal waves. They are located above topography and could be in close contact with wind-induced NIWs when the topography is a tall ridge, like in the Luzon Strait of the northern South China Sea (SCS). A natural question arises as to whether there is significant interaction between wind- and tide-induced NIWs. By using moored velocity observations, a satellite-tracked surface drifter dataset, and idealized numerical simulations, we find that in the presence of tide-induced NIWs, the wind can inject slightly more near-inertial energy (NIE), while in the presence of wind-induced NIWs, significantly more tidal energy is transferred to NIWs. Thus, wind- and tide-induced NIWs can mutually enhance each other, producing more NIE than a linear superposition of that generated by wind and tide forcing alone. Increasing wind intensity and tidal excursion lead to saturation of NIE enhancement, while a taller ridge leads to stronger enhancement. The high mixed layer NIE near Luzon Strait is mostly generated by the wind, while the mutual enhancement between wind- and tide-induced NIWs can further enhance this pattern. The interaction between wind- and tide-induced NIWs leads to an enhancement of 25% more NIE. If tide-induced NIWs are neglected, as is usually the case in the estimation of NIE, the total NIE will be underestimated by almost 50%. This might imply that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait.
Significance Statement
Near-inertial internal waves (NIWs) usually occupy the most kinetic energy of internal waves and contribute significantly to ocean mixing. Near the surface they are usually generated by wind forcing, but near the bottom they can be generated by geostrophic or tidal flow interacting with topography. Above the tall ridge in Luzon Strait, wind- and tide-induced NIWs are in close contact, leading to potential interactions. It is found that these NIWs can mutually enhance each other, with most of the additional near-inertial energy (NIE) coming from the tides. If tide-induced NIWs are neglected, the total NIE will be underestimated by almost 50%. This suggests that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait.
Abstract
Tide-induced near-inertial internal waves (NIWs) are generated by tide–topography interaction and are energized by internal tides through triadic resonant interaction of internal waves. They are located above topography and could be in close contact with wind-induced NIWs when the topography is a tall ridge, like in the Luzon Strait of the northern South China Sea (SCS). A natural question arises as to whether there is significant interaction between wind- and tide-induced NIWs. By using moored velocity observations, a satellite-tracked surface drifter dataset, and idealized numerical simulations, we find that in the presence of tide-induced NIWs, the wind can inject slightly more near-inertial energy (NIE), while in the presence of wind-induced NIWs, significantly more tidal energy is transferred to NIWs. Thus, wind- and tide-induced NIWs can mutually enhance each other, producing more NIE than a linear superposition of that generated by wind and tide forcing alone. Increasing wind intensity and tidal excursion lead to saturation of NIE enhancement, while a taller ridge leads to stronger enhancement. The high mixed layer NIE near Luzon Strait is mostly generated by the wind, while the mutual enhancement between wind- and tide-induced NIWs can further enhance this pattern. The interaction between wind- and tide-induced NIWs leads to an enhancement of 25% more NIE. If tide-induced NIWs are neglected, as is usually the case in the estimation of NIE, the total NIE will be underestimated by almost 50%. This might imply that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait.
Significance Statement
Near-inertial internal waves (NIWs) usually occupy the most kinetic energy of internal waves and contribute significantly to ocean mixing. Near the surface they are usually generated by wind forcing, but near the bottom they can be generated by geostrophic or tidal flow interacting with topography. Above the tall ridge in Luzon Strait, wind- and tide-induced NIWs are in close contact, leading to potential interactions. It is found that these NIWs can mutually enhance each other, with most of the additional near-inertial energy (NIE) coming from the tides. If tide-induced NIWs are neglected, the total NIE will be underestimated by almost 50%. This suggests that tide-induced NIWs are important for the energetics of NIWs in Luzon Strait.
Abstract
Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Indirect microstructure measurements of in situ turbulence suggest that mixing is bottom enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. Tracer release experiments (TREs), in which inert tracers are purposefully released and their dispersion is surveyed over time, have been used to independently infer turbulent diffusivities—but typically provide estimates in excess of microstructure ones. In an attempt to reconcile these differences, Ruan and Ferrari derived exact tracer-weighted buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. A tracer’s diapycnal displacement rate is exactly twice the tracer-averaged buoyancy velocity, itself a convolution of an asymmetric upwelling/downwelling dipole. The tracer’s diapycnal spreading rate, however, involves both the expected positive contribution from the tracer-averaged in situ diffusion as well as an additional nonlinear diapycnal distortion term, which is caused by correlations between buoyancy and the buoyancy velocity, and can be of either sign. Distortion is generally positive (stretching) due to bottom-enhanced mixing in the stratified interior but negative (contraction) near the bottom. Our simulations suggest that these two effects coincidentally cancel for the Brazil Basin Tracer Release Experiment, resulting in negligible net distortion. By contrast, near-bottom tracers experience leading-order distortion that varies in time. Errors in tracer moments due to realistically sparse sampling are generally small (<20%), especially compared to the
Abstract
Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Indirect microstructure measurements of in situ turbulence suggest that mixing is bottom enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. Tracer release experiments (TREs), in which inert tracers are purposefully released and their dispersion is surveyed over time, have been used to independently infer turbulent diffusivities—but typically provide estimates in excess of microstructure ones. In an attempt to reconcile these differences, Ruan and Ferrari derived exact tracer-weighted buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. A tracer’s diapycnal displacement rate is exactly twice the tracer-averaged buoyancy velocity, itself a convolution of an asymmetric upwelling/downwelling dipole. The tracer’s diapycnal spreading rate, however, involves both the expected positive contribution from the tracer-averaged in situ diffusion as well as an additional nonlinear diapycnal distortion term, which is caused by correlations between buoyancy and the buoyancy velocity, and can be of either sign. Distortion is generally positive (stretching) due to bottom-enhanced mixing in the stratified interior but negative (contraction) near the bottom. Our simulations suggest that these two effects coincidentally cancel for the Brazil Basin Tracer Release Experiment, resulting in negligible net distortion. By contrast, near-bottom tracers experience leading-order distortion that varies in time. Errors in tracer moments due to realistically sparse sampling are generally small (<20%), especially compared to the
Abstract
The abyssal overturning circulation is thought to be primarily driven by small-scale turbulent mixing. Diagnosed water-mass transformations are dominated by rough topography “hotspots,” where the bottom enhancement of mixing causes the diffusive buoyancy flux to diverge, driving widespread downwelling in the interior—only to be overwhelmed by an even stronger upwelling in a thin bottom boundary layer (BBL). These water-mass transformations are significantly underestimated by one-dimensional (1D) sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this 1D boundary layer approach to three-dimensional eddying flows over realistically rough topography. When applied to the Mid-Atlantic Ridge in the Brazil Basin, the idealized simulation results are roughly consistent with available observations. Integral buoyancy budgets isolate the physical processes that contribute to realistically strong BBL upwelling. The downward diffusion of buoyancy is primarily balanced by upwelling along the sloping canyon sidewalls and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies and by the blocking of along-ridge thermal wind within the canyon. Major topographic sills block along-thalweg flows from restratifying the canyon trough, resulting in the continual erosion of the trough’s stratification. We propose simple modifications to the 1D boundary layer model that approximate each of these three-dimensional effects. These results provide local dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the nonlocal coupling to the basin-scale circulation.
Abstract
The abyssal overturning circulation is thought to be primarily driven by small-scale turbulent mixing. Diagnosed water-mass transformations are dominated by rough topography “hotspots,” where the bottom enhancement of mixing causes the diffusive buoyancy flux to diverge, driving widespread downwelling in the interior—only to be overwhelmed by an even stronger upwelling in a thin bottom boundary layer (BBL). These water-mass transformations are significantly underestimated by one-dimensional (1D) sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this 1D boundary layer approach to three-dimensional eddying flows over realistically rough topography. When applied to the Mid-Atlantic Ridge in the Brazil Basin, the idealized simulation results are roughly consistent with available observations. Integral buoyancy budgets isolate the physical processes that contribute to realistically strong BBL upwelling. The downward diffusion of buoyancy is primarily balanced by upwelling along the sloping canyon sidewalls and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies and by the blocking of along-ridge thermal wind within the canyon. Major topographic sills block along-thalweg flows from restratifying the canyon trough, resulting in the continual erosion of the trough’s stratification. We propose simple modifications to the 1D boundary layer model that approximate each of these three-dimensional effects. These results provide local dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the nonlocal coupling to the basin-scale circulation.
Abstract
The diabatic transformations of the middepth meridional overturning circulation (MOC) as it exits and reenters the South Atlantic to close the AMOC are studied using a state estimate assimilating data into a dynamically consistent ocean model. Virtual Lagrangian parcels in the lower branch of the MOC are followed in their global tour as they return to the upper branch of the MOC. Three return pathways are identified. The first pathway enters the abyssal Indo-Pacific as Circumpolar Deep Water, directly from the northern Antarctic Circumpolar Current (ACC), and before sampling the Antarctic margin. The second pathway sinks to abyssal densities exclusively in the Southern Ocean, then upwells while circulating within the ACC and eventually enters the Indo-Pacific or Atlantic at mid- to upper depths. The third pathway never reaches densities in the abyssal range. Parcels sinking in the Antarctic Bottom Water range upwell to mid- to upper depths south of 55°S. Parcels in all three pathways experience additional diabatic transformations after upwelling in the Southern Ocean, with more diabatic changes north of about 30°S than elsewhere. Diabatic changes are predominantly in the mixed layer of the tropical and subpolar gyres, enabled by Ekman suction. A simple model of the wind-driven flow illustrates that parcels always reach the surface in the tropical and subpolar gyres, regardless of their initial condition, because of coupling among gyres, the Ekman transport, and its return.
Abstract
The diabatic transformations of the middepth meridional overturning circulation (MOC) as it exits and reenters the South Atlantic to close the AMOC are studied using a state estimate assimilating data into a dynamically consistent ocean model. Virtual Lagrangian parcels in the lower branch of the MOC are followed in their global tour as they return to the upper branch of the MOC. Three return pathways are identified. The first pathway enters the abyssal Indo-Pacific as Circumpolar Deep Water, directly from the northern Antarctic Circumpolar Current (ACC), and before sampling the Antarctic margin. The second pathway sinks to abyssal densities exclusively in the Southern Ocean, then upwells while circulating within the ACC and eventually enters the Indo-Pacific or Atlantic at mid- to upper depths. The third pathway never reaches densities in the abyssal range. Parcels sinking in the Antarctic Bottom Water range upwell to mid- to upper depths south of 55°S. Parcels in all three pathways experience additional diabatic transformations after upwelling in the Southern Ocean, with more diabatic changes north of about 30°S than elsewhere. Diabatic changes are predominantly in the mixed layer of the tropical and subpolar gyres, enabled by Ekman suction. A simple model of the wind-driven flow illustrates that parcels always reach the surface in the tropical and subpolar gyres, regardless of their initial condition, because of coupling among gyres, the Ekman transport, and its return.
Abstract
An idealized width-averaged model is employed to study the influence of wind stress on subtidal salt intrusion and stratification in well-mixed and partially stratified estuaries. We show that even in mild conditions, wind forcing can influence the estuarine salinity structure in a substantial way. By studying the role of wind forcing on dominant salt transport balances and associated salt transport regimes, we unify and clarify ambiguous observations from previous authors regarding the influence of wind stress: the response of the estuarine salinity structure to wind forcing is different depending on the underlying dominant salt transport balance, which in turn was found to determine whether wind-induced salinity shear, wind-induced modulation of the longitudinal salt distribution, or wind-induced mixing dominates.
Significance Statement
The purpose of this idealized study is to better understand how wind influences the salinity distribution in estuaries on large time scales. This is important because a change in winds can move saline water further inland, threatening freshwater availability and the natural balance of delicate ecosystems. We clarify the sometimes ambiguous observations regarding the influence of wind on the salt distribution and highlight the importance of including average wind forcing in analyses of estuarine dynamics on large time scales.
Abstract
An idealized width-averaged model is employed to study the influence of wind stress on subtidal salt intrusion and stratification in well-mixed and partially stratified estuaries. We show that even in mild conditions, wind forcing can influence the estuarine salinity structure in a substantial way. By studying the role of wind forcing on dominant salt transport balances and associated salt transport regimes, we unify and clarify ambiguous observations from previous authors regarding the influence of wind stress: the response of the estuarine salinity structure to wind forcing is different depending on the underlying dominant salt transport balance, which in turn was found to determine whether wind-induced salinity shear, wind-induced modulation of the longitudinal salt distribution, or wind-induced mixing dominates.
Significance Statement
The purpose of this idealized study is to better understand how wind influences the salinity distribution in estuaries on large time scales. This is important because a change in winds can move saline water further inland, threatening freshwater availability and the natural balance of delicate ecosystems. We clarify the sometimes ambiguous observations regarding the influence of wind on the salt distribution and highlight the importance of including average wind forcing in analyses of estuarine dynamics on large time scales.
Abstract
Complex small-scale processes and energetic turbulence are observed at a sill located on the I-Lan Ridge that spans across the strong Kuroshio off Taiwan. The current speed above the sill is strong (1.5 m s−1) and unsteady (±0.5 m s−1) due to the Kuroshio being modulated by the semidiurnal tide. Above the sill crest, isothermal domes, with vertical scales of ∼20 and ∼50 m during the low and high tides, respectively, are generated by turbulent mixing as a result of shear instability in the bottom boundary layer. Tidally modulated hydraulic character modifies the small-scale processes occurring on the leeward side of the sill. Criticality analysis, performed by solving the Taylor–Goldstein equation, suggests that the observed lee waves and intermediate layer sandwiched by two free shear layers are related to the mode-1 and mode-2 critical control between the sill crest and immediate lee, respectively. Around high tide, lee waves are advected further downstream, and only mode-1 critical control can occur, leading to a warm water depression. The shear instabilities ensuing from the hydraulic transition processes continuously mediate flow kinetic energy to turbulence such that the status of marginal instability where the Richardson number converges at approximately 0.25 is reached. The resultant eddy diffusivity Kρ is concentrated from O(10−4) to O(10−3) m2 s−1 and has a maximum value of 10 m2 s−1. The sill on the western flank of the Kuroshio is a hotspot for energetic mixing of Kuroshio waters and South China Sea waters.
Abstract
Complex small-scale processes and energetic turbulence are observed at a sill located on the I-Lan Ridge that spans across the strong Kuroshio off Taiwan. The current speed above the sill is strong (1.5 m s−1) and unsteady (±0.5 m s−1) due to the Kuroshio being modulated by the semidiurnal tide. Above the sill crest, isothermal domes, with vertical scales of ∼20 and ∼50 m during the low and high tides, respectively, are generated by turbulent mixing as a result of shear instability in the bottom boundary layer. Tidally modulated hydraulic character modifies the small-scale processes occurring on the leeward side of the sill. Criticality analysis, performed by solving the Taylor–Goldstein equation, suggests that the observed lee waves and intermediate layer sandwiched by two free shear layers are related to the mode-1 and mode-2 critical control between the sill crest and immediate lee, respectively. Around high tide, lee waves are advected further downstream, and only mode-1 critical control can occur, leading to a warm water depression. The shear instabilities ensuing from the hydraulic transition processes continuously mediate flow kinetic energy to turbulence such that the status of marginal instability where the Richardson number converges at approximately 0.25 is reached. The resultant eddy diffusivity Kρ is concentrated from O(10−4) to O(10−3) m2 s−1 and has a maximum value of 10 m2 s−1. The sill on the western flank of the Kuroshio is a hotspot for energetic mixing of Kuroshio waters and South China Sea waters.
Abstract
The spatially averaged frequency spectrum of sea level has been computed at 4 cycle-per-year resolution and a Nyquist frequency of 0.5 cycles per hour using dual-satellite crossover data from the Jason and CryoSat-2 satellite altimeter missions. The novelty of the analysis is that it reveals unambiguous peaks due to high-frequency tidal signals, even after removing the predicted barotropic tide, without the usual aliasing caused by altimeter sampling. The tidal continuum, that is, a tidal cusp, is present in the spectrum in the diurnal and semidiurnal tidal bands, and a Lorentzian model spectrum has been fit within each band to identify the properties of the non-phase-locked tidal variability. An interesting feature of the semidiurnal tidal continuum is the unambiguous presence of an inner and an outer band, characterized by different Lorentzian bandwidths of roughly (180 day)−1 and (30 day)−1. Considering different latitude ranges, it is clear that the tidal continuum is most prominent in the range from −30° to 30° latitude. Within this range, it is found that 1.05-cm2 variance is associated with the semidiurnal continuum, and slightly less than half of this variance, 0.41 cm2, is associated with the slower, (180 day)−1 bandwidth, variability. The ratio of non-phase-locked to total baroclinic variability is about 62% in this latitude band, a value that is consistent with previous model-based estimates for this quantity. Quantification of the properties of the tidal continuum poleward of 30° latitude is not possible with the present data, due to the small size of the tidal signal compared to the mesoscale variability and other sources of noise.
Abstract
The spatially averaged frequency spectrum of sea level has been computed at 4 cycle-per-year resolution and a Nyquist frequency of 0.5 cycles per hour using dual-satellite crossover data from the Jason and CryoSat-2 satellite altimeter missions. The novelty of the analysis is that it reveals unambiguous peaks due to high-frequency tidal signals, even after removing the predicted barotropic tide, without the usual aliasing caused by altimeter sampling. The tidal continuum, that is, a tidal cusp, is present in the spectrum in the diurnal and semidiurnal tidal bands, and a Lorentzian model spectrum has been fit within each band to identify the properties of the non-phase-locked tidal variability. An interesting feature of the semidiurnal tidal continuum is the unambiguous presence of an inner and an outer band, characterized by different Lorentzian bandwidths of roughly (180 day)−1 and (30 day)−1. Considering different latitude ranges, it is clear that the tidal continuum is most prominent in the range from −30° to 30° latitude. Within this range, it is found that 1.05-cm2 variance is associated with the semidiurnal continuum, and slightly less than half of this variance, 0.41 cm2, is associated with the slower, (180 day)−1 bandwidth, variability. The ratio of non-phase-locked to total baroclinic variability is about 62% in this latitude band, a value that is consistent with previous model-based estimates for this quantity. Quantification of the properties of the tidal continuum poleward of 30° latitude is not possible with the present data, due to the small size of the tidal signal compared to the mesoscale variability and other sources of noise.
