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In global ocean simulations, forward (non-data-assimilative) tide models generally feature large sea-surface-height errors near Hudson Strait in the North Atlantic Ocean with respect to altimetry-constrained tidal solutions. These errors may be associated with tidal resonances that are not well resolved by the complex coastal-shelf bathymetry in low-resolution simulations. An online two-way nesting framework has been implemented to improve global surface tides in the HYbrid Coordinate Ocean Model (HYCOM). In this framework, a high-resolution child domain, covering Hudson Strait, is coupled with a relatively low-resolution parent domain for computational efficiency. Data such as barotropic pressure and velocity are exchanged between the child and parent domains with the external coupler OASIS3-MCT. The developed nesting framework is validated with semi-idealized basin-scale model simulations. The M2 sea-surface heights show very good accuracy in the one-way and two-way nesting simulations in Hudson Strait, where large tidal elevations are observed. In addition, the mass and tidal energy flux are not adversely impacted at the nesting boundaries in the semi-idealized simulations. In a next step, the nesting framework is applied to a realistic global tide simulation. In this simulation, the resolution of the child domain (1/75°) is three times as fine as that of the parent domain (1/25°). The M2 sea-surface-height root-mean-square errors with tide gauge data and the altimetry-constrained global FES2014 and TPXO9-atlas tidal solutions are evaluated for the nesting and no-nesting solutions. The better resolved coastal bathymetry and the finer grid in the child domain improve the local tides in Hudson Strait and Bay, and the back-effect of the coastal tides induces an improvement of the barotropic tides in the open ocean of the Atlantic.

Tidal mixing fronts, which represent boundaries between stratified and tidally mixed waters, are locations of enhanced biological activity. They occur in summer shelf seas when, in the presence of strong tidal currents, mixing due to bottom friction balances buoyancy production due to seasonal heat flux. In this paper we examine the occurrence and fidelity of tidal mixing fronts in shelf seas generated within a global 3-dimensional simulation of the HYbrid Coordinate Ocean Model (HYCOM) that is simultaneously forced by atmospheric fields and the astronomical tidal potential. We perform a first order assessment of shelf sea tides in global HYCOM through comparison of sea surface temperature, sea surface tidal elevations, and tidal currents with observations. HYCOM was tuned to minimize errors in M2 sea surface heights in deep water. Over the global coastal and shelf seas (depths

The evidence for, baroclinic energetics of, and geographic distribution of parametric subharmonic instability (PSI) arising from both diurnal and semidiurnal tides in a global ocean general circulation model is investigated using 1/12.5° and 1/25° simulations that are forced by both atmospheric analysis fields and the astronomical tidal potential. The paper examines whether PSI occurs in the model, and whether it accounts for a significant fraction of the tidal baroclinic energy loss. Using energy transfer calculations and bispectral analyses, evidence is found for PSI around the critical latitudes of the tides. The intensity of both diurnal and semidiurnal PSI in the simulations is greatest in the upper ocean, consistent with previous results from idealized simulations, and quickly drops off about 5° from the critical latitudes. The sign of energy transfer depends on location; the transfer is positive (from the tides to subharmonic waves) in some locations and negative in others. The net globally integrated energy transfer is positive in all simulations and is 0.5%–10% of the amount of energy required to close the baroclinic energy budget in the model. The net amount of energy transfer is about an order of magnitude larger in the 1/25° semidiurnal simulation than the 1/12.5° one, implying the dependence of the rate of energy transfer on model resolution.

This letter investigates the relationship between wind, sea surface temperature (SST), and thermocline in the Seychelles–Chagos Thermocline Ridge (SCTR, 5°S–10°S, 50°E–80°E) using a combination of satellite data and a reanalysis version of the HYbrid Coordinate Ocean Model from 1993 to 2012. The asymmetry of this relationship during positive and negative Indian Ocean Dipole (IOD) events and the impacts on the SST—thermocline depth (represented by the 20 °C isotherm depth, D20) relationship—are examined. On interannual timescales, an asymmetric relation between SST and zonal wind stress causes a strengthening of easterlies that enhances anticyclonic wind stress curl and local Ekman downwelling, which in turn deepens the D20 and increases the heat content during positive IOD (PIOD) events. In contrast, during negative IOD (NIOD) events, the winds reverse to be westerlies and cause a three times greater impact on remotely generated upwelling Rossby waves. Subsequently, these asymmetric relations cause an asymmetric D20–SST feedback in the SCTR such that a shoaling D20 is ~2.5 times more effective at lowering SST during NIOD events than a deepening D20 is at raising the SST during PIOD events. The changes to D20 are observed to extend into the year following IOD events, persisting into the end of the year following a PIOD event (due to stronger asymmetric reinforcing effects of warm SST anomalies on zonal wind anomalies) but only to May–June of the year following NIOD events.

Two global ocean models ranging in horizontal resolution from 1/12° to 1/48° are used to study the space- and time-scales of sea surface height (SSH) signals associated with internal gravity waves (IGWs). Frequency-horizontal wavenumber SSH spectral densities are computed over seven regions of the world ocean from three simulations of the HYbrid Coordinate Ocean Model (HYCOM) and two simulations of the Massachusetts Institute of Technology general circulation model (MITgcm). High-wavenumber, high-frequency SSH variance follows the predicted IGW linear dispersion curves. The realism of high-frequency motions (>0.87cpd) in the models is tested through comparison of the frequency spectral density of dynamic height variance computed from the highest resolution runs of each model (1/25° HYCOM and 1/48° MITgcm) with dynamic height variance frequency spectral density computed from 9 in-situ profiling instruments. These high-frequency motions are of particular interest because of their contributions to the small-scale SSH variability that will be observed on a global scale in the upcoming Surface Water and Ocean Topography (SWOT) satellite altimetry mission. The variance at supertidal frequencies can be comparable to the tidal and low-frequency variance for high-wavenumbers (length scales smaller than ∼50km), especially in the higher resolution simulations. In the highest resolution simulations, the high-frequency variance can be greater than the low-frequency variance at these scales.

Motivated by the substantial sensitivity of eddies in two-layer quasigeostrophic (QG) turbulence models to the strength of bottom drag, this study explores the sensitivity of eddies in more realistic ocean general circulation model (OGCM) simulations to bottom drag strength. The OGCM results are interpreted using previous results from horizontally homogeneous, two-layer, flat-bottom, f-plane, doubly periodic QG turbulence simulations and new results from two-layer, β-plane QG turbulence simulations run in a basin geometry with both flat and rough bottoms. Baroclinicity in all of the simulations varies greatly with drag strength, with weak drag corresponding to more barotropic flow and strong drag corresponding to more baroclinic flow. The sensitivity of the baroclinicity in the QG basin simulations to bottom drag is considerably reduced, however, when rough topography is used in lieu of a flat bottom. Rough topography reduces the sensitivity of the eddy kinetic energy amplitude and horizontal length scales in the QG basin simulations to bottom drag to an even greater degree. The OGCM simulation behavior is qualitatively similar to that in the QG rough-bottom basin simulations, in that baroclinicity is more sensitive to bottom drag strength than are eddy amplitudes or horizontal length scales. Rough topography therefore appears to mediate the sensitivity of eddies in models to the strength of bottom drag. The sensitivity of eddies to parameterized topographic internal lee wave drag, which has recently been introduced into some OGCMs, is also briefly discussed. Wave drag acts like a strong bottom drag in that it increases the baroclinicity of the flow, without strongly affecting eddy horizontal length scales.

The jets in the equatorial Pacific Ocean of a realistically forced global circulation model with a horizontal resolution of 1/12.5° cause a strong loss of phase coherence in semidiurnal internal tides that propagate equatorward from the French Polynesian Islands and Hawaii. This loss of coherence is quantified with a baroclinic energy analysis, in which the semidiurnal-band terms are separated into coherent, incoherent, and cross terms. For time scales longer than a year, the coherent energy flux approaches zero values at the equator, while the total flux is ∼500 W/m. The time variability of the incoherent energy flux is compared with the internal-tide travel-time variability, which is based on along-beam integrated phase speeds computed with the Taylor-Goldstein equation. The variability of monthly mean Taylor-Goldstein phase speeds agrees well with the phase speed variability inferred from steric sea surface height phases extracted with a plane-wave fit technique. On monthly time scales, the loss of phase coherence in the equatorward beams from the French Polynesian Islands is attributed to the time variability in the vertically sheared background flow associated with the jets and tropical instability waves. On an annual time scale, the effect of stratification variability is of equal or greater importance than the shear variability is to the loss of coherence. In the model simulations, low-frequency equatorial jets do not noticeably enhance the dissipation of the internal tide, but merely decohere and scatter it.

We examine the temporal means and variability of the semidiurnal internal tide energy fluxes in 1/25° global simulations of the Hybrid Coordinate Ocean Model (HYCOM) and in a global archive of 79 historical moorings. Low-frequency flows, a major cause of internal tide variability, have comparable kinetic energies at the mooring sites in model and observations. The computed root-mean-square (RMS) variability of the energy flux is large in both model and observations and correlates positively with the time-averaged flux magnitude. Outside of strong generation regions, the normalized RMS variability (the RMS variability divided by the mean) is nearly independent of the flux magnitudes in the model, and of order 23% or more in both the model and observations. The spatially averaged flux magnitudes in observations and the simulation agree to within a factor of about 1.4 and 2.4 for vertical mode-1 and mode-2, respectively. The difference in energy flux computed from the full-depth model output versus model output subsampled at mooring instrument depths is small. The global historical archive is supplemented with six high-vertical resolution moorings from the Internal Waves Across the Pacific (IWAP) experiment. The model fluxes agree more closely with the high-resolution IWAP fluxes than with the historical mooring fluxes. The high variability in internal tide energy fluxes implies that internal tide fluxes computed from short observational records should be regarded as realizations of a highly variable field, not as “means” that are indicative of conditions at the measurement sites over all time.

The effects of a parameterized linear internal wave drag on the semidiurnal barotropic and baroclinic energetics of a realistically forced, three-dimensional global ocean model are analyzed. Although the main purpose of the parameterization is to improve the surface tides, it also influences the internal tides. The relatively coarse resolution of the model of ~8 km only permits the generation and propagation of the first three vertical modes. Hence, this wave drag parameterization represents the energy conversion to and the subsequent breaking of the unresolved high modes. The total tidal energy input and the spatial distribution of the barotropic energy loss agree with the Ocean Topography Experiment (TOPEX)/Poseidon (TPXO) tidal inversion model. The wave drag overestimates the high-mode conversion at ocean ridges as measured against regional high-resolution models. The wave drag also damps the low-mode internal tides as they propagate away from their generation sites. Hence, it can be considered a scattering parameterization, causing more than 50% of the deep-water dissipation of the internal tides. In the near field, most of the baroclinic dissipation is attributed to viscous and numerical dissipation. The far-field decay of the simulated internal tides is in agreement with satellite altimetry and falls within the broad range of Argo-inferred dissipation rates. In the simulation, about 12% of the semidiurnal internal tide energy generated in deep water reaches the continental margins.

The impact of topographic internal lee wave drag (wave drag hereafter) on several aspects of the low-frequency circulation in a high-resolution global ocean model forced by winds and air-sea buoyancy fluxes is examined here. The HYbrid Coordinate Ocean Model (HYCOM) is run at two different horizontal resolutions (one nominally 1/12° and the other 1/25°). Wave drag, which parameterizes both topographic blocking and the generation of lee waves arising from geostrophic flow impinging upon rough topography, is inserted into the simulations as they run. The parameterization used here affects the momentum equations and hence the structure of eddy kinetic energy. Lee waves also have implications for diapycnal mixing in the ocean, though the parameterization does not directly modify the density. Total near-bottom energy dissipation due to wave drag and quadratic bottom boundary layer drag is nearly doubled, and the energy dissipation due to quadratic bottom drag is reduced by about a factor of two, in simulations with an inserted wave drag compared to simulations having only quadratic bottom drag. With the insertion of wave drag, the kinetic energy is reduced in the abyss and in a three-dimensional global integral. Deflection by partial topographic blocking is inferred to be one reason why the near-bottom kinetic energy can increase in locations where there is little change in dissipation by quadratic bottom drag. Despite large changes seen in the abyss, the changes that occur near the sea surface are relatively small upon insertion of wave drag into the simulations. Both the sea surface height variance and geostrophic surface kinetic energy are reduced on global average by more than twice the seasonal variability in these diagnostics. Alterations in the intensified jet positions brought about by inserting wave drag are not distinguishable from the temporal variability of jet positions. Various statistical measures suggest that applying wave drag only within a fixed distance from the seafloor is not detrimental to the model performance relative to observations. However, the introduction of a novel diagnostic suggests that one way to improve the wave drag parameterization is to allow the vertical deposition of lee wave momentum flux to be spatially heterogeneous.

The ocean tidal velocity and elevation can be estimated concurrently with the ocean circulation by adding the astronomical tidal forcing, parameterized topographic internal wave drag, and self-attraction and loading to the general circulation physics. However, the accuracy of these tidal estimates does not yet match accuracies in the best data-assimilative barotropic tidal models. This paper investigates the application of an augmented state ensemble Kalman Filter (ASEnKF) to improve the accuracy of M2 barotropic tides embedded in a 1/12.5° three-dimensional ocean general circulation model. The ASEnKF is an alternative to the techniques typically used with linearized tide-only models; such techniques cannot be applied to the embedded tides in a nonlinear eddying circulation. An extra term, meant to correct for errors in the tide model due to imperfectly known topography and damping terms, is introduced into the tidal forcing. Ensembles of the model are created with stochastically generated forcing correction terms. The discrepancies for each ensemble member with TPXO, an existing data-assimilative tide model, are computed. The ASEnKF method yields an optimal estimate of the model forcing correction terms, that minimizes resultant root mean square (RMS) tidal sea surface elevation error with respect to TPXO, as well as an estimate of the tidal elevation. The deep-water, global area-averaged RMS sea surface elevation error of the principal lunar semidiurnal tide M2 is reduced from 4.4 cm in a best-case non-assimilative solution to 2.6 cm. The largest elevation errors in both the non-assimilative and ASEnKF solutions are in the North Atlantic, a highly resonant basin. Possible pathways for achieving further reductions in the RMS error are discussed.

The effects of horizontal resolution and wave drag damping on the semidiurnal M 2 tidal energetics are studied for two realistically-forced global HYbrid Coordinate Ocean Model (HYCOM) simulations with 41 layers and horizontal resolutions of 8 km ( 1 ∕ 12 . 5 ∘ ; H12) and 4 km ( 1 ∕ 25 ∘ ; H25). In both simulations, the surface tidal error is minimized by tuning the strength of the linear wave drag, which is a parameterization of the surface-tide energy conversion to the unresolved baroclinic wave modes. In both simulations the M 2 surface tide error with TPXO8-atlas, an altimetry constrained model, is 2.6 cm. Compared to H12, the surface tide energy conversion to the resolved vertical modes is increased by 50% in H25. This coincides with an equivalent reduction in the tuned loss of energy from the surface tide to the wave drag. For the configurations studied here, the horizontal and not the vertical resolution is the factor limiting the number of vertical modes that are resolved in most of the global ocean: modes 1–2 in H12 and modes 1–5 in H25. The wave drag also dampens the resolved internal tides. The 40% reduction in wave-drag strength does not result in a proportional increase in the mode-1 energy density in H25. In the higher-resolution simulations, topographic mode-scattering and wave–wave interactions are better resolved. This allows for an energy flux out of mode 1 to the higher modes, mitigating the need for an internal tide damping term. The HYCOM simulations are validated with analytical conversion models and altimetry-inferred sea-surface height, fluxes, and surface tide dissipation. H25 agrees best with these data sets to within ∼ 10%. To facilitate the comparison of stationary tide signals extracted from time series with different durations, we successfully apply a spatially-varying correction factor.

Semi-diurnal internal waves generated by tides in a high resolution numerical model that includes the Andaman Sea archipelago are found to propagate into the central Bay of Bengal and reach the coasts of India and Sri Lanka. The waves are also present in subsurface temperature records from RAMA moorings, and their propagation speed across the Bay of Bengal agrees well with satellite remote sensing from MODIS imagery in spite of the hydrostatic nature of the model. The internal waves are simulated by a fully coupled ocean-atmosphere prediction system, exchanging surface fluxes between the air and sea at high frequency and at high resolution. For the ocean, a hydrostatic model including diurnal and semi-diurnal tides provides a 2 km resolution representation of the entire Bay of Bengal. In the ocean model simulations, the semi-diurnal internal waves interact with the mesoscale circulation and surface waves and modify the flow and the stratification. By comparing coupled ocean-wave model runs with tides and without tides, but forced by identical surface fluxes from the atmosphere, it is demonstrated that the inclusion of diurnal and semi-diurnal tides act to cool the core of the thermocline while increasing the temperature above and below it along the pathway of the internal waves, a result that likely is due to vertical mixing by the waves.

The relative influence of both temperature and salinity on the mixed layer depth (MLD) is evaluated using a relationship of binned regressions of MLD on vertical density compensation and isothermal layer depth (ILD) from a global set of in situ profile observations. Our approach is inspired by the observations of the difference between the MLD and the sonic layer depth (SLD) that evolve seasonally around the global ocean. In this article, we hypothesize that vertical density compensation governs SLD-MLD differences and can be used for mapping the relative influence of temperature and salinity on upper ocean structure. The Turner angle, computed between the surface and 200 m (bulk Turner angle, BTA), serves as a measure of vertical density compensation that quantifies times and areas where either temperature or salinity is destabilizing. For temperature destabilization the ocean exhibits cool/fresh overlying hot/salty water. For salinity destabilization the ocean exhibits hot/salty overlying cool/fresh water. These two classes of density compensation have seasonal variability with different geographical characteristics. Profiles with salinity controlled stable density and destabilizing temperature gradient are found most often at high latitudes. Profiles with temperature controlled stable density and destabilizing salinity gradient are found in the tropics and subtropics of all oceans. Results indicate that about half of the ocean has vertical density compensation that is a necessary condition for SLD-MLD differences. While density compensation is necessary, it is not a sufficient condition for predicting the dependence of MLD on BTA. Density compensation is the dominant factor in MLD variability in heavy river input and subduction regions that cover only ~14% of the ocean.

A global high-resolution ocean circulation model forced by atmospheric fields and the M2 tidal constituent is used to explore plausible scenarios for the damping of low-mode internal tides. The plausibility of different damping scenarios is tested by comparing the modeled barotropic tides with TPXO8, a highly accurate satellite-altimetry-constrained tide model, and by comparing the modeled coherent baroclinic tide amplitudes against along-track altimetry. Five scenarios are tested: (1) a topographic internal wave drag, argued here to represent the breaking of unresolved high vertical modes, applied to the bottom flow (default configuration), (2) a wave drag applied to the barotropic flow, (3) absence of wave drag, (4) a substantial increase in quadratic bottom friction along the continental shelves (with wave drag turned off), and (5) application of wave drag to the barotropic flow at the same time that quadratic bottom friction is substantially increased along the shelves. Of the scenarios tested here, the default configuration (1) yields the most accurate tides. In all other scenarios (2–5), the lack of damping on open ocean baroclinic motions yields baroclinic tides that are too energetic and travel too far from their sources, despite the presence of a vigorous mesoscale eddy field which can scatter and decohere internal tides in the model. The barotropic tides are also less accurate in the absence of an open ocean damping on barotropic motions, that is, in scenarios (3) and (4). The results presented here suggest that low-mode internal tides experience substantial damping in the open ocean.

High-resolution global ocean models forced by atmospheric fields and tides are beginning to display realistic internal gravity wave spectra, especially as model resolution increases. This paper examines internal waves in global simulations with 0.08 Degrees and 0.04 Degrees (approximately 8 and 4 km) horizontal resolutions, respectively. Frequency spectra of internal wave horizontal kinetic energy in the North Pacific lie closer to observations in the 0.04 Degrees simulation than in the 0.08 Degrees simulation. The horizontal wave number and frequency (K- omega) kinetic energy spectra contain peaks in the semidiurnal tidal band and near-inertial band, along with a broadband frequency continuum aligned along the linear dispersion relations of low-vertical-mode internal waves. Spectral kinetic energy transfers describe the rate at which nonlinear mechanisms remove or supply kinetic energy in specific K-omega ranges. Energy is transferred out of low-mode inertial and semidiurnal internal waves into a broad continuum of higher-frequency and higher-wave number internal waves.

In Monterey Bay, CA, during northwesterly, upwelling favorable winds, the development of a southward flowing cold jet along the entrance to the Bay is often observed. This dense cold jet separates warm waters of the anticyclonic circulation offshore from the water masses inside the Bay. Interactions between the cold jet and the offshore anticyclonic circulation generate ageostrophic secondary circulation (ASC) cells due to submesoscale processes as, for example, flow interaction with the development of surface frontogenesis and nonlinear Ekman pumping. Based on observations and modeling studies, we evaluate the impact of these submesoscale processes on the formation of chlorophyll a filaments during late spring-earlier summer, and late summer time frames. We show that during the late summer time frame, ASC leads to the development of filaments with high values of chlorophyll a concentration along the edge of the cold jet–in contrast to the earlier summer time, when the ASC mixes phytoplankton much deeper to the area below of the euphotic depth, and chlorophyll a filaments are 3–4 times weaker.

Strongly nonlinear surface eddies are identified and analyzed in a general circulation model. Agulhas rings and Gulf Stream cold-core eddies are examples of eddies that cannot be properly characterized using linear geostrophic dynamics. These eddies are compact, highly circular, persistent in time, and travel long distances while maintaining their characteristics. The nonlinear eddies can be identified by a large Rossby number and high circularity. The majority of the anomalous eddies are anticyclones. Calculation of the balance of forces on these eddies demonstrates that the centrifugal force associated with strong curvature is significant, and the force balance shifts from geostrophy toward a gradient wind balance. Using geostrophy instead of the gradient wind balance produces large errors in estimates of rotational velocity of these eddies. The gradient wind velocity can be calculated from geostrophic velocity and eddy radius. Comparison between the results demonstrates that even when only sea surface height and associated geostrophic velocities are available, strongly nonlinear eddies can be identified and properly characterized. This analysis is then applied to altimetric maps of sea surface height. Nonlinear eddies are present in the altimetric maps, but are less common and not as strongly nonlinear. This analysis demonstrates that by properly accounting for the dynamics of the eddy field, a more complete statistical description including nonlinear terms can be obtained from readily available observations.

A global tuning experiment for the semidiurnal tide is performed with a barotropic model. The model is forced with the M2 equilibrium tide and accounts for the self-attraction and loading (SAL) term. In addition to a quadratic drag, various linear internal wave drag terms adjusted by a scale factor of Oð1Þ are applied. The drag terms include the original Nycander (2005) tensor scheme, the Nycander tensor scheme reduced at supercritical slopes, and their scalar sisters, a Nycander scalar scheme computed for additional abyssal hill roughness, and the Jayne and St. Laurent (2001) scalar scheme. The Nycander scheme does not have a tunable parameter, but to obtain the best tidal solutions, it is demonstrated that some tuning is unavoidable. It is shown that the scalar Nycander schemes yield slightly lower root-mean square (RMS) elevation errors vs. the data-assimilative TPXO tide model than the tensor schemes. Although the simulation with the optimally tuned original Nycander scalar yields dissipation rates close to TPXO, the RMS error is among the highest. The RMS error is lowered for the reduced schemes, which place relatively more dissipation in deeper water. The inclusion of abyssal hill roughness improves the regional agreement with TPXO dissipation rates, without changing the RMS errors. It is difficult to have each ocean basin optimally tuned with the application of a constant scale factor. The relatively high RMS error in the Atlantic Ocean is reduced with a spatially varying scale factor with a larger value in the Atlantic. Our best global mean RMS error of 4.4 cm for areas deeper than 1000 m and equatorward of 66 is among the lowest obtained in a forward barotropic tide model.

Ocean mesoscale eddies are non-deterministic in that small errors grow in time so that accurate prediction is not possible without continual correction from observations. Ocean frontogenesis can be forced by mesoscale eddies through straining of buoyancy gradients, which produces filaments of surface divergence related to ageostrophic upwelling. The upwelling can result in thinning of the mixed layer. The frontogenesis predictability is tested through a series of Observation System Experiments (OSEs), the results of which indicate that if the strength and location of the mesoscale eddies are accurately predicted, then the associated frontogenesis features can be predicted. The frontogenesis features have a ‘conditional deterministic predictability’. The OSEs are started with perturbed initial conditions, and the OSEs assimilate an increasing number of satellite altimeter data streams. One experiment uses all available data to provide the most accurate analysis, which is labeled as the nature run. Relative to the nature run, ocean steric height correlations increases from about 0.87 with one altimeter and asymptotically reaches 0.99 with four altimeters, showing increasing skill in mesoscale prediction. Satellite data provide no information to dynamically correct frontogenesis processes in the numerical models. Even though not corrected by data, as the number of satellite altimeters increases from 1 to 4, the spatial correlation to the nature run of the frontogenesis forcing increases linearly from 0.27 to 0.59, the surface divergence correlation increases linearly from 0.27 to 0.57 and mixed layer depth correlation increases linearly from 0.67 to 0.89. The conclusion is that within the simulations the frontogenesis filaments are deterministically predictable conditioned on accurate prediction of the mesoscale.

The stationarity of the internal tides generated in a global eddy-resolving ocean circulation model forced by realistic atmospheric fluxes and the luni-solar gravitational potential is explored. The root mean square (RMS) variability in the M2 internal tidal amplitude is approximately 2 mm or less over most of the ocean and exceeds 2 mm in regions with larger internal tidal amplitude. The M2 RMS variability approaches the mean amplitude in weaker tidal areas such as the tropical Pacific and eastern Indian Ocean, but is smaller than the mean amplitude near generation regions. Approximately 60% of the variance in the complex M2 tidal amplitude is due to amplitude-weighted phase variations. Using the RMS tidal amplitude variations normalized by the mean tidal amplitude (normalized RMS variability (NRMS)) as a metric for stationarity, low-mode M2 internal tides with NRMS

[1] We apply several skill tests to assess tidal currents within a three-dimensional, eddy resolving, global ocean circulation model compared to over 5000 observational velocity records spanning 40 years. We examine the skill of the HYbrid Coordinate Ocean Model (HYCOM) on a regional, basin, and global scale and in deep versus shallow water. On a global scale, we examine the model tidal kinetic energy (KE) compared to the tidal KE estimated from the observational velocity records. We examine the vertical structure of the model tidal KE by averaging over predetermined depth bins. We also investigate the ability of the model to satisfy the 95% confidence intervals of the individual tidal ellipse parameters. On a basin scale, we determine if any bias exists in model performance with regards to a particular part of the global ocean and further investigate if any variability of model skill exists within the ocean basins by testing the model against smaller subsets of the observations. Our results show that the skill of the nondata assimilative HYCOM is comparable to the skill of the altimetric-constrained model TPXO7.2. HYCOM is shown to have up to 20% higher skill in resolving the Greenwich phase of the tides on a global basis and demonstrates moderate skill in replicating the vertical structure of the tidal currents as represented by the current meters. HYCOM demonstrates up to 20% higher skill than TPXO7.2 for some ocean basins and some ocean regions but exhibits up to 20% weaker skill in the Southern Ocean.

Ocean tides, and the atmospherically forced oceanic general circulation and its associated mesoscale eddy field, have long been run separately in high-resolution global models. They are now being simulated concurrently in a high-resolution version of the HYbrid Coordinate Ocean Model (HYCOM). The incorporation of horizontally varying stratification with the addition of atmospheric forcing yields internal tides (internal waves of tidal frequency) in high-latitude, low-stratification regions that are qualitatively different from those in earlier global internal tide models, in which atmospheric forcing and horizontally variable stratification were absent. The internal tides in the new concurrent HYCOM simulations compare well with those measured in along-track satellite altimeter data. The new concurrent simulations demonstrate that the wavenumber spectrum of sea surface height --a measure of the energy contained in different length scales is dominated in some locations by internal tides and in others by mesoscale eddies. Tidal kinetic energies in the new concurrent simulations compare well with those in current-meter observations, as long as sufficient spatial averaging is performed. The new concurrent simulations are being used in the planning of future-generation satellite altimeters, in the provision of boundary conditions for coastal ocean models, and in studies of ocean mixing.

Jacques Verron et al., © 2015, Copyright © Taylor & Francis Group, LLC. The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedi-cated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. Secondary objectives include ice sheet and inland waters monitoring. One year after launch, the results widely confirm the nominal expectations in terms of accuracy, data quality and data availability in general. Today's performances are compliant with specifications with an overall observed performance for the Sea Surface Height RMS of 3.4 cm to be compared to a 4 cm requirement. Some scientific examples are provided that illustrate some salient features of today's SARAL/AltiKa data with regard to standard altimetry: data availability, data accuracy at the mesoscales, data usefulness in costal area, over ice sheet, and for inland waters.

Paleoceanographic evidence points to the Southern Ocean as a strong sink for atmospheric CO2 during the Last Glacial Maximum (LGM), but no consensus about the responsible mechanism has yet been reached. Martin (Paleoceanography 5 (1990) 1) postulated that greater iron input during the LGM could have stimulated phytoplankton to consume the surface nutrients in the Southern Ocean, increasing carbon export substantially. We use a simple ecological model to elucidate the extent to which iron availability affects export production in the southwest Pacific sector. The model includes the effect of iron in a semi-explicit way. Based on the physiological response of the photosynthetic apparatus, the simulated phytoplankton growth rates are explicitly dependent on iron. The cycling of iron in the food web is not tracked, since uncertainties persist about the dynamics of iron uptake, transformation and release processes within the pelagic community. A simulation of the present ocean that uses the semi-explicit approach to include iron is compared with a simulation that accounts for iron only implicitly by using model parameters that are typical for low iron conditions. The simulations agree within the range of available observations. Glacial scenarios are simulated (assuming an increase in iron supply) and compared to the modern ocean simulation. Primary and export production increase in the glacial simulations, in particular if we assume an adaptation of the Si:N cell quota of diatoms to the higher iron levels. In this case the export doubles north of the Polar Front and in the Seasonal Ice Zone.

The Southern Ocean is the largest high-nutrient, low-chlorophyll region in the world's ocean and a potentially important site for the sequestration of carbon. We present a one-dimensional physical/biogeochemical model that integrates biogeochemical measurements obtained during the AESOPS (U.S. JGOFS) study in the southwest Pacific sector to elucidate the controls of primary productivity and export. The model is applied to a series of four stations along 170°W spanning the different biogeochemical subsystems in the Polar Frontal Zone, the Polar Front, and the Seasonal Ice Zone south of the Polar Front. Since horizontal fluxes of heat, freshwater and nutrients are found to be important but cannot be resolved explicitly in a one-dimensional model, we employ a restoration of temperature, salinity and nutrients. The surface fluxes of light and momentum are modified during ice-covered periods to account for the effects of sea ice. The biological model component includes the elemental cycles of nitrogen and silica. Diatoms are represented as a separate phytoplankton group, and small phytoplankton and zooplankton are tightly coupled. The effect of the low iron availability in the region is implicitly taken into account by using typical phytoplankton growth rates and a typical, high Si:N stoichiometry of 4 for the diatoms. The model captures the essential features of the distinct subsystems including the low-chlorophyll condition north of the Polar Front, the diatom blooms in the vicinity of the Polar Front and to its south, the differential drawdown of nitrate and silicic acid, and the seasonal patterns of biogenic silica, primary production and vertical particle flux. “Top-down” control of the small phytoplankton by efficient microzooplankton grazing and “bottom-up” control of diatoms by light and silicic acid are the main factors for the simulated behavior. A sensitivity analysis of the biological model component shows that the growth parameters for the two phytoplankton groups are most important in constraining primary productivity and overall model behavior. This implies that changes in growth rates induced by variations in iron supply as assumed over glacial–interglacial transitions can affect primary and export production substantially.

Profiles of absolute velocity are difficult to obtain in the ocean, especially over long periods of time at the same location. This paper presents a method of estimating full water column absolute horizontal velocity profiles as a function of time by combining historical hydrography with the measurements from two separate instruments, the inverted echo sounder (IES) and the horizontal electric field recorder (HEFR). Hydrography is used to construct temperature, salinity, and specific volume anomaly characteristics as functions of the independent variables pressure and seafloor-to-sea-surface round-trip acoustic travel time (τ). Each IES measured τ is combined with these two-dimensional characteristics to estimate the profile of specific volume anomaly, which then is integrated vertically to obtain profiles of geopotential height anomaly (Δϕ). Profiles of Δϕ from adjacent IES sites are differenced to yield vertical profiles of relative geostrophic velocity. Horizontal electric fields arising from ...

Develop and implement a fully coupled global atmosphere/wave/ocean/land/ice prediction system capable of providing daily predictions out to 10 days and weekly predictions out to 30-90 days. Initial Operational Capability is targeted as 2018. Predictions will provide environmental information to meet Navy and DoD operations and planning needs throughout the globe from undersea to the upper atmosphere and from the tropics to the poles. The system will be implemented on Navy operational computer systems, and the necessary processing infrastructure will be put in place to provide products for Navy fleet user consumption. Develop and implement a coupling infrastructure and interoperability layer to enable the component models to exchange information needed to make global earth system predictions. For each of the component models, develop and test coupled physical parameterizations and emphasize testing of new feedback loops that arise in the coupled system. Implement the tripolar grid for WaveWatch-III and wave forcing in the ocean. Incorporate time-dependent, radiatively active, and cloud-nucleating aerosols into NAVGEM for use in long-term simulations and forecasts and for use in the full coupled system.

Motivated by the potential of oceanic mesoscale eddies to drive intrinsic low-frequency variability, this paper examines geostrophic turbulence in the frequency–wavenumber domain. Frequency–wavenumber spectra, spectral fluxes, and spectral transfers are computed from an idealized two-layer quasigeostrophic (QG) turbulence model, a realistic high-resolution global ocean general circulation model, and gridded satellite altimeter products. In the idealized QG model, energy in low wavenumbers, arising from nonlinear interactions via the well-known inverse cascade, is associated with energy in low frequencies and vice versa, although not in a simple way. The range of frequencies that are highly energized and engaged in nonlinear transfer is much greater than the range of highly energized and engaged wavenumbers. Low-frequency, low-wavenumber energy is maintained primarily by nonlinearities in the QG model, with forcing and friction playing important but secondary roles. In the high-resolution ocean model, nonlinearities also generally drive kinetic energy to low frequencies as well as to low wavenumbers. Implications for the maintenance of low-frequency oceanic variability are discussed. The cascade of surface kinetic energy to low frequencies that predominates in idealized and realistic models is seen in some regions of the gridded altimeter product, but not in others. Exercises conducted with the general circulation model suggest that the spatial and temporal filtering inherent in the construction of gridded satellite altimeter maps may contribute to the discrepancies between the direction of the frequency cascade in models versus gridded altimeter maps seen in some regions. Of course, another potential reason for the discrepancy is missing physics in the models utilized here.

The accuracy of state-of-the-art global barotropic tide models is assessed using bottom pressure data, coastal tide gauges, satellite altimetry, various geodetic data on Antarctic ice shelves, and independent tracked satellite orbit perturbations. Tide models under review include empirical, purely hydrodynamic ("forward"), and assimilative dynamical, i.e., constrained by observations. Ten dominant tidal constituents in the diurnal, semidiurnal, and quarter-diurnal bands are considered. Since the last major model comparison project in 1997, models have improved markedly, especially in shallow-water regions and also in the deep ocean. The root-sum-square differences between tide observations and the best models for eight major constituents are approximately 0.9, 5.0, and 6.5 cm for pelagic, shelf, and coastal conditions, respectively. Large intermodel discrepancies occur in high latitudes, but testing in those regions is impeded by the paucity of high-quality in situ tide records. Long-wavelength components of models tested by analyzing satellite laser ranging measurements suggest that several models are comparably accurate for use in precise orbit determination, but analyses of GRACE intersatellite ranging data show that all models are still imperfect on basin and subbasin scales, especially near Antarctica. For the M-2 constituent, errors in purely hydrodynamic models are now almost comparable to the 1980-era Schwiderski empirical solution, indicating marked advancement in dynamical modeling. Assessing model accuracy using tidal currents remains problematic owing to uncertainties in in situ current meter estimates and the inability to isolate the barotropic mode. Velocity tests against both acoustic tomography and current meters do confirm that assimilative models perform better than purely hydrodynamic models.

The Antarctic Polar Front is a complex set of meandering jets, which appear to support enhanced primary productivity. The US Joint Global Ocean Flux Study conducted a series of survey and process studies in part to study the processes regulating primary productivity in this high nutrient, low chlorophyll region. We deployed a set of surface velocity drifters, some of which were equipped with bio-optical sensors, to study the temporal and spatial scales of biological and physical processes in the Antarctic Polar Frontal Zone. There were two primary sets of deployments: November 1997 before the spring bloom and January 1998 after the spring bloom. The November deployment revealed a strong spring bloom that lasted about 10 days. In late spring, when incoming solar radiation began to increase, the vertical motions associated with the meanders strongly affected the accumulation of phytoplankton biomass, primarily through their impact on light availability. Weaker meandering was observed in the January deployment, and chlorophyll values remained relatively constant. As the bloom began to decay, it appears that nutrient availability became more important in regulating phytoplankton photosynthesis. Some of the drifters in the November deployment were deployed in coherent clusters, thus allowing us to calculate vertical velocities associated with the meanders. Estimates of fluorescence/chlorophyll suggest that areas of upwelling and downwelling alternately decrease and increase photosynthetic stress, perhaps as a result of changes in the availability of iron or light during the formation of the bloom.

The US Joint Global Ocean Flux Study (JGOFS) conducted a series of survey and process studies in part to understand the processes regulating primary productivity and carbon flux in the APFZ, which is a high-nutrient, low-chlorophyll (HNLC) region. We deployed a high-resolution array of 12 moorings (average horizontal spacing 30 km) equipped with bio-optical and physical sensors to study the temporal and spatial scales of biological and physical processes in the APFZ. The moorings collected data from November 1997 to March 1998, effectively observing the growing season. Estimates of chlorophyll and sun-stimulated fluorescence/chlorophyll (F/C) were derived from the bio-optical measurements. Each mooring showed a strong spring bloom beginning in early December as the upper ocean began to stratify, with chlorophyll levels nearly quadrupling. The time series, along with ship studies, suggest that phytoplankton were initially light-limited as a result of deep, late spring mixing, followed by intense zooplankton grazing or silicate limitation, which controlled the maximum chlorophyll concentration, and finally by iron limitation, which led to increasing photoadaptive stress. These results suggest that phytoplankton in the APFZ are regulated by a confluence of processes involving light, grazing, silicate, and iron, and that models comprising a single mechanism may not be sufficient. The spring bloom in the APFZ is a transient event, persisting for only a few weeks, and therefore it is difficult to draw conclusions from sporadic ship cruises. Moreover, its spatial scales are also small so that widely spaced hydrographic stations can easily overlook critical processes.

The location of the Antarctic Polar Front (PF) was mapped over a 7-year period (1987–1993) within images of satellite-derived sea surface temperature. The mean path of the PF is strongly steered by the topographic features of the Southern Ocean. The topography places vorticity constraints on the dynamics of the PF that strongly affect spatial and temporal variability. Over the deep ocean basins the surface expression of the PF is weakened, and the PF meanders over a wide latitudinal range. Near large topographic features, width and temperature change across the front increase, and large-scale meandering is inhibited. Elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the PF encounters large topographic features. The strong correlations between topography and PF dynamics can be understood in the context of the planetary potential vorticity (PPV or f/H) field. Mean PPV at the PF varies by more than a factor of 2 along its circumpolar path. However, at the mesoscale the PF remains within a relatively narrow range of PPV values around the local mean. Away from large topographic features, the PF returns to a preferred PPV value of ∼25 × 10−9 m−1 s−1 despite large latitudinal shifts. The mean paths of the surface and subsurface expressions of the PF are closely coupled over much of the Southern Ocean.

The path of the Antarctic Polar Front (PF) is mapped using satellite sea surface temperature data from the NOAA/NASA Pathfinder program. The mean path and variability of the PF are strongly influenced by bathymetry. Meandering intensity is weaker where the bathymetry is steeply sloped and increases in areas where the bottom is relatively flat. There is an inverse relationship between meandering intensity and both the width of the front and the change in temperature across it There is a persistent, large separation between the surface and subsurface expressions of the PF at Ewing Bank on the Falkland Plateau.

The long-term goal is to develop and implement a fully coupled global atmosphere/wave/ocean/land/ice prediction system capable of providing daily predictions out to 10 days and weekly predictions out to 30 days. Initial Operational Capability is targeted as 2018. Predictions will provide environmental information to meet Navy and DoD operations and planning needs throughout the globe from undersea to the upper atmosphere and from the tropics to the poles. The system will be implemented on Navy operational computer systems, and the necessary processing infrastructure will be put in place to provide products for Navy fleet user consumption.

The long-term goal is to determine the impact of observations on the performance of ocean circulation forecast models and provide guidance on the design and implementation of ocean observing systems. The primary objective for the Trident Warrior 2013 Exercise is determining the impact of on scene data on the performance of forecast systems for prediction EM propagation characteristics. A number of supplemental projects were added for the exercise with a focus on air-sea interaction and shelf processes. However, the shelf circulation is impacted by Gulf Stream and slope circulation in this region. Additional airborne temperature profiles were obtained as part of this project to test the performance of the forecast models in the region near the separation of the Gulf Stream from the coast at Cape Hatteras. The observations are used to evaluate a new vertical projection scheme, Improved Synthetic Ocean Profiles, and test a new hypothesis on conditional predictability of small scale frontogenesis with improved mesoscale prediction.

Local, one-dimensional (1-D) simulations of the upper-ocean structure observed at the Hawaii Ocean Time Series (HOTS) mooring located 100 km north of Oahu are compared with output from the operational Global NCOM ocean forecast system at the location of the mooring. The observations from the mooring indicate that the upper-ocean density structure and mixed-layer depth (MLD) in this area are significantly affected by the extensive eddy field that exists there. Local, 1-D simulations do not account for the modulation of the upper-ocean density structure by the eddies. However, Global NCOM, along with the data assimilation that it uses, provides a reasonably accurate simulation of the eddies and does a fairly good job of accounting for the effect of the eddies on the upper-ocean density structure and MLD. Comparison of atmospheric fields from NOGAPS, which are used by Global NCOM, with observations from the mooring show fairly good agreement, which contributes to the skill shown by Global NCOM in simulating the sea-surface temperature and MLD observed at the mooring.

Eddies ; Nonlinear dynamics ; Ocean dynamics ; Satellite observations ; Ocean models; International audience; Motivated by the recent interest in ocean energetics, the widespread use of horizontal eddy viscosity in models, and the promise of high horizontal resolution data from the planned wide-swath satellite altimeter, this paper explores the impacts of horizontal eddy viscosity and horizontal grid resolution on geostrophic turbulence, with a particular focus on spectral kinetic energy fluxes Π(K) computed in the isotropic wavenumber (K) domain. The paper utilizes idealized two-layer quasigeostrophic (QG) models, realistic high-resolution ocean general circulation models, and present-generation gridded satellite altimeter data. Adding horizontal eddy viscosity to the QG model results in a forward cascade at smaller scales, in apparent agreement with results from present-generation altimetry. Eddy viscosity is taken to roughly represent coupling of mesoscale eddies to internal waves or to submesoscale eddies. Filtering the output of either the QG or realistic models before computing Π(K) also greatly increases the forward cascade. Such filtering mimics the smoothing inherent in the construction of present-generation gridded altimeter data. It is therefore difficult to say whether the forward cascades seen in present-generation altimeter data are due to real physics (represented here by eddy viscosity) or to insufficient horizontal resolution. The inverse cascade at larger scales remains in the models even after filtering, suggesting that its existence in the models and in altimeter data is robust. However, the magnitude of the inverse cascade is affected by filtering, suggesting that the wide-swath altimeter will allow a more accurate determination of the inverse cascade at larger scales as well as providing important constraints on smaller-scale dynamics.

[1] The slopes of the wavenumber spectra of sea surface height (SSH) and kinetic energy (KE) have been used to infer “interior” or surface quasi-geostrophic (QG or SQG) dynamics of the ocean. However, inspection of spectral slopes for altimeter SSH in the mesoscale band of 70 to 250 km shows much flatter slopes than the QG or SQG predictions over most of the ocean. Comparison of altimeter wavenumber spectra with spectra from an eddy resolving global ocean circulation model (the HYbrid Coordinate Ocean Model, HYCOM, at 1/12.5° equatorial resolution), which has embedded tides, suggests that the flatter slopes of the altimeter SSH may arise from three possible sources: (1) presence of strong internal tides, (2) shift of the inertial sub-range to smaller scales and (3) altimeter noise. Artificially adding noise to the model tends to flatten the spectra for low KE regions. Near internal tide generating regions, spectral slopes in the presence of internal waves are much flatter than QG or SQG predictions. Separating the variability into high and low frequency (around periods of 2 days), then a different pattern emerges with a flat high-frequency wavenumber spectrum and a steeper low-frequency wavenumber spectrum. For low mesoscale KE, the inertial sub-range, defined by the nearly flat enstrophy band, moves to smaller scales and the mesoscale band of 70 to 250 km no longer represents the inertial sub-range. The model wavenumber spectra are consistent with QG and SQG theory when internal waves and inertial sub-range shifts are taken into consideration.

The parameterization of direct wave mixing proposed by Qiao et al. (2004) was tested with data from the Ocean Weathership Station (OWS) Papa in the northeast Pacific and with data from three NOAA buoys, two in the northeast Pacific and one in the northwest Atlantic. Previous testing has indicated that the commonly used Mellor-Yamada-type mixed-layer models tend to underpredict the mixed-layer depth (MLD) in the open ocean. Adding the vertical mixing proposed by Qiao et al. (2004) to the vertical mixing predicted by the Mellor-Yamada Level 2 turbulence model increased the predicted MLD and improved the agreement between the predicted and observed sea-surface temperature (SST) at Papa. However, the results of the tests showed two significant problems with the parameterization of the wave mixing. At OWS Papa, the wave mixing caused too much diffusion of heat through the seasonal thermocline below the mixed layer, and too much diffusion of the thermocline itself. At the NOAA buoys and, less conclusively, at OWS Papa, the wave mixing inhibited the formation of shallow mixed layers and their associated SST spikes during periods of light winds due to the presence of swell.