Near-inertial waves modulated by background flow in realistic global ocean simulations

2021 ◽  
Author(s):  
Keshav Raja ◽  
Maarten Buijsman ◽  
Oladeji Siyanbola ◽  
Miguel Solano ◽  
Jay Shriver ◽  
...  
Keyword(s):  
Kinetic Energy ◽  
Normal Modes ◽  
Deep Ocean ◽  
Ocean Model ◽  
Global Ocean ◽  
Inertial Waves ◽  
Flux Divergence ◽  
Large Wave ◽  
Wind Input ◽  
Anticyclonic Eddies

<p>Wind generated near-inertial waves (NIWs) are a major source of energy for deep-ocean mixing by transmitting wind energy from the ocean surface into the interior. Recently, it has been established that the NIW energy transmission to ocean depths is significantly modulated by background mesoscale vorticity. Thus, understanding NIW energetics in the presence of mesoscale eddies on a global scale is crucial.</p><p>We study the generation, propagation and dissipation of NIWs in global 1/25<sup>o</sup> Hybrid Coordinate Ocean Model (HYCOM) simulations with realistic tidal forcing. The model has 41 layers with uniform vertical coordinates in the mixed layer and isopycnal coordinates in the ocean interior. The model is forced by 1/3hr wind from the NAVGEM atmospheric model. We analyze one month of model data for May-June 2019. The 3D HYCOM fields are projected on vertical normal modes to compute the wind input, wave kinetic energy (KE), flux divergence and dissipation per mode.</p><p>We find that the globally integrated wind input in surface near-inertial motions is 0.21 TW for the 30-day period and is consistent with previous studies. The sum of the wind input to the first 5 modes accounts to only 31% of the total wind input while the sum of the NIW kinetic energy in the first 5 modes adds up to 60% of the total NIW KE. The difference in the fraction of the total between the wind input and NIW KE (31% and 60%) suggests that a significant portion of wind-induced near-inertial motions is dissipated close to the surface without being projected onto modes. We also find that NIW horizontal fluxes diverge from areas with cyclonic vorticity and converge in areas with anticyclonic vorticity, i.e., anticyclonic eddies are a sink for NIW energy in the global ocean.</p><p>The residual NIW KE that does not project onto modes is found to be largely trapped in anticyclonic eddies. In a next step, we will study the fate of this energy, which most likely propagate downward as beam-like features with large wave numbers. We will compute the near-inertial wave energy balance for fixed subsurface layers and consider the energy exchange between these layers to understand the vertical structure of NIW energy dissipation. We find that the downward NIW radiation to the ocean interior at 500 m depth is 19% of the surface near-inertial wind input for the 30-day period.</p>

scholarly journals Recent development of the Met Office operational ocean forecasting system: an overview and assessment of the new Global FOAM forecasts

2014 ◽  
Vol 7 (6) ◽  
pp. 2613-2638 ◽  
Author(s):  
E. W. Blockley ◽  
M. J. Martin ◽  
A. J. McLaren ◽  
A. G. Ryan ◽  
J. Waters ◽  
...  
Keyword(s):  
Sea Ice ◽  
Forecast Accuracy ◽  
Deep Ocean ◽  
Ocean Model ◽  
Global Ocean ◽  
Surface Boundary ◽  
Observation Window ◽  
Sea Ice Concentration ◽  
Near Surface ◽  
Shelf Sea

Abstract. The Forecast Ocean Assimilation Model (FOAM) is an operational ocean analysis and forecast system run daily at the Met Office. FOAM provides modelling capability in both deep ocean and coastal shelf sea regimes using the NEMO (Nucleus for European Modelling of the Ocean) ocean model as its dynamical core. The FOAM Deep Ocean suite produces analyses and 7-day forecasts of ocean tracers, currents and sea ice for the global ocean at 1/4° resolution. Satellite and in situ observations of temperature, salinity, sea level anomaly and sea ice concentration are assimilated by FOAM each day over a 48 h observation window. The FOAM Deep Ocean configurations have recently undergone a major upgrade which has involved the implementation of a new variational, first guess at appropriate time (FGAT) 3D-Var, assimilation scheme (NEMOVAR); coupling to a different, multi-thickness-category, sea ice model (CICE); the use of coordinated ocean-ice reference experiment (CORE) bulk formulae to specify the surface boundary condition; and an increased vertical resolution for the global model. In this paper the new FOAM Deep Ocean system is introduced and details of the recent changes are provided. Results are presented from 2-year reanalysis integrations of the Global FOAM configuration including an assessment of short-range ocean forecast accuracy. Comparisons are made with both the previous FOAM system and a non-assimilative FOAM system. Assessments reveal considerable improvements in the new system to the near-surface ocean and sea ice fields. However there is some degradation to sub-surface tracer fields and in equatorial regions which highlights specific areas upon which to focus future improvements.

scholarly journals Deterministic Model of the Eddy Dynamics for a Midlatitude Ocean Model

2021 ◽  
Author(s):  
Takaya Uchida ◽  
Bruno Deremble ◽  
Stephane Popinet
Keyword(s):  
Deterministic Model ◽  
North Atlantic Ocean ◽  
Ocean Model ◽  
Global Ocean ◽  
Mean Flow ◽  
Flux Divergence ◽  
Time Step ◽  
Weather System ◽  
Basin Scale ◽  
The Mean

Mesoscale eddies, the weather system of the oceans, although being on the scales of O(20-100km), have a disproportionate role in shaping the mean jets such as the separated Gulf Stream in the North Atlantic Ocean, which is on the scale of O(1000km) in the along-jet direction. With the increase in computational power, we are now able to partially resolve the eddies in basin-scale and global ocean simulations, a model resolution often referred to as mesoscale permitting. It is well known, however, that due to grid-scale numerical viscosity, mesoscale permitting simulations have less energetic eddies and consequently weaker eddy feedback onto the mean flow. In this study, we run a quasi-geostrophic model at mesoscale resolving resolution in a double gyre configuration and formulate a deterministic parametrization for the eddy rectification term of potential vorticity (PV), namely, the eddy PV flux divergence. We have moderate success in reproducing the spatial patterns and magnitude of eddy kinetic and potential energy diagnosed from the model. One novel point about our approach is that we account for non-local eddy feedbacks onto the mean flow by solving the eddy PV equation prognostically in addition to the mean flow. In return, we are able to parametrize the variability in total (mean+eddy) PV at each time step instead of solely the mean PV. A closure for the total PV is beneficial as we are able to account for both the mean state and extreme events.

scholarly journals Recent development of the Met Office operational ocean forecasting system: an overview and assessment of the new Global FOAM forecasts

2013 ◽  
Vol 6 (4) ◽  
pp. 6219-6278 ◽  
Author(s):  
E. W. Blockley ◽  
M. J. Martin ◽  
A. J. McLaren ◽  
A. G. Ryan ◽  
J. Waters ◽  
...  
Keyword(s):  
Sea Ice ◽  
Forecast Accuracy ◽  
Deep Ocean ◽  
Ocean Model ◽  
Global Ocean ◽  
Surface Boundary ◽  
Observation Window ◽  
Sea Ice Concentration ◽  
Near Surface ◽  
The North

Abstract. The Forecast Ocean Assimilation Model (FOAM) is an operational ocean analysis and forecast system run daily at the Met Office. FOAM provides modelling capability in both deep ocean and coastal shelf seas regimes using the NEMO ocean model as its dynamical core. The FOAM Deep Ocean suite produces analyses and 7 day forecasts of ocean tracers, currents and sea ice for the global ocean at 1/4° resolution and at 1/12° resolution in the North Atlantic, Indian Ocean and Mediterranean Sea. Satellite and in-situ observations of temperature, salinity, sea level anomaly and sea ice concentration are assimilated by FOAM each day over a 48 h observation window. The FOAM Deep Ocean configurations have recently undergone a major upgrade which has involved: the implementation of a new variational, first guess at appropriate time 3D-Var, assimilation scheme (NEMOVAR); coupling to a different, multi-thickness-category, sea ice model (CICE); the use of CORE bulk formulae to specify the surface boundary condition; and an increased vertical resolution for the global model. In this paper the new FOAM Deep Ocean system is introduced and details of the recent changes are provided. Results are presented from 2 yr reanalysis integrations of the Global FOAM configuration including an assessment of forecast accuracy. Comparisons are made with both the previous FOAM system and a non-assimilative FOAM system. Assessments reveal considerable improvements in the new system to the near-surface ocean and sea ice fields. However there is some degradation to sub-surface tracer fields and in equatorial regions which highlight specific areas upon which to focus future improvements.

A synthesis of upper ocean geostrophic kinetic energy spectra from a global submesoscale permitting simulation

2021 ◽  
Author(s):  
Hemant Khatri ◽  
Stephen Griffies ◽  
Takaya Uchida ◽  
Han Wang ◽  
Dimitris Menemenlis
Keyword(s):  
Kinetic Energy ◽  
Mixed Layer ◽  
Seasonal Variability ◽  
Ocean Model ◽  
Global Ocean ◽  
Upper Ocean ◽  
Late Winter ◽  
Second Phase ◽  
Ocean Turbulence ◽  
Two Phases

<p>In the upper ocean, submesoscale turbulence shows seasonal variability and is pronounced in winter. We analyze geostrophic KE spectra in a submesoscale-permitting global ocean model to study the seasonal variability in the upper ocean turbulence. Submesoscale processes peak in winter and, consequently, geostrophic kinetic energy (KE) spectra tend to be relatively shallow in winter (<em>k</em><sup>-2</sup>) with steeper spectra in summer (<em>k</em><sup>-3</sup>). The roles of frontogenesis processes and mixed-layer instabilities in submesoscale turbulence and their effects on the evolution of KE spectra over an annual cycle are discussed. It is shown that this transition in KE spectral scaling has two phases. In the first phase (late autumn), KE spectra show a presence of two spectral regimes: <em>k</em><sup>-3</sup> scaling in mesoscales (100-300 km) and <em>k</em><sup>-2</sup> scaling in submesoscales (< 50 km), indicating the coexistence of QG, surface-QG, and frontal dynamics. In the second phase (late winter), mixed-layer instabilities convert available potential energy into KE, which cascades upscale leading to flattening of the KE spectra at larger scales, and <em>k</em><sup>-2</sup> power-law develops in mesoscales too.</p>

The dissipation of the internal tide inferred from a global ocean model, altimetry, and in-situ observations

2020 ◽  
Author(s):  
Maarten Buijsman ◽  
Harpreet Kaur ◽  
Zhongxiang Zhao ◽  
Amy Waterhouse ◽  
Caitlin Whalen
Keyword(s):  
Correction Factor ◽  
Internal Tide ◽  
Horizontal Resolution ◽  
Ocean Model ◽  
Internal Tides ◽  
Global Ocean ◽  
Flux Divergence ◽  
Data Set ◽  
Tidal Dissipation ◽  
Dissipation Rates

<p>In this presentation we combine several model and observational data sets to better understand the dissipation of the diurnal and semidiurnal internal tide in the global ocean, which is relevant for maintaining the global overturning circulation. We compute depth-integrated internal tide dissipation rates from a realistically-forced global HYbrid Coordinate Ocean Model (HYCOM) simulation with a horizontal resolution of 4 km (1/25 degrees) and 41 layers. We also compute dissipation rates from altimetry in two ways: 1) from the low-mode flux divergence away from topography and 2) by fitting exponential decay curves along low-mode internal tide beams. The internal-tide sea-surface height amplitude is computed with a least-squares harmonic analysis over a 20+ year altimetry data set. Hence, the altimetry-inferred dissipation rates both reflect the tidal dissipation and the energy scattered from the stationary to the nonstationary internal tide. To account for the dissipation of the nonstationary tide, we apply a spatially-varying correction factor to the stationary dissipation inferred from altimetry.  This correction factor is computed from a global 8-km HYCOM simulation with a duration of 6 years, from which the stationary and nonstationary internal tides can be easily isolated. We compare the simulated and the corrected altimetry-inferred dissipation rates with dissipation rates from finescale and microstructure observations. Preliminary results show that the simulated dissipation is up to a factor of two larger than the depth-integrated dissipation rates inferred from finescale methods, but smaller than the dissipation rates from microstructure.</p>

scholarly journals On the assimilation of absolute geodetic dynamic topography in a global ocean model: impact on the deep ocean state

2018 ◽  
Vol 93 (2) ◽  
pp. 141-157 ◽  
Author(s):  
Alexey Androsov ◽  
Lars Nerger ◽  
Reiner Schnur ◽  
Jens Schröter ◽  
Alberta Albertella ◽  
...  
Keyword(s):  
Deep Ocean ◽  
Ocean Model ◽  
Global Ocean ◽  
Dynamic Topography

scholarly journals Global Calculation of Tidal Energy Conversion into Vertical Normal Modes

2014 ◽  
Vol 44 (12) ◽  
pp. 3225-3244 ◽  
Author(s):  
Saeed Falahat ◽  
Jonas Nycander ◽  
Fabien Roquet ◽  
Moundheur Zarroug
Keyword(s):  
Normal Modes ◽  
Internal Tide ◽  
Deep Ocean ◽  
Direct Calculation ◽  
Tidal Energy ◽  
Horizontal Distribution ◽  
Internal Tides ◽  
Global Ocean ◽  
Vertical Scale ◽  
Vertical Distributions

Abstract A direct calculation of the tidal generation of internal waves over the global ocean is presented. The calculation is based on a semianalytical model, assuming that the internal tide characteristic slope exceeds the bathymetric slope (subcritical slope) and the bathymetric height is small relative to the vertical scale of the wave, as well as that the horizontal tidal excursion is smaller than the horizontal topographic scale. The calculation is performed for the M2 tidal constituent. In contrast to previous similar computations, the internal tide is projected onto vertical eigenmodes, which gives two advantages. First, the vertical density profile and the finite ocean depth are taken into account in a fully consistent way, in contrast to earlier work based on the WKB approximation. Nevertheless, the WKB-based total global conversion follows closely that obtained using the eigenmode decomposition in each of the latitudinal and vertical distributions. Second, the information about the distribution of the conversion energy over different vertical modes is valuable, since the lowest modes can propagate over long distances, while high modes are more likely to dissipate locally, near the generation site. It is found that the difference between the vertical distributions of the tidal conversion into the vertical modes is smaller for the case of very deep ocean than the shallow-ocean depth. The results of the present work pave the way for future work on the vertical and horizontal distribution of the mixing caused by internal tides.

scholarly journals Eddy-Modulated Internal Waves and Mixing on a Midocean Ridge

2012 ◽  
Vol 42 (7) ◽  
pp. 1242-1248 ◽  
Author(s):  
Xinfeng Liang ◽  
Andreas M. Thurnherr
Keyword(s):  
Kinetic Energy ◽  
Energy Content ◽  
World Ocean ◽  
Low Frequency ◽  
Mesoscale Eddies ◽  
Deep Ocean ◽  
Small Scale ◽  
Inertial Waves ◽  
Topographic Roughness ◽  
Geostrophic Flows

Abstract Mesoscale eddies are ubiquitous in the World Ocean and dominate the energy content on subinertial time scales. Recent theoretical and numerical studies suggest a connection between mesoscale eddies and diapycnal mixing in the deep ocean, especially near rough topography in regions of strong geostrophic flow. However, unambiguous observational evidence for such a connection has not yet been found, and it is still unclear what physical processes are responsible for transferring energy from mesoscale to small-scale processes. Here, the authors present observations demonstrating that finescale variability near the crest of the East Pacific Rise is strongly modulated by low-frequency geostrophic flows, including those due to mesoscale eddies. During times of strong subinertial flows, the authors observed elevated kinetic energy on vertical scales <50 m and in the near-inertial band, predominantly upward-propagating near-inertial waves, and increased incidence of layers with Richardson number . In contrast, during times of weak subinertial flows, kinetic energy in the finescale and near-inertial bands is lower, Ri values are higher, and near-inertial waves propagate predominantly downward through the water column. Diapycnal diffusivities estimated indirectly from a simple Ri-based parameterization are consistent with results from a tracer-release experiment and a microstructure survey bracketing the mooring measurements. These observations are consistent with energy transfer (a “cascade”) from subinertial flows, including mesoscale eddies, to near-inertial oscillations, turbulence, and mixing. This interpretation suggests that, in addition to topographic roughness and tidal forcing, parameterization of deep-ocean mixing should also take subinertial flows into account. The findings presented here are expected to be useful for validating and improving numerical-model parameterizations of turbulence and mixing in the ocean.

scholarly journals Water isotope variations in the global ocean model MPI-OM

2012 ◽  
Vol 5 (3) ◽  
pp. 809-818 ◽  
Author(s):  
X. Xu ◽  
M. Werner ◽  
M. Butzin ◽  
G. Lohmann
Keyword(s):  
Surface Waters ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Deep Ocean ◽  
Ocean Surface ◽  
Ocean Model ◽  
Global Ocean ◽  
Stable Water Isotopes ◽  
Tropical Regions

Abstract. The stable water isotopes H218O and HDO are incorporated as passive tracers into the oceanic general circulation model MPI-OM, and a control simulation under present-day climate conditions is analyzed in detail. Both δ18O and δD distributions at the ocean surface and deep ocean are generally consistent with available observations on the large scale. The modelled δD-δ 18O relations in surface waters slightly deviates from the slope of the global meteoric water line in most basins, and a much steeper slope is detected in Arctic Oceans. The simulated deuterium excess of ocean surface waters shows small variations between 80° S and 55° N, and a strong decrease north of 55° N. The model is also able to capture the quasi-linear relationship between δ18O and salinity S, as well as δD and S, as seen in observational data. Both in the model results and observations, the surface δ−S relations show a steeper slope in extra-tropical regions than in tropical regions, which indicates relatively more addition of isotopically depleted water at high latitudes.

scholarly journals Water isotope variations in the global ocean model MPI-OM

2012 ◽  
Vol 5 (1) ◽  
pp. 277-307
Author(s):  
X. Xu ◽  
M. Werner ◽  
M. Butzin ◽  
G. Lohmann
Keyword(s):  
Surface Waters ◽  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Deep Ocean ◽  
Ocean Surface ◽  
Ocean Model ◽  
Global Ocean ◽  
Stable Water Isotopes ◽  
Tropical Regions

Abstract. The stable water isotopes H218O and HDO are incorporated as passive tracers into the oceanic general circulation model MPI-OM, and a control simulation under present-day climate conditions is analyzed in detail. Both δ18O and δD distributions at the ocean surface and deep ocean are generally consistent with available observations on the large scale. The modelled δD-δ18O relations in surface waters slightly deviates from the slope of the global meteoric water line in most basins, and a much steeper slope is detected in Arctic Oceans. The simulated deuterium excess of ocean surface waters shows small variations between 80° S and 55° N, and a strong decrease north of 55° N. The model is also able to capture the quasi-linear relationship between δ18O and salinity S, as well as δD and S, as seen in observational data. Both in the model results and observations, the surface δ–S relations show a steeper slope in extra-tropical regions than in tropical regions, which indicates relatively more addition of isotopically depleted water at high latitudes.

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