Global Ocean Circulation Model Research Articles

Regional Sea Level Response to External Forcings from the Twentieth to the Twenty-First Century

Abstract Using a range of Detection and Attribution Model Intercomparison Project (DAMIP) simulations from phase 6 of Coupled Model Intercomparison Project (CMIP6), we study the response of dynamic sea level (DSL) to external anthropogenic climate forcing [greenhouse gases (GHGs), aerosols, and stratospheric ozone] with a focus on the differences over the twentieth and twenty-first century. In the second half of the twentieth century, the DSL nonuniformity in the Northern Hemisphere (NH) was relatively small due to a cancellation between the effects of increasing GHGs and aerosols. In contrast, the DSL signal in the Southern Hemisphere (SH) over this period was large because stratospheric ozone depletion reinforced the effects of increasing GHGs. In the twenty-first century, the DSL response has been intensified in the NH because the warming effects of diminishing aerosols have acted to reinforce the effects of increasing GHGs. Meanwhile, the distribution of SH DSL has also become uneven although stratospheric ozone recovery has partially offset the effects of rising GHGs. Using a global ocean circulation model, we decompose the changes in the twenty-first century DSL into distinct responses to surface forcings including sea surface temperature, salinity, and wind stress. Our results show that the dipole-like pattern of DSL in the North Pacific can be attributed largely to sea surface warming, while the dipole-like pattern in the North Atlantic is attributed to subpolar surface salinity freshening. The belted pattern of DSL changes in the Southern Ocean is induced by both surface warming and intensifying/poleward-shifting westerly winds.

The Vertical Structure of Internal Lee-Wave-Driven Benthic Mixing Hotspots

Abstract In global ocean circulation and climate models, bottom-enhanced turbulent mixing is often parameterized such that the vertical decay scale of the energy dissipation rate ζ is universally constant at 500 m. In this study, using a nonhydrostatic two-dimensional numerical model in the horizontal–vertical plane that incorporates a monochromatic sinusoidal seafloor topography and the Garrett–Munk (GM) background internal wave field, we find that ζ of the internal lee-wave-driven bottom-enhanced mixing is actually variable depending on the magnitude of the steady flow U0, the horizontal wavenumber kH, and the height hT of the seafloor topography. When the steepness parameter (Sp = NhT/U0 where N is the buoyancy frequency near the seafloor) is less than 0.3, internal lee waves propagate upward from the seafloor while interacting with the GM internal wave field to create a turbulent mixing region with ζ that extends farther upward from the seafloor as U0 increases, but is nearly independent of kH. In contrast, when Sp exceeds 0.3, inertial oscillations (IOs) not far above the seafloor are enhanced by the intermittent supply of internal lee-wave energy Doppler-shifted to the near-inertial frequency, which occurs depending on the sign and magnitude of the background IO shear. The composite flow, consisting of the superposition of U0 and the IOs, interacts with the seafloor topography to efficiently generate internal lee waves during the period centered on the time of the composite flow maximum, but their upward propagation is inhibited by the increased IO shear, creating a turbulent mixing region of small ζ.

An Asynchronous Parallel I/O Framework for Mass Conservation Ocean Model

I/O is often a performance bottleneck in global ocean circulation models with fine spatial resolution. In this paper, we present an asynchronous parallel I/O framework and demonstrate its efficacy in the Mass Conservation Ocean Model (MaCOM) as a case study. By largely reducing I/O operations in computing processes and overlapping output in I/O processes with computation in computing processes, this framework significantly improves the performance of the MaCOM. Through both reordering output data for maintaining data continuity and combining file access for reducing file operations, the I/O optimizing algorithms are provided to improve output bandwidth. In the case study of the MaCOM, the cost of output in I/O processes can be overlapped by up to 99% with computation in computing processes as decreasing output frequency. The 1D data output bandwidth with these I/O optimizing algorithms is 3.1 times faster than before optimization at 16 I/O worker processes. Compared to the synchronous parallel I/O framework, the overall performance of MaCOM is improved by 38.8% at 1024 computing processes for a 7-day global ocean forecast with 1 output every 2 h through the asynchronous parallel I/O framework presented in this paper.

Assessing the Potential of Backscattering as a Proxy for Phytoplankton Carbon Biomass

AbstractDespite phytoplankton contributing roughly half of the photosynthesis on earth and fueling marine food‐webs, field measurements of phytoplankton biomass remain scarce. The particulate backscattering coefficient (bbp) has often been used as an optical proxy to estimate phytoplankton carbon biomass (Cphyto). However, total observed bbp is impacted by phytoplankton size, cell composition, and non‐algal particles. The lack of phytoplankton field data has prevented the quantification of uncertainties driven by these factors. Here, we first review and discuss existing bbp algorithms by applying them to bbp data from the BGC‐Argo array in surface waters (<10 m). We find a bbp threshold where estimated Cphyto differs by more than an order of magnitude. Next, we use a global ocean circulation model (the MITgcm Biogeochemical and Optical model) that simulates plankton dynamics and associated inherent optical properties to quantify and understand uncertainties from bbp‐based algorithms in surface waters. We do so by developing and calibrating an algorithm to the model. Simulated error‐estimations show that bbp‐based algorithms overestimate/underestimate Cphyto between 5% and 100% in surface waters, depending on the location and time. This is achieved in the ideal scenario where Cphyto and bbp are known precisely. This is not the case for algorithms derived from observations, where the largest source of uncertainty is the scarcity of phytoplankton biomass data and related methodological inconsistencies. If these other uncertainties are reduced, the model shows that bbp could be a relatively good proxy for phytoplankton carbon biomass, with errors close to 20% in most regions.

Pacific Decadal Oscillation Influences Tropical Oxygen Minimum Zone Extent and Obscures Anthropogenic Changes

AbstractObservations suggest that the tropical Pacific Ocean has lost oxygen since the 1960s leading to the expansion of its oxygen minimum zone (OMZ). Attribution to anthropogenic forcing is, however, difficult because of limited data availability and the large natural variability introduced by the Pacific Decadal Oscillation (PDO). Here, we evaluate the PDO influence on oxygen dynamics and OMZ extent using observations and hindcast simulations from two global ocean circulation models (NEMO‐PISCES, MOM6‐COBALT). In both models, the tropical Pacific oxygen content decreases by about 30 Tmol.decade−1 and the OMZ volume expands by 1.3 × 105 km3.decade−1 during PDO positive phases, while variations of similar magnitude but opposite sign are simulated during negative phases. Changes in equatorial advective oxygen supply, partially offset by biological demand, control the oxygen response to PDO. Observations which cover 39% of the tropical Pacific volume only partially capture spatio‐temporal variability, hindering the separation of anthropogenic trend from natural variations.

Long-Term Trend and Variability of Volume Transport and Advective Heat Flux through the Boundaries of the Java Sea Based on a Global Ocean Circulation Model (1950–2013)

The variability and trend of volume and advective heat flux, in addition to the net inflow and outflow of advective heat flux in the Java Sea for 64 years (1950–2013), along with its relationship with the monsoon, ENSO (El Niño Southern Oscillation), and IOD (Indian Ocean Dipole), have all been studied. A simulation of the 3D hydrodynamic HYCOM (HYbrid Coordinate Ocean Model) with a 1/8° grid resolution was performed in this study. Judging from the simulated results, the seasonal variability, which has a period of 12 months, has a very significant impact on contributing to the variability and trend of volume and advective heat flux, as well as the net inflow and outflow of advective heat flux in the Java Sea for 64 years. This is followed by interannual variability, which has a time range of 1.5–6.5 years, and interdecadal variability, with a period of 21.3–32 years. The interannual variability in the Java Sea is strongly caused by ENSO and IOD. El Niño and a positive IOD caused a weakening of southward transport through Karimata and the Bangka Strait. On the contrary, southward transport strengthened during La Niña and the negative IOD. Furthermore, La Niña and a positive IOD both strengthen (weaken) the transport westward (eastward) in the Sunda Strait (Eastern Java). On the other hand, El Niño and a negative IOD weaken (strengthen) the westward (eastward) transport in the Sunda Strait (Eastern Java). According to the findings, the IOD effect is stronger than the ENSO effect in the Java Sea. The inflow and outflow of volume transport in the Java Sea are in balance, but not the advective heat flux. The advective heat transported through Karimata and Bangka Strait to the Java Sea is up to 0.216 PW, while the total advective heat flux through the outflow straits (Sunda Strait and Eastern Java) is 0.220 PW. Thus, the net advective heat flux out of the Java Sea is 0.004 PW, allegedly obtained from an atmosphere–sea interaction in which the sea received heat from the atmosphere.

Fast Spin‐Up of Geochemical Tracers in Ocean Circulation and Climate Models

AbstractOcean geochemical tracers such as radiocarbon, protactinium and thorium isotopes, and noble gases are widely used to constrain a range of physical and biogeochemical processes in the ocean. However, their routine simulation in global ocean circulation and climate models is hindered by the computational expense of integrating them to a steady state. Here, a new approach to this long‐standing “spin‐up” problem is introduced to efficiently compute equilibrium distributions of such tracers in seasonally‐forced models. Based on “Anderson Acceleration,” a sequence acceleration technique developed in the 1960s to solve nonlinear integral equations, the new method is entirely “black box” and offers significant speed‐up over conventional direct time integration. Moreover, it requires no preconditioning, ensures tracer conservation and is fully consistent with the numerical time‐stepping scheme of the underlying model. It thus circumvents some of the drawbacks of other schemes such as matrix‐free Newton Krylov that have been proposed to address this problem. An implementation specifically tailored for the batch HPC systems on which ocean and climate models are typically run is described, and the method illustrated by applying it to a variety of geochemical tracer problems. The new method, which provides speed‐ups by over an order of magnitude, should make simulations of such tracers more feasible and enable their inclusion in climate change assessments such as IPCC.

Can We Estimate Air‐Sea Flux of Biological O2 From Total Dissolved Oxygen?

AbstractIn this study, we compare mechanistic and empirical approaches to reconstruct the air‐sea flux of biological oxygen () by parameterizing the physical oxygen saturation anomaly (ΔO2[phy]) in order to separate the biological contribution from total oxygen. The first approach matches ΔO2[phy] to the monthly climatology of the argon saturation anomaly from a global ocean circulation model's output. The second approach derives ΔO2[phy] from an iterative mass balance model forced by satellite‐based physical drivers of ΔO2[phy] prior to the sampling day by assuming that air‐sea interactions are the dominant factors driving the surface ΔO2[phy]. The final approach leverages the machine‐learning technique of Genetic Programming (GP) to search for the functional relationship between ΔO2[phy] and biophysicochemical parameters. We compile simultaneous measurements of O2/Ar and O2 concentration from 14 cruises to train the GP algorithm and test the validity and applicability of our modeled ΔO2[phy] and . Among the approaches, the GP approach, which incorporates ship‐based measurements and historical records of physical parameters from the reanalysis products, provides the most robust predictions (R2 = 0.74 for ΔO2[phy] and 0.72 for ; RMSE = 1.4% for ΔO2[phy] and 7.1 mmol O2 m−2 d−1 for ). We use the empirical formulation derived from GP approach to reconstruct regional, inter‐annual, and decadal variability of based on historical oxygen records. Overall, our study represents a first attempt at deriving from snapshot measurements of oxygen, thereby paving the way toward using historical O2 data and a rapidly growing number of O2 measurements on autonomous platforms for independent insight into the biological pump.

Marine Heatwaves and Their Depth Structures on the Northeast U.S. Continental Shelf

Marine Heatwaves (MHWs) are ocean extreme events, characterized by anomalously high temperatures, which can have significant ecological impacts. The Northeast U.S. continental shelf is of great economical importance as it is home to a highly productive ecosystem. Local warming rates exceed the global average and the region experienced multiple MHWs in the last decade with severe consequences for regional fisheries. Due to the lack of subsurface observations, the depth-extent of MHWs is not well-known, which hampers the assessment of impacts on pelagic and benthic ecosystems. This study utilizes a global ocean circulation model with a high-resolution (1/20°) nest in the Atlantic to investigate the depth structure of MHWs and associated drivers on the Northeast U.S. continental shelf. It is shown that MHWs exhibit varying spatial extents, with some only occurring at depth. The highest intensities are found around 100 m depth with temperatures exceeding the climatological mean by up to 7°C, while surface intensities are typically smaller (around 3°C). Distinct vertical structures are associated with different spatial MHW patterns and drivers. Investigation of the co-variability of temperature and salinity reveals that over 80% of MHWs at depth (>50 m) coincide with extreme salinity anomalies. Two case studies provide insight into opposing MHW patterns at the surface and at depth, being forced by anomalous air-sea heat fluxes and Gulf Stream warm core ring interaction, respectively. The results highlight the importance of local ocean dynamics and the need to realistically represent them in climate models.

Local Drivers of Extreme Upper Ocean Marine Heatwaves Assessed Using a Global Ocean Circulation Model

The growing threat of Marine heatwaves (MHWs) to ecosystems demands that we better understand their physical drivers. This information can be used to improve the performance of ocean models in predicting major events so more appropriate management decisions can be made. Air-sea heat fluxes have been found to be one of the dominant drivers of MHWs but their impact are expected to decrease for MHWs extending deeper into the water column. In this study, we examine the most extreme MHWs occurring within an upper ocean layer and quantify the relative contributions of oceanic and atmospheric processes to their onset and decay phases. The base of the upper ocean layer is defined as the local winter mixed layer depth so that summer events occurring within a shallower mixed layer are also included. We perform a local upper ocean heat budget analysis at each grid point of a global ocean general circulation model. Results show that in 78% of MHWs, horizontal heat convergence is the main driver of MHW onset. In contrast, heat fluxes dominate the formation of MHWs in 11% of cases, through decreased latent heat cooling and/or increased solar radiation. These air-sea heat flux driven events occur mostly in the tropical regions where the upper ocean layer is shallow. In terms of MHW decay, heat advection is dominant in only 31% of MHWs, while heat flux dominance increases to 23%. For the majority of remaining events, advection and air-sea heat flux anomalies acted together to dissipate the excessive heat. This shift toward a comparable contribution of advection and air-sea heat flux is a common feature of extreme MHW decay globally. The anomalous air-sea heat flux cooling is mostly due to an increased latent heat loss feedback response to upper ocean temperature anomalies. Extreme upper ocean MHWs coincided with SST MHWs consistently, but with lower intensity in extra-tropical regions, where the upper ocean layer is deeper. This suggests that the upper ocean heat accumulation may pre-condition the SST MHWs in these regions. Our analysis provides valuable insights into the local physical processes controlling the onset and decay of extreme MHWs.

Acute Sensitivity of Global Ocean Circulation and Heat Content to Eddy Energy Dissipation Timescale

AbstractThe global ocean overturning circulation, critically dependent on the global density stratification, plays a central role in regulating climate evolution. While it is well known that the global stratification profile exhibits a strong dependence to Southern Ocean dynamics and in particular to wind and buoyancy forcing, we demonstrate here that the stratification is also acutely sensitive to the mesoscale eddy energy dissipation timescale. Within the context of a global ocean circulation model with an energy constrained mesoscale eddy parameterization, it is shown that modest variations in the eddy energy dissipation timescale lead to significant variations in key metrics relating to ocean circulation, namely the Antarctic Circumpolar Current transport, Atlantic Meridional Overturning Circulation strength, and global ocean heat content, over long timescales. The results highlight a need to constrain uncertainties associated with eddy energy dissipation for climate model projections over centennial timescales and also for paleoclimate simulations over millennial timescales.

Empirical Lagrangian parametrization for wind-driven mixing of buoyant particles at the ocean surface

Abstract. Turbulent mixing is a vital component of vertical particulate transport, but ocean global circulation models (OGCMs) generally have low-resolution representations of near-surface mixing. Furthermore, turbulence data are often not provided in OGCM model output. We present 1D parametrizations of wind-driven turbulent mixing in the ocean surface mixed layer that are designed to be easily included in 3D Lagrangian model experiments. Stochastic transport is computed by Markov-0 or Markov-1 models, and we discuss the advantages and disadvantages of two vertical profiles for the vertical diffusion coefficient Kz. All vertical diffusion profiles and stochastic transport models lead to stable concentration profiles for buoyant particles, which for particles with rise velocities of 0.03 and 0.003 m s−1 agree relatively well with concentration profiles from field measurements of microplastics when Langmuir-circulation-driven turbulence is accounted for. Markov-0 models provide good model performance for integration time steps of Δt≈30 s and can be readily applied when studying the behavior of buoyant particulates in the ocean. Markov-1 models do not consistently improve model performance relative to Markov-0 models and require an additional parameter that is poorly constrained.

Ice-shelf ocean boundary layer dynamics from large-eddy simulations

Abstract. Small-scale turbulent flow below ice shelves is regionally isolated and difficult to measure and simulate. Yet these small-scale processes, which regulate heat and salt transfer between the ocean and ice shelves, can affect sea-level rise by altering the ability of Antarctic ice shelves to “buttress” ice flux to the ocean. In this study, we improve our understanding of turbulence below ice shelves by means of large-eddy simulations at sub-meter resolution, capturing boundary layer mixing at scales intermediate between laboratory experiments or direct numerical simulations and regional or global ocean circulation models. Our simulations feature the development of an ice-shelf ocean boundary layer through dynamic ice melting in a regime with low thermal driving, low ice-shelf basal slope, and strong shear driven by the geostrophic flow. We present a preliminary assessment of existing ice-shelf basal melt parameterizations adopted in single component or coupled ice-sheet and ocean models on the basis of a small parameter study. While the parameterized linear relationship between ice-shelf melt rate and far-field ocean temperature appears to be robust, we point out a little-considered relationship between ice-shelf basal slope and melting worthy of further study.

Slower Long‐Term Coastal Warming Drives Dampened Trends in Coastal Marine Heatwave Exposure

AbstractLong‐term temperature changes drive coastal Marine Heatwave (MHW) trends globally. Here, we provide a more comprehensive global analysis of cross‐shore gradients of MHW and Sea Surface Temperatures (SST) changes using an ensemble of three satellite SST products during recent decades. Our analysis reveals depressed onshore SST trends in more than 2/3 of coastal pixels, including both eastern and western boundary current systems. These were well correlated with depressed trends of MHW exposure and severity, ranging from a −2 to −10 decrease in MHW days per decade and from a −2.5°C to −15°C.days per decade decrease in cumulative intensity. Results were consistent across all satellite products, indicating that these cross‐shore gradients are a robust feature of observations. ERA reanalysis data show that neither air‐sea heat fluxes nor wind driven upwelling were found to be consistent drivers. Global ocean circulation models (OFAM3 and ACCESS‐OM2) have limited ability to simulate the depressed onshore trends. A heat budget analysis performed in the Chilean coast region, where models agree with observations, showed that the gradient of temperature change was controlled by an onshore increase of longwave radiative cooling, despite an increase in upwelling. This highlights the complexity of small‐scale coastal ocean‐atmosphere feedbacks, which coarser resolution climate models do not resolve. Here, we show that global coastal regions may act as thermal refugia for marine ecosystems from aspects of climate change and pulsative (MHW) changes. Contrary to the literature, our results suggest that driving mechanisms are region dependant, stressing the necessity to improve climate models resolution.

Assessing the sequestration time scales of some ocean-based carbon dioxide reduction strategies

Ocean-based carbon dioxide (CO2) removal (CDR) strategies are an important part of the portfolio of approaches needed to achieve negative greenhouse gas emissions. Many ocean-based CDR strategies rely on injecting CO2 or organic carbon (that will eventually become CO2) into the ocean interior, or enhancing the ocean’s biological pump. These approaches will not result in permanent sequestration, because ocean currents will eventually return the injected CO2 back to the surface, where it will be brought into equilibrium with the atmosphere. Here, a model of steady state global ocean circulation and mixing is used to assess the time scales over which CO2 injected in the ocean interior remains sequestered from the atmosphere. There will be a distribution of sequestration times for any single discharge location due to the infinite number of pathways connecting a location at depth with the sea surface. The resulting probability distribution is highly skewed with a long tail of very long transit times, making mean sequestration times much longer than typical time scales. Deeper discharge locations will sequester purposefully injected CO2 much longer than shallower ones and median sequestration times are typically decades to centuries, and approach 1000 years in the deep North Pacific. Large differences in sequestration times occur both within and between the major ocean basins, with the Pacific and Indian basins generally having longer sequestration times than the Atlantic and Southern Oceans. Assessments made over a 50 year time horizon illustrates that most of the injected carbon will be retained for injection depths greater than 1000 m, with several geographic exceptions such as the Western North Atlantic. Ocean CDR strategies that increase upper ocean ecosystem productivity with the goal of exporting more carbon to depth will have mainly a short-term influence on atmospheric CO2 levels because ∼70% will be transported back to the surface ocean within 50 years. The results presented here will help plan appropriate ocean CDR strategies that can help limit climate damage caused by fossil fuel CO2 emissions.

Improving the accuracy of barotropic and internal tides embedded in a high-resolution global ocean circulation model of MITgcm

In recent years, high-resolution global ocean circulation models have begun to include astronomical tidal forcing to concurrently simulate ocean circulation, barotropic and internal tides. The inclusion of tides is of great significance for understanding three-dimensional (3-D) ocean mixing and developing internal tide prediction. However, due to uncertainty in topography and damping terms, improper representation of the self-attraction and loading term, barotropic tidal estimate cannot match that in the data-constrained barotropic tidal model. In this study, the TPXO8 data are assimilated to optimize the tides embedded in a global 9-km 3-D general circulation model of MITgcm as follows. For each time step of the MITgcm, the previous 25-hour-averaged sea surface height (SSH) is subtracted from the instantaneous SSH to obtain the MITgcm surface tidal elevation. Then, the difference of surface tidal elevation between the MITgcm and TPXO8 together with a damping coefficient acts as an additional term for the right-hand side of the SSH equation in order to correct the SSH field. Results show that the global M2 (K1) error of the corrected surface tidal elevation relative to the TPXO8 reaches 2.12 cm (0.89 cm). Compared with tide gauge data, the accuracy of surface tidal elevation after using the correction scheme is almost the same as that of TPXO8 for all areas except for some continental shelf and coastal areas. The internal tidal signals are subsequently improved compared to the along-track altimetry and TAO observations. Compared to previous tidal assimilation scheme, the implementation of the correction scheme used in this study is simple and more suitable for realistic applications.

Fate of floating plastic debris released along the coasts in a global ocean model

Marine plastic pollution is a global issue, from the shores to the open ocean. Understanding the pathway and fate of plastic debris is fundamental to manage and reduce plastic pollution. Here, the fate of floating plastic pollution discharged along the coasts is studied by comparing two sources, one based on river discharges and the other on mismanaged waste from coastal populations, using a Lagrangian numerical analysis in a global ocean circulation model. About 1/3 of the particles end up in the open ocean and 2/3 on beaches. The input scenario largely influences the accumulation of particles toward the main subtropical convergence zones, with the South Pacific and North Atlantic being mostly fed by the coastal population inputs. The input scenario influences the number of beached particles that end up in several coastal areas. Beaching occurs mainly locally, although a significant number of particles travel long distances, allowing for global connectivity.

Analysis of sea conditions for manned submersible operation in Southwest Indian Ocean

The southwest Indian Ocean is an important area for deep-sea investigation and exploration. In this paper, the spatial distribution and temporal variation characteristics of wave elements in the sea area were analyzed by using the high resolution reanalysis data (ERA) from the European Center, the optimal path set of tropical cyclones provided by the Joint Warning Center (JTWC) and the global ocean circulation model (OFES). The results show that the monthly mean effective wave height in the southwest Indian Ocean is relatively large, ranging from 3.5m to 5.5m. the wave season is relatively stable, with the highest mean wave height in winter and the lowest mean wave height in summer; The frequency of tropical cyclones is higher in summer and autumn and lower in winter; The most suitable operation time of manned submersible in the southwest Indian Ocean is from November to march of the next year, and other time needs to select good sea conditions for operation.

Observations of the Low‐Mode Internal Tide and Its Interaction With Mesoscale Flow South of the Azores

AbstractUnderstanding the temporal variability of internal tides plays a crucial role in identifying sources and sinks of energy in the ocean. Using a 10‐month‐long time series from moored instruments inside a tidal beam south of the Azores, the magnitude and the underlying causes of temporal variability in the first two modes of the internal tide energy flux was studied. We analyzed changes of the direction and coherence of the energy flux, its modal structure, and the impact of two eddies. Semidiurnal energy fluxes were further compared with estimates from a 1/10° ocean global circulation model, as well as with fluxes derived from satellite altimetry. All energy fluxes correlate reasonably well in direction, deviations from its fixed phase relation to astronomical forcing, and modal composition while model and satellite underestimate the total energy flux. A pronounced damping of the in situ fluxes coincides with the passing of two eddies. In the presence of a surface‐intensified eddy, the coherent part of the energy flux in the first two modes is lowered by more than 40%, a subsurface eddy coincides with a decrease of the energy flux mainly in the second mode. These observations support the hypothesis that eddy interactions increase the incoherent part of the energy flux and transfer energy from low modes into higher modes, which can lead to increased local dissipation. It remains an open question how much of the energy converted from lower to higher modes results in local dissipation, a crucial part in creating energetically consistent ocean‐climate models.

Investigation of the Internal Tides in the Northwest Pacific Ocean Considering the Background Circulation and Stratification

AbstractA global ocean circulation and tide model with nonuniform resolution is used in this work to resolve the ocean circulation globally as well as mesoscale eddies and internal tides regionally. Focusing on the northwest Pacific Ocean (NWP, 0°–35°N, 105°–150°E), a realistic experiment is conducted to simulate internal tides considering the background circulation and stratification. To investigate the influence of a background field on the generation and propagation of internal tides, idealized cases with horizontally homogeneous stratification and zero surface fluxes are also implemented for comparison. By comparing the realistic cases with idealized ones, the astronomical tidal forcing is found to be the dominant factor influencing the internal tide conversion rate magnitude, whereas the stratification acts as a secondary factor. However, stratification deviations in different areas can lead to an error exceeding 30% in the local internal tide energy conversion rate, indicating the necessity of a realistic stratification setting for simulating the entire NWP. The background shear is found to refract propagating diurnal internal tides by changing the effective Coriolis frequencies and phase speeds, while the Doppler-shifting effect is remarkable for introducing biases to semidiurnal results. In addition, nonlinear baroclinic tide energy equations considering the background circulation and stratification are derived and diagnosed in this work. The mean flow–baroclinic tide interaction and nonlinear energy flux are the most significant nonlinear terms in the derived equations, and nonlinearity is estimated to contribute approximately 5% of the total internal tide energy in the greater Luzon Strait area.