Simulations Of Ocean Circulation Research Articles

Assessment of Global Ocean Biogeochemistry Models for Ocean Carbon Sink Estimates in RECCAP2 and Recommendations for Future Studies

AbstractThe ocean is a major carbon sink and takes up 25%–30% of the anthropogenically emitted CO2. A state‐of‐the‐art method to quantify this sink are global ocean biogeochemistry models (GOBMs), but their simulated CO2 uptake differs between models and is systematically lower than estimates based on statistical methods using surface ocean pCO2 and interior ocean measurements. Here, we provide an in‐depth evaluation of ocean carbon sink estimates from 1980 to 2018 from a GOBM ensemble. As sources of inter‐model differences and ensemble‐mean biases our study identifies (a) the model setup, such as the length of the spin‐up, the starting date of the simulation, and carbon fluxes from rivers and into sediments, (b) the simulated ocean circulation, such as Atlantic Meridional Overturning Circulation and Southern Ocean mode and intermediate water formation, and (c) the simulated oceanic buffer capacity. Our analysis suggests that a late starting date and biases in the ocean circulation cause a too low anthropogenic CO2 uptake across the GOBM ensemble. Surface ocean biogeochemistry biases might also cause simulated anthropogenic fluxes to be too low, but the current setup prevents a robust assessment. For simulations of the ocean carbon sink, we recommend in the short‐term to (a) start simulations at a common date before the industrialization and the associated atmospheric CO2 increase, (b) conduct a sufficiently long spin‐up such that the GOBMs reach steady‐state, and (c) provide key metrics for circulation, biogeochemistry, and the land‐ocean interface. In the long‐term, we recommend improving the representation of these metrics in the GOBMs.

Eddy‐Mediated Turbulent Mixing of Oxygen in the Equatorial Pacific

AbstractIn the tropical Pacific, weak ventilation and intense microbial respiration at depth give rise to a low dissolved oxygen (O2) environment that is thought to be ventilated primarily by the equatorial current system (ECS). The role of mesoscale eddies and vertical mixing as potential pathways of O2 supply in this region, however, remains poorly known due to sparse observations and coarse model resolution. Using an eddy resolving simulation of ocean circulation and biogeochemistry, we assess the contribution of these processes to the O2 budget balance and find that vertical mixing of O2, which is modulated by the surface wind speed and the vertical shear of the eddying currents, contributes substantially to the replenishment of O2 in the upper equatorial Pacific thermocline, complementing the advective supply of O2 by the ECS and meridional circulation at depth. These transport processes vary seasonally in conjunction with the wind: mixing of O2 into the upper thermocline is strongest during boreal summer and fall when the vertical shear and eddy kinetic energy are intensified. The relationship between eddy activity and the downward mixing of O2 arises from the modulation of equatorial turbulence by Tropical Instability Waves via their impacts on the vertical shear. This interaction of processes across scales sustains a local pathway of O2 delivery into the equatorial Pacific interior and highlights the need for adequate observations and models of turbulent mixing and mesoscale processes for understanding and predicting the fate of the tropical Pacific O2 content in a warmer and more stratified ocean.

Simulated ocean circulation and its variability in the Northern Banda Sea from CROCO model

A simulation of 1/24º resolution 3-dimension regional ocean circulation of CROCO was successfully performed from 2015 to 2022 (8 years), covering the ENSO year in El Niño 2015 and La Niña 2022 in the northern Banda Sea (NBS). The model and data comparison of sea surface temperature/height reproduced the observed satellite datasets well, with correlation coefficients above 0.9. It is shown that the mean circulation in the NBS is fed by two inflows from western Buru and the Manipa Strait, resulting in a meandering eastward mean circulation with warmer water accumulated in the eastern NBS, associated with high current variance south of Buru and along the Manipa Strait. Seasonal variations in the oceanographic parameters were dominant in the study area. For example, during the southeast (northwest) monsoon period, the seawater temperature is minimum (maximum) and the salinity is maximum (minimum). Furthermore, the interannual timescale ENSO and IOD significantly modulated seawater temperature and salinity variation, particularly at the thermocline layer (110 m). Much cooler and saltier water is associated with El Niño 2015 and a positive IOD, in contrast to warmer and fresher water during La Niña 2022 and a negative IOD.

Deep ocean circulation in the subpolar North Atlantic observed by acoustically-tracked floats

The deep circulation in the subpolar North Atlantic determines the spread and mixing of high latitude climate signals to lower latitudes. However, our current understanding of the subpolar deep circulation has been limited due to relatively sparse observational data. To improve that understanding, we construct gridded fields of mean velocity and eddy kinetic energy (EKE) in the deep (1800–2800 dbar) subpolar North Atlantic using direct velocity measurements from 122 subsurface acoustically-tracked floats that drifted during June 2014–January 2019. The mean velocity field reveals a relatively strong deep boundary current around Greenland and in the Labrador Sea, with a weaker deep boundary current over the eastern flank of the Reykjanes Ridge, and near-zero mean flow over the western flank, implying a discontinuous deep boundary current across the subpolar basin. The deep EKE, albeit with smaller magnitudes, generally resembles the EKE pattern at the ocean surface, including relatively high values along pathways of the North Atlantic Current and west of Greenland where the Irminger Rings are formed. A surprising finding about deep EKE is an elevated band east of Greenland that parallels the coast and is not present in the surface EKE field. This high EKE band is possibly attributed to the combined influence from propagating Denmark Strait Overflow Cyclones, variability of the wind-driven recirculation offshore of southeast Greenland, and/or topographic waves. The float-based flow fields constructed in this study provide an unprecedented quantitative view of the kinematic properties of the large-scale deep circulation in the subpolar North Atlantic. Combined with cross-basin Eulerian measurements, we believe these recent observations provide a benchmark for testing and improving numerical simulations of deep ocean circulation and the Meridional Overturning Circulation, which are urgently needed for climate change predictions.

The Enhancement of Submesoscale Ageostrophic Motion on the Mesoscale Eddies in the South China Sea

AbstractAgeostrophic motion regulates the transformation of energy between large‐scale geostrophic motion and dissipate scale motion. By combining surface drifters and satellite altimeter data, we present the characteristics of the submesoscale ageostrophic motion (SMAM) with a temporal scale of 2–7 day in the South China Sea (SCS). The SMAM enhances the strength of the mesoscale eddies, inducing negative vorticity for anticyclonic eddies and positive vorticity for cyclonic eddies. Spatially, SMAM is significant and explains more than 20% of the total kinetic energy in the Kuroshio branch zone. The geostrophic eddy kinetic energy is higher in western SCS than the Kuroshio zone, but the submesoscale EKE is lower, which is suggested to be influenced by the developing stage of mesoscale eddies. The dominant oscillation period of SMAM in the SCS and northwestern Pacific is 7‐to 30 day intraseasonal variation. This spatiotemporal scale of SMAM is expected to be a good indicator for the simulation of oceanic circulation in submesoscale eddy‐resolving models.

The impact of ocean data assimilation on the simulation of mesoscale eddies at São Paulo plateau (Brazil) using the regional ocean modeling system

This work presents simulations of ocean circulation in the southwestern South Atlantic Ocean with emphasis on the São Paulo Plateau region, a unique physiographic feature at the foot of the continental slope offshore southeast Brazil, where the flow is dominated by ocean eddies and dipoles. Surface dynamics south of 23°S presents a heterogeneous circulation pattern intimately related to the presence of mesoscale features, such as meanders of the Brazil Current along the continental slope, and eddies that interacts with deep ocean features. The reproduction of these eddies at the correct time and spatial structure are quite challenging in the context of a non-assimilative model, considering the limitations imposed by the model’s numerical schemes, physical parameterizations, boundary conditions and surface forcing. In this sense, the Regional Ocean Modeling System (ROMS) 4-dimensional Variational Data Assimilation (4D-Var) module was used to increase model skill and minimize deviations from oceanic observations. Weekly observations of Sea Surface Height (SSH), daily Sea Surface Temperature (SST) fields and in situ vertical profiles of temperature and salinity from multiple sources were used in consecutive assimilation cycles to produce an accurate representation of ocean features and their variability. Both the assimilative and free run were evaluated against observations, including in situ current measurements. Results showed significant improvements of the assimilation procedure on both assimilated and non-assimilated variables, with RMSE reductions of 27% and 47% for SST and SSH, respectively, and 51% for current speed measurements during a dipole event in the São Paulo Plateau. The assimilative run was able to reproduce a remarkable event of mesoscale eddies interaction (dipole) and consequent current intensification, leading to important improvements on model skill. The correct use of observational information of open ocean surface features through data assimilation is a key factor for a proper simulation of such eddy dipole events.

Exploring drift simulations from ocean circulation experiments: application to cod eggs and larval drift

Drift models are commonly used to study the transport of early life stages of fish and other marine organisms. Various approaches may be applied to examine the distribution and variability of ocean trajectory pathways. In the present study, we compare results using passive Eulerian tracers and Lagrangian float trajectories that are embedded in numerical models. We supplement this analysis by applying an offline model for drift computations. The contrasts in the results from the various configurations are mainly due to differences in drift depth. Simulations were performed using horizontal resolutions of 4 and 0.8 km. The higher-resolution experiment gives somewhat more realistic results for the drift time from Lofoten to the Tromsøflaket bank at the southwestern entrance of the Barents Sea. Furthermore, differences in results between simulation years are much larger than the differences that arise from the choice of model configuration. Climate variability at high latitudes on a multi-decadal time scale is dominated by large interannual variability superimposed on an underlying moderate warming trend. We conclude that a properly configured offline drift model using hourly or 2-hourly results from a simulation with a horizontal resolution of 1 km or finer is the best approach for investigations of trajectory pathways. The flexibility of an offline drift model is also highly advantageous in biological contexts, as it easily allows for a variety of ways in which behavioural characteristics can be parameterized, including descriptions that are defined after the ocean circulation simulation has been executed.

On the control of subantarctic stratification by the ocean circulation

A shallow mixed layer depth bias in Austral winter in the Subantarctic Zone is a common feature of Coupled Model Intercomparison Project (CMIP5) models, including the Community Earth System Model (CESM). The bias is related to other deficiencies in the model solution, including too-weak Subantarctic Mode water formation and excessive leakage of Agulhas waters into the Atlantic instead of into the Indian Ocean and Subantarctic Frontal Zone. This work investigates the hypothesis that the shallow bias is due to errors in the simulated ocean circulation. Results from a model with low resolution ocean component (1° grid) are compared against: (i) an experiment with an eddy resolving (0.1°) grid and (ii) experiments using the 1° grid and employing an adiabatic nudging of the ocean pressure field to observations that is referred to as a semi-prognostic method. For both the higher resolution and semi-prognostic experiments, improved horizontal advection of warm and salty water near the surface leads to a more realistic sea surface temperature (SST) and salinity front associated with the Agulhas Return Current and Antarctic Circumpolar Current. The warmer surface waters in the high-resolution model and semi-prognostic model lead to stronger air–sea heat loss, with a response of about 40 Wm−2 °C−1 in winter in the area of deep mixed layers. Budgets of the temperature and salinity stratification show that the deeper mixed layers in the high-resolution experiment are primarily a result of the increased surface heat loss, whereas in the semi-prognostic case salinity advection is the main factor leading to destabilization of the water column. Both results indicate that ocean circulation is a key factor in wintertime deep Southern Ocean mixing, associated with advection of water masses and air–sea feedbacks.

Simulation of Ocean Circulation of Dongsha Water Using Non-Hydrostatic Shallow-Water Model

A two-dimensional non-hydrostatic shallow-water model for weakly dispersive waves is developed using the least-squares finite-element method. The model is based on the depth-averaged, nonlinear and non-hydrostatic shallow-water equations. The non-hydrostatic shallow-water equations are solved with the semi-implicit (predictor-corrector) method and least-squares finite-element method. In the predictor step, hydrostatic pressure at the previous step is used as an initial guess and an intermediate velocity field is calculated. In the corrector step, a Poisson equation for the non-hydrostatic pressure is solved and the final velocity and free-surface elevation is corrected for the new time step. The non-hydrostatic shallow-water model is verified and applied to both wave and flow driven fluid flows, including solitary wave propagation in a channel, progressive sinusoidal waves propagation over a submerged bar, von Karmann vortex street, and ocean circulations of Dongsha Atolls. It is found hydrostatic shallow-water model is efficient and accurate for shallow water flows. Non-hydrostatic shallow-water model requires 1.5 to 3.0 more cpu time than hydrostatic shallow-water model for the same simulation. Model simulations reveal that non-hydrostatic pressure gradients could affect the velocity field and free-surface significantly in case where nonlinearity and dispersion are important during the course of wave propagation.

Reproducibility and variability of submesoscale frontal eddies on a broad, low-energy shelf of freshwater influence

A multi-decadal simulation of ocean circulation in the northern Gulf of Mexico produces strong submesoscale instabilities in the Mississippi/Atchafalaya plume fronts. The model skill in reproducing these submesoscale frontal eddies over the Texas-Louisiana shelf is assessed using simulated and observed salinity and velocity fields as a way to investigate simulation accuracy and quantify the variability of frontal eddies. The model successfully reproduces mean salinity structure observed in multi-year densely sampled CTD profiles. Variability associated with submesoscale eddies is the largest source of error in predicted salinity. On the other hand, the model is statistically able to reproduce the magnitude and characteristics of frontal eddies; metrics for eddy kinetic energy are similar between the observations and simulation, and observed horizontal salinity gradients have similar occurrence rates in the model when sampled in a manner similar to the observations. Seasonal and inter-annual variability of frontal eddies is associated with the volume of freshwater onto the shelf and wind stress. Wind stress, the highest in winter and lowest in summer, contributes to the suppression of baroclinic instability during non-summer seasons. River streamflow, highest in spring, creates strong horizontal and vertical density gradients. These strong horizontal density gradients, along with weak seasonal upwelling-favorable winds that tend to broaden the plume, are the primary factors in exciting submesoscale instabilities during summer on the Texas-Louisiana shelf. At decadal scales, streamflow, EKE, and salinity gradients have a positive correlation suggesting that long-term variability of frontal eddies may be influenced remotely by inter-annual variability in the Mississippi River outflow.

Mixing by Oceanic Lee Waves

Oceanic lee waves are generated in the deep stratified ocean by the flow of ocean currents over sea floor topography, and when they break, they can lead to mixing in the stably stratified ocean interior. While the theory of linear lee waves is well established, the nonlinear mechanisms leading to mixing are still under investigation. Tidally driven lee waves have long been observed in the ocean, along with associated mixing, but observations of lee waves forced by geostrophic eddies are relatively sparse and largely indirect. Parameterizations of the mixing due to ocean lee waves are now being developed and implemented in ocean climate models. This review summarizes current theory and observations of lee wave generation and mixing driven by lee wave breaking, distinguishing between steady and tidally oscillating forcing. The existing parameterizations of lee wave–driven mixing informed by theory and observations are outlined, and the impacts of the parameterized lee wave–driven mixing on simulations of large-scale ocean circulation are summarized.

Simulation and Improvements of Oceanic Circulation and Sea Ice by the Coupled Climate System Model FGOALS-f3-L

This study documents simulated oceanic circulations and sea ice by the coupled climate system model FGOALS-f3-L developed at the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, under historical forcing from phase 6 of the Coupled Model Intercomparison Project (CMIP6). FGOALS-f3-L reproduces the fundamental features of global oceanic circulations, such as sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), vertical temperature and salinity, and meridional overturning circulations. There are notable improvements compared with the previous version, FGOALS-s2, such as a reduction in warm SST biases near the western and eastern boundaries of oceans and salty SSS biases in the tropical western Atlantic and eastern boundaries, and a mitigation of deep MLD biases at high latitudes. However, several obvious biases remain. The most significant biases include cold SST biases in the northwestern Pacific (over 4°C), freshwater SSS biases and deep MLD biases in the subtropics, and temperature and salinity biases in deep ocean at high latitudes. The simulated sea ice shows a reasonable distribution but stronger seasonal cycle than observed. The spatial patterns of sea ice are more realistic in FGOALS-f3-L than its previous version because the latitude-longitude grid is replaced with a tripolar grid in the ocean and sea ice model. The most significant biases are the overestimated sea ice and underestimated SSS in the Labrador Sea and Barents Sea, which are related to the shallower MLD and weaker vertical mixing.

Marine biogeochemical cycling and oceanic CO<sub>2</sub> uptake simulated by the NUIST Earth System Model version 3 (NESM v3)

Abstract. In this study, we evaluate the performance of the Nanjing University of Information Science and Technology (NUIST) Earth System Model version 3 (hereafter NESM v3) in simulating the marine biogeochemical cycle and carbon dioxide (CO2) uptake. Compared with observations, the NESM v3 reproduces the large-scale patterns of biogeochemical fields reasonably well in the upper ocean, including nutrients, alkalinity, dissolved inorganic, chlorophyll, and net primary production. Some discrepancies between model simulations and observations are identified and the possible causes are investigated. In the upper ocean, the simulated biases in biogeochemical fields are mainly associated with shortcomings in the simulated ocean circulation. Weak upwelling in the Indian Ocean suppresses the nutrient entrainment to the upper ocean, thus reducing biological activities and resulting in an underestimation of net primary production and the chlorophyll concentration. In the Pacific and the Southern Ocean, nutrients are overestimated as a result of strong iron limitation and excessive vertical mixing. Alkalinity is also overestimated in high-latitude oceans due to excessive convective mixing. The major discrepancy in biogeochemical fields is that the model overestimates nutrients, alkalinity, and dissolved inorganic carbon in the deep North Pacific, which is caused by the excessive deep ocean remineralization. The model reasonably reproduces present-day oceanic CO2 uptake. Model-simulated cumulative oceanic CO2 uptake is 149 PgC between 1850 and 2016, which compares well with data-based estimates of 150±20 PgC. In the 1 % yr−1 CO2 increase (1ptCO2) experiment, the diagnosed carbon-climate (γ=-7.9 PgC K−1) and carbon-concentration sensitivity parameters (β=0.88 PgC ppm−1) in the NESM v3 are comparable with those in Coupled Model Intercomparison Project phase 5 (CMIP5) models (β: 0.69 to 0.91 PgC ppm−1; γ: −2.4 to −12.1 PgC K−1). The nonlinear interaction between carbon-concentration and carbon-climate sensitivity in the NESM v3 accounts for 10.3 % of the total carbon uptake, which is within the range of CMIP5 model results (3.6 %–10.6 %). Overall, the NESM v3 can be employed as a useful modeling tool to investigate large-scale interactions between the ocean carbon cycle and climate change.

Impact of Transmission Scheme of Visible Solar Radiation on Temperature and Mixing in the Upper Water Column With Inputs for Transmission Derived From Ocean Color Remote Sensing

AbstractThe penetration of visible solar radiation (VSR) in the upper ocean contributes to heating in the upper water column, and this process is modulated by constituents in water such as phytoplankton. Various schemes have been developed to propagate surface VSR to deeper depths, which are incorporated in ocean circulation models to study basin‐scale impacts on ocean and atmosphere by phytoplankton. However, none of these studies evaluated sensitivity of the schemes of VSR transmission to upper‐water dynamics, especially when the required input is derived from ocean color remote sensing. We used an idealized one‐dimensional (1‐D) Regional Ocean Modeling System (ROMS) to simulate profiles of upper water column temperature at six locations with different ocean color characteristics (containing different water constituents). We incorporated and tested four different schemes of VSR transmission: the default water‐type scheme within the ROMS, two schemes based on concentrations of chlorophyll (Chl), and one scheme based on water's inherent optical properties (IOPs). The results showed that, although using the same ocean color information and the same circulation model, the IOPs‐ and Chl‐based schemes resulted in very different vertical temperature and upper‐water mixing compared to those using the default scheme, with differences in sea surface temperature up to 1.5–2°C. These results highlight the importance of developing and incorporating more appropriate VSR transmission schemes into large‐scale ocean circulation simulations, where the inputs for the transmission are inherently estimates from ocean color—the actual measurement from satellite remote sensing.

Numerical Simulation of Ocean Circulation with Ultrahigh Spatial Resolution

This paper provides an overview of publications related to the study of submesoscale ocean variability. The significant progress achieved recently in understanding the special role of submesoscale processes in the formation of ocean circulation and ocean–atmosphere interaction, including modulation of the heat and gas exchange of the two media and their impact on the flux of biogenic elements into the top layer of the ocean, is emphasized. The necessity of conducting a study of submesoscale ocean variability in order to improve the quality of short-term marine forecasts is noted. Marine forecasts of the new level will be based on models with ultrahigh spatial resolution. The models will allow one to describe the reverse cascade of energy in the submesoscale range. In connection with the fact that new instruments capable of conducting 2–10 km surface observations of seas and oceans will be launched in the next five years, the possibilities of using the Black Sea as a testing area for the development of numerical models of circulation in a basin with a resolution of no less than a kilometer and improving methods of analysis and assimilation of observational technologies of the next generation are discussed.

Latitudinal Structure of the Meridional Overturning Circulation Variability on Interannual to Decadal Time Scales in the North Atlantic Ocean

AbstractThe latitudinal structure of the Atlantic meridional overturning circulation (AMOC) variability in the North Atlantic is investigated using numerical results from three ocean circulation simulations over the past four to five decades. We show that AMOC variability south of the Labrador Sea (53°N) to 25°N can be decomposed into a latitudinally coherent component and a gyre-opposing component. The latitudinally coherent component contains both decadal and interannual variabilities. The coherent decadal AMOC variability originates in the subpolar region and is reflected by the zonal density gradient in that basin. It is further shown to be linked to persistent North Atlantic Oscillation (NAO) conditions in all three models. The interannual AMOC variability contained in the latitudinally coherent component is shown to be driven by westerlies in the transition region between the subpolar and the subtropical gyre (40°–50°N), through significant responses in Ekman transport. Finally, the gyre-opposing component principally varies on interannual time scales and responds to local wind variability related to the annual NAO. The contribution of these components to the total AMOC variability is latitude-dependent: 1) in the subpolar region, all models show that the latitudinally coherent component dominates AMOC variability on interannual to decadal time scales, with little contribution from the gyre-opposing component, and 2) in the subtropical region, the gyre-opposing component explains a majority of the interannual AMOC variability in two models, while in the other model, the contributions from the coherent and the gyre-opposing components are comparable. These results provide a quantitative decomposition of AMOC variability across latitudes and shed light on the linkage between different AMOC variability components and atmospheric forcing mechanisms.

A novel sigma coordinate system to simulate abrupt changes of underwater terrain in a hydrodynamic model: application to Lake Mead, USA.

The sigma (SIG) coordinate system in ocean circulation simulation models results inevitably in horizontal pressure gradient error. This problem also emerges in models of deep lakes or reservoirs with the same characteristics of underwater terrain mutation. SIG coordinates reflect vertical relative stratification but cannot be used to calculate horizontal pressure gradient force in places with drastic topographic changes; this results in vertical water temperature and circulation errors. In deep lakes or reservoirs, differences in water density caused by the temperature difference between upper and lower water bodies is the primary cause of thermal stratification phenomena. Lake Mead was used as a case study on steep topography based on Environmental Fluid Dynamics Code (EFDC) model in this study. SIG coordinates result in close agreement between the calibrated temperature time series at the top and middle water layers, but disparity in the bottom water layer. The error emerges in the horizontal pressure gradient error due to the SIG coordinate transformation. Neither increasing the vertical resolution nor adjusting the horizontal viscosity coefficient resolve this error. We test the sigma-zed (SGZ) coordinate which combines Z coordinate and SIG coordinate as a replacement for the SIG coordinate to find that they effectively reduce the model's runtime and simulation efficiency. The vertical temperature distribution in SGZ coordinate mode is more accurate than the distribution in SIG coordinate mode. The Navier-Stokes horizontal gradient and advection diffusion equation results under SIG coordinates are very sensitive to the pressure gradient. The replacement also enhances resolution near the thermocline, facilitates reclosing of the water bottom and the equal sigma surface, lends significant advantages in terms of vertical temperature in the simulation for local deep water with steep terrain, and shortens runtime for 0.14h. SGZ mixed coordinates are recommended in the simulation of deep lakes or reservoirs wherein the underwater topography is large (with abundant continuous deep trenches or reefs).

A non-breaking-wave-generated turbulence mixing scheme for a global ocean general circulation model

A novel vertical mixing scheme to describe the influence of the non-breaking surface waves in ocean general circulation models is proposed based on the second-order turbulence closure model and a mathematical relationship fitted between the in situ observations of the turbulence kinetic energy dissipation rate and the calculated velocity shear module of non-breaking surface waves. The non-breaking-wave-generated turbulence mixing coefficients can be calculated empirically in terms of the wave spectral parameters including ΦSW, K, and ω, where ΦSW is the wave number spectrum of the significant wave height, K is the wave number, ω is the surface wave frequency. The effect of the new mixing scheme on global ocean circulation simulation was tested using the MArine Science and NUmerical Modeling ocean model. The results indicate significant improvement in the upper-ocean temperature structure and the mixed layer depth, i.e., the non-breaking-wave-generated turbulence mixing plays an important role in the upper ocean at high latitudes in summer for both hemispheres compared with traditional Mellor-Yamada 2.5 scheme.

Origin of Circumpolar Deep Water intruding onto the Amundsen and Bellingshausen Sea continental shelves

Melting of West Antarctic ice shelves is enhanced by Circumpolar Deep Water (CDW) intruding onto the Amundsen and Bellingshausen Seas (ABS) continental shelves. Despite existing studies of cross-shelf and on-shelf CDW transports, CDW pathways onto the ABS originating from further offshore have never been investigated. Here, we investigate CDW pathways onto the ABS using a regional ocean model. Simulated CDW tracers from a zonal section across 67°S (S04P) circulate along the Antarctic Circumpolar Current (ACC) and Ross Gyre (RG) and travel into ABS continental shelf after 3–5 years, but source locations are shifted westward by ~900 km along S04P in 2001–2006 compared to 2009–2014. We find that simulated on- and off-shelf CDW is ~0.1–0.2 °C warmer in the 2009–2014 case than in the 2001–2006 case together with changes in simulated ocean circulation. These differences are primarily caused by lateral, rather than surface, boundary conditions, implying that large-scale atmospheric and ocean circulations are able to control CDW pathways and thus off- and on-shelf CDW properties.

Mechanisms of Mixed‐Layer Salinity Seasonal Variability in the Indian Ocean

AbstractBased on a joint analysis of an ensemble mean of satellite sea surface salinity retrievals and the output of a high‐resolution numerical ocean circulation simulation, physical processes are identified that control seasonal variations of mixed‐layer salinity (MLS) in the Indian Ocean, a basin where salinity changes dominate changes in density. In the northern and near‐equatorial Indian Ocean, annual salinity changes are mainly driven by respective changes of the horizontal advection. South of the equatorial region, between 45°E and 90°E, where evaporation minus precipitation has a strong seasonal cycle, surface freshwater fluxes control the seasonal MLS changes. The influence of entrainment on the salinity variance is enhanced in mid‐ocean upwelling regions but remains small. The model and observational results reveal that vertical diffusion plays a major role in precipitation and river runoff dominated regions balancing the surface freshwater flux. Vertical diffusion is important as well in regions where the advection of low salinity leads to strong gradients across the mixed‐layer base. There, vertical diffusion explains a large percentage of annual MLS variance. The simulation further reveals that (1) high‐frequency small‐scale eddy processes primarily determine the salinity tendency in coastal regions (in particular in the Bay of Bengal) and (2) shear horizontal advection, brought about by changes in the vertical structure of the mixed layer, acts against mean horizontal advection in the equatorial salinity frontal regions. Observing those latter features with the existing observational components remains a future challenge.