High-resolution Ocean Simulation Research Articles

Southern Ocean biogenic blooms freezing-in Oligocene colder climates

Crossing a key atmospheric CO2 threshold triggered a fundamental global climate reorganisation ~34 million years ago (Ma) establishing permanent Antarctic ice sheets. Curiously, a more dramatic CO2 decline (~800–400 ppm by the Early Oligocene(~27 Ma)), postdates initial ice sheet expansion but the mechanisms driving this later, rapid drop in atmospheric carbon during the early Oligocene remains elusive and controversial. Here we use marine seismic reflection and borehole data to reveal an unprecedented accumulation of early Oligocene strata (up to 2.2 km thick over 1500 × 500 km) with a major biogenic component in the Australian Southern Ocean. High-resolution ocean simulations demonstrate that a tectonically-driven, one-off reorganisation of ocean currents, caused a unique period where current instability coincided with high nutrient input from the Antarctic continent. This unrepeated and short-lived environment favoured extreme bioproductivity and enhanced sediment burial. The size and rapid accumulation of this sediment package potentially holds ~1.067 × 1015 kg of the ‘missing carbon’ sequestered during the decline from an Eocene high CO2-world to a mid-Oligocene medium CO2-world, highlighting the exceptional role of the Southern Ocean in modulating long-term climate.

Effect of Small-Scale Topography on Eddy Dissipation in the Northern South China Sea

Abstract Mesoscale eddies are ubiquitous dynamical features, accounting for over 90% of the total kinetic energy of the ocean. However, the pathway for eddy energy dissipation has not been fully understood. Here we investigate the effect of small-scale topography on eddy dissipation in the northern South China Sea by comparing high-resolution ocean simulations with smooth and synthetically generated rough topography. The presence of rough topography is found to 1) significantly enhance viscous dissipation and instabilities within a few hundred meters above the rough bottom, especially in the slope region, and 2) change the relative importance of energy dissipation by bottom frictional drag and interior viscosity. The role of lee wave generation in eddy energy dissipation is investigated using a Lagrangian filter method. About one-third of the enhanced viscous energy dissipation in the rough topography experiment is associated with lee wave energy dissipation, with the remaining two-thirds explained by nonwave energy dissipation, at least partly as a result of the nonpropagating form drag effect.

Direct and Rectified Effects of Tropical Instability Waves on the Eastern Tropical Pacific Mean State in a Regional Ocean Model

Abstract Tropical instability waves (TIWs) are oceanic features propagating westward along the northern front of the Pacific cold tongue. Observational and modeling studies suggest that TIWs may have a large impact on the eastern tropical Pacific background state from seasonal to interannual time scales through heat advection and mixing. However, observations are coarse or limited to surface data, and modeling studies are often based on the comparison of low- versus high-resolution simulations. In this study, we perform a set of regional high-resolution ocean simulations (CROCO 1/12°) in which we strongly damp (NOTIWs-RUN) or not (TIWs-RUN) TIW propagation, by nudging meridional current velocities in the TIW region toward their monthly climatological values. This approach, while effectively removing TIW mesoscale activity, does not alter the model internal physics in particular related to the equatorial Kelvin wave dynamics. The impact of TIWs on the oceanic mean state is then assessed by comparing the two simulations. While the well-known direct effect of TIW heat advection is to weaken the meridional temperature gradient by warming up the cold tongue (0.34°C month−1), the rectified effect of TIWs onto the mean state attenuates this direct effect by cooling down the cold tongue (−0.10°C month−1). This rectified effect occurs through the TIW-induced deepening and weakening of the Equatorial Undercurrent, which subsequently modulates the mean zonal advection and counterbalances the TIWs’ direct effect. This approach allows quantifying the rectified effect of TIWs without degrading the model horizontal resolution and may lead to a better characterization of the eastern tropical Pacific mean state and to the development of TIW parameterizations in Earth system models. Significance Statement Tropical instability waves (TIWs), meandering features at the surface of the equatorial Pacific Ocean, have long been recognized as a key component of the climate system that can even impact marine ecosystems. Yet, they are still hardly simulated in coupled global climate models. Here, we introduce a new framework to isolate and quantify their complex influence on the tropical Pacific background climate. This approach allows revealing a so far overlooked effect of TIWs on the mean circulation and heat transport in this region that should be accounted for in the next generation of global coupled climate models through parameterization or increased resolution.

Using Machine Learning at scale in numerical simulations with SmartSim: An application to ocean climate modeling

We demonstrate the first climate-scale, numerical ocean simulations improved through distributed, online inference of Deep Neural Networks (DNN) using SmartSim. SmartSim is a library dedicated to enabling online analysis and Machine Learning (ML) for high performance, numerical simulations. In this paper, we detail the SmartSim architecture and provide benchmarks including online inference with a shared ML model, EKE-ResNet, on heterogeneous HPC systems. We demonstrate the capability of SmartSim by using it to run a 12-member ensemble of global-scale, high-resolution ocean simulations, each spanning 19 compute nodes, all communicating with the same ML architecture at each simulation timestep. In total, 970 billion inferences are collectively served by running the ensemble for a total of 120 simulated years. The inferences are used to predict the oceanic eddy kinetic energy (EKE), which is a variable that is used to tune different turbulence closures in the model and thus directly affects the simulation. The root-mean-square of the error in EKE (as compared to an eddy-resolving simulation) is 20% lower when using the ML-prediction than the previous state of the art. This demonstration is an example of how machine learning methods can be integrated into traditional numerical simulations, replace prognostic equations, and preserve overall simulation stability without significantly affecting the time to solution.

Tracking Irminger Rings’ properties using a sub-mesoscale ocean model

A 1/60° numerical simulation is carried out within the Labrador Sea to investigate eddies produced along the western coast of Greenland. These eddies, known as Irminger Rings, carry relatively buoyant water from the West Greenland Current system into the interior Labrador Sea. These eddies can survive for up to 2 years; we detect and track 232 eddies produced within our 14 year simulation to investigate how they evolve during their lifetime. Irminger Rings start with a significant layer of freshwater (median 4.4 m) that quickly erodes during the convective winter. The freshwater layer, as opposed to the warm Irminger Water layer, constitutes the majority of the stratification within each eddy. Eddies generally travel southwestwards after formation, and eddies whose trajectory is close to the continental slope tend to have a reduced lifespan and quicker speed than those which drift into the interior deep basin. We find that eddies which spawn further north are more likely to end up influenced by the boundary currents, while those which form to the south are more likely to live longer and enter the deep interior basin. While the formation rate of eddies is generally uniform across our 2005–2018 simulation, Irminger Rings are far more likely to decay during the convective wintertime.We find that most eddies quickly decay within a few months, although some survive long enough to endure two convective winters. All Irminger Rings increase the local stratification in the Labrador Sea, limiting convection. However, the eddies which endure some part of two winters experience a significant buoyancy loss over a long time span such that they may produce Labrador Sea Water within their core during their second winter. This constitutes a small but non-negligible volume of Labrador Sea Water (0.02 to 0.09 Sv) and updates our understanding of Irminger Ring’s role on stratifying the Labrador Sea.Plain language summary:The Labrador Sea, between Canada and Greenland, experiences deep convection, a process where the surface water cools and becomes denser than the water at depth. This forces the surface water to sink and mix with the water below, homogenizing the water column and producing deepwater, an important component in the distribution of heat between the equatorial and polar regions. Deep convection is also important in ventilating the deep ocean with oxygen and carbon dioxide, sequestering these gasses for thousands of years. Deepwater is strongly influenced by the local weather as well as the current systems which surround the Labrador Sea. Oceanic eddies are produced from these current systems and bring their water mass properties into the region where deepwater is formed. These eddies are relatively buoyant, hindering deepwater formation, but also survive up to 2 years before decaying. We use a high-resolution ocean simulation to resolve these eddies so we can further understand how they evolve after being formed. Our simulation suggests that these eddies decay faster during winter, although eddies which survived to experience two winters often experienced deep convection and the production of Labrador Sea Water. This paints the picture that these eddies not only significantly reduce deepwater formation, but can also act as local sources of deepwater formation.

Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4 °C, up to 5 °C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.

Coastal trapped waves propagation along the Southwestern Atlantic Continental Shelf

This study investigates the propagation of coastal trapped waves (CTWs) along the Brazilian continental shelf between 34°S and 11°S using in situ data combined with the outputs from a high-resolution ocean simulation with HYCOM. The CTWs generation area covers a wide region ranging from the Patagonian shelf to the southern Brazilian shelf. The spectral analysis of coastal sea level series between 54°S and 10.5°S shows three bands of high energy associated with periods from 5 to 12 days, 15–22 days, and 25–40 days. The energy of the CTWs decreases along their propagation for all frequency bands, showing a drastic reduction north of 22°S, due to abrupt variations in the width and depth of the continental shelf between Tubarão Bight and Abrolhos Bank. Their phase speed propagation varies along the coast, being faster (>25 m/s) in the southernmost region (between 42°S and 41°S), reaching ~11 m/s north of 41°S, and reducing to ~3 m/s further north (equatorward of 24°S). The free Continental Shelf Wave theory supports the notion that the intense deceleration north of 24°S can be explained by the narrowing of the continental shelf. The stratification parameter indicates that the Brazilian continental shelf has a barotropic response to wind-generated disturbances.

Modulation of Cross-Isothermal Velocities with ENSO in the Tropical Pacific Cold Tongue

AbstractThe tropical Pacific Ocean cold tongue (CT) plays a major role in the global climate system. The strength of the CT sets the zonal temperature gradient in the Pacific that couples with the atmospheric Walker circulation. This coupling is an essential component of the El Niño–Southern Oscillation (ENSO). The CT is supplied with cold water by the Equatorial Undercurrent that follows the thermocline as it shoals toward the east, adiabatically transporting cold water toward the surface. As the thermocline shoals, its water is transformed through diabatic processes, producing water mass transformation (WMT) that allows water to cross mean isotherms. Here, we examine WMT in the cold-tongue region from a global high-resolution ocean simulation with saved budget terms that close its heat budget exactly. Using the terms of the heat budget, we quantify each individual component of WMT (vertical mixing, horizontal mixing, eddy fluxes, and solar penetration) and find that vertical mixing is the single most important contribution in the thermocline and solar heating dominates close to the surface. Horizontal diffusion is much smaller. During El Niño events, vertical mixing, and hence cross-isothermal flow as a whole, are much reduced, whereas, during La Niña periods, strong vertical mixing leads to strong WMT, thereby cooling the surface. This analysis demonstrates the enhancement of diabatic processes during cold events, which in turn enhances cooling of the CT from below the surface.

Visualization of riverine water and vortex dynamics around the Naruto Strait based on high-resolution ocean simulation and satellite images

Visualization of riverine water and vortex dynamics around the Naruto Strait based on high-resolution ocean simulation and satellite images

Three-dimensional structure of mesoscale eddies in the western tropical Pacific as revealed by a high-resolution ocean simulation

The three-dimensional structure of mesoscale eddies in the western tropical Pacific (6°S–20°N, 120°E–150°E) is investigated using a high-resolution ocean model simulation. Eddy detection and eddy tracking algorithms are applied to simulated horizontal velocity vectors, and the anticyclonic and cyclonic eddies identified are composited to obtain their three-dimensional structures. The mean lifetime of all long-lived eddies is about 52 days, and their mean diameter is 147 km. Two typical characteristics of mesoscale eddies are revealed and possible dynamic explanations are analyzed. One typical characteristic is that surface eddies are generally separated from subthermocline eddies along the bifurcation latitude (~13°N) of the North Equatorial Current in the western tropical Pacific, which may be associated with different eddy energy sources and vertical eddy energy fluxes in subtropical and tropical gyres. Surface eddies have maximum swirl velocities of 8–9 cm s−1 and can extend to about 1500 m depth. Subthermocline eddies occur below 200 m, with their cores at about 400–600 m depth, and their maximum swirl velocities can reach 10 cm s−1. The other typical characteristic is that the meridional velocity component of the eddy is much larger than the zonal component. This characteristic might be due to more zonal eddy pairs (two eddies at the same latitude), which is also supported by the zonal wavelength (about 200 km) in the high-frequency meridional velocity component of the horizontal velocity.

The relationship between Arabian Sea upwelling and Indian Monsoon revisited in a high resolution ocean simulation

Studies based on sediment records, sea-surface temperature and wind suggest that upwelling along the western coast of Arabian Sea is strongly affected by the Indian summer Monsoon. We examine this relationship directly in an eddy-resolving global ocean simulation STORM driven by atmospheric reanalysis over the last 61 years. With its very high spatial resolution (10 km), STORM allows us to identify characteristics of the upwelling system. We analyse the co-variability between upwelling and meteorological and oceanic variables from 1950 to 2010. The analysis reveals high interannual correlations between coastal upwelling and along-shore wind-stress (r = 0.73) as well as with sea-surface temperature (r = −0.83). However, the correlation between the upwelling and the Monsoon is small. We find an atmospheric circulation pattern different from the one that drives the Monsoon as the main modulator of the upwelling variability. In spite of this, the patterns of temperature anomalies that are either linked to Arabian Sea upwelling or to the Monsoon are spatially quite similar, although the physical mechanisms of these links are different. In addition, no long-term trend is detected in our modelled upwelling in the Arabian Sea.

P-CSI v1.0, an accelerated barotropic solver for the high-resolution ocean model component in the Community Earth System Model v2.0

Abstract. In the Community Earth System Model (CESM), the ocean model is computationally expensive for high-resolution grids and is often the least scalable component for high-resolution production experiments. The major bottleneck is that the barotropic solver scales poorly at high core counts. We design a new barotropic solver to accelerate the high-resolution ocean simulation. The novel solver adopts a Chebyshev-type iterative method to reduce the global communication cost in conjunction with an effective block preconditioner to further reduce the iterations. The algorithm and its computational complexity are theoretically analyzed and compared with other existing methods. We confirm the significant reduction of the global communication time with a competitive convergence rate using a series of idealized tests. Numerical experiments using the CESM 0.1° global ocean model show that the proposed approach results in a factor of 1.7 speed-up over the original method with no loss of accuracy, achieving 10.5 simulated years per wall-clock day on 16 875 cores.

Decadal variability and trends of the Benguela upwelling system as simulated in a high-resolution ocean simulation

Abstract. Detecting the atmospheric drivers of the Benguela upwelling systems is essential to understand its present variability and its past and future changes. We present a statistical analysis of a high-resolution (0.1°) ocean-only simulation driven by observed atmospheric fields over the last 60 years with the aim of identifying the large-scale atmospheric drivers of upwelling variability and trends. The simulation is found to reproduce well the seasonal cycle of upwelling intensity, with a maximum in the June–August season in North Benguela and in the December–February season in South Benguela. The statistical analysis of the interannual variability of upwelling focuses on its relationship to atmospheric variables (sea level pressure, 10 m wind, wind stress). The relationship between upwelling and the atmospheric variables differ somewhat in the two regions, but generally the correlation patterns reflect the common atmospheric pattern favouring upwelling: southerly wind/wind stress, strong subtropical anticyclone, and an ocean–land sea level pressure gradient. In addition, the statistical link between upwelling and large-scale climate variability modes was analysed. The El Niño–Southern Oscillation and the Antarctic Oscillation exert some influence on austral summer upwelling velocities in South Benguela. The decadal evolution and the long-term trends of simulated upwelling and of ocean-minus-land air pressure gradient do not agree with Bakun's hypothesis that anthropogenic climate change should generally intensify coastal upwelling.

Ocean Mixed Layer responses to intense meteorological events during HyMeX-SOP1 from a high-resolution ocean simulation

The western Mediterranean Sea is a source of heat and humidity for the atmospheric low-levels in autumn. Large exchanges take place at the air–sea interface, especially during intense meteorological events, such as heavy precipitation and/or strong winds. The Ocean Mixed Layer (OML), which is quite thin at this time of year (∼20m-depth), evolves rapidly under such intense fluxes.This study investigates the ocean responses under intense meteorological events that occurred during HyMeX SOP1 (5 September–6 November 2012). The OML conditions and tendencies are derived from a high-resolution ocean simulation using the sub-regional eddy-resolving NEMO-WMED36 model (1/36°-resolution), driven at the surface by hourly air–sea fluxes from the AROME-WMED forecasts (2.5km-resolution). The high space–time resolution of the atmospheric forcing allows the highly variable surface fluxes, which induce rapid changes in the OML, to be well represented and linked to small-scale atmospheric processes.First, the simulation results are compared to ocean profiles from several platforms obtained during the campaign. Then, this study focuses on the short-term OML evolution during three events. In particular, we examine the OML cooling and mixing under strong wind events, potentially associated with upwelling, as well as the surface freshening under heavy precipitation events, producing low-salinity lenses. Tendencies demonstrate the major role of the surface forcing in the temperature and/or salinity anomaly formation. At the same time, mixing [restratification] rapidly occurs. As expected, the sign of this tendency term is very dependent on the local vertical stratification which varies at fine scale in the Mediterranean. It also controls [disables] the vertical propagation. In the Alboran Sea, the strong dynamics redistribute the OML anomalies, sometimes up to 7days after their formation. Elsewhere, despite local amplitude modulations due to internal wave excitation by strong winds, the integrated effect of the horizontal advection is almost null on the anomalies’ spread and decay. Finally, diffusion has a small contribution.

Interannual variations of the Hawaiian Lee Countercurrent induced by potential vorticity variability in the subsurface

Interannual variations of the Hawaiian Lee Countercurrent (HLCC) in the 2000s were investigated using satellite and Argo profiling float observations. The satellite-observed sea surface height shows that the geostrophic eastward current was anomalously strong to the west away from Hawaii in 2003 and 2005. However, the trade winds and the orographic wind curl dipole in the lee of Hawaii that drives the climatological mean HLCC were not particularly strong in these years, suggesting that the accelerations of the HLCC were not caused by the wind stress curl forcing around Hawaii and subsequent Rossby wave propagation. Using Argo observations, we found negative potential vorticity (PV) anomalies in the subsurface north of the HLCC in these 2 years. The pycnocline is lifted northward as low PV waters of different densities stack up in the vertical, and the HLCC is then accelerated via the thermal wind. The intensification and/or southward intrusion of the eastern subtropical mode water and subtropical mode water seem to have induced negative PV anomalies in 2003 and 2005, respectively. Using high-resolution ocean simulations, we confirmed the migrations of PV anomalies and their contributions to the HLCC accelerations. Although the HLCC is located away from the cores of major mode waters, our results suggest that interannual variations of the HLCC are affected by those of mode waters.

Formation of Subtropical Mode Water in a high-resolution ocean simulation of the Kuroshio Extension region

A high-resolution numerical model is used to examine the formation and variability of the North Pacific Subtropical Mode Water (STMW) over a 3-year period. The STMW distribution is found to be highly variable in both space and time, a characteristic often unexplored because of sparse observations or the use of coarse resolution simulations. Its distribution is highly dependent on eddies, and where it was renewed during the previous winter. Although the potential vorticity fluxes associated with down-front winds can be of the same order of magnitude or even greater than the diabatic ones due to air–sea temperature differences, the latter dominate the potential vorticity budget on regional and larger scales. Air–sea fluxes, however, are dominated by a few strong wind events, emphasizing the importance of short time scales in the formation of mode waters. In the Kuroshio Extension region, both advection and mixing play important roles to remove the STMW from the formation region.

Lagrangian flow in the California Undercurrent, an observation and model comparison

During the period 1992–1998, 38 isobaric RAFOS floats were deployed to sample the subsurface flow of the California Undercurrent. The deployments, released over the California continental slope west of San Francisco, have sampled robust year-round poleward subsurface flow associated with the Undercurrent most seasons and the combined inshore current and Undercurrent in winter. Two other types of flow have been seen: a region of weak flow with little net displacement just west of the California Undercurrent, and an active westward propagating eddy field. This eddy field appears to be the primary mechanism for moving floats from the Undercurrent into the ocean interior. The observations and statistics from the RAFOS floats are compared with Lagrangian estimates of particles tracked in a global high resolution ocean simulation in order to evaluate the fidelity of the model along an eastern boundary. The results show that the model reproduces the general character of the flow reasonably well, but underestimates both the mean and eddy energies by a substantial amount.