Ocean General Circulation Model Research Articles

Ecosystem relocation on Snowball Earth: Polar−alpine ancestry of the extant surface biosphere?

Geological observations informed by climate dynamics imply that the oceans were 99.9% covered by light-blocking ice shelves during two discrete, self-reversing Snowball Earth epochs spanning a combined 60 to 70 Myr of the Cryogenian Period (720 to 635 Ma). The timescale for initial ice advances across the tropical oceans is ~300 y in an ice−atmosphere−ocean general circulation model in Cryogenian paleogeography. Areas of optically thin oceanic ice are usually invoked to account for fossil marine phototrophs, including macroscopic multicellular eukaryotes, before and after each Snowball, but different taxa. Ecosystem relocation is a scenario that does not require thin marine ice. Assume that long before Cryogenian Snowballs, diverse supra- and periglacial biomes were established in polar−alpine regions. When the Snowball onsets occurred, those biomes migrated in step with their ice margins to the equatorial zone of net sublimation. There, they prospered and evolved, their habitat areas expanded, and the cruelty of winter reduced. Nutrients were supplied by dust (loess) derived from cozonal ablative lands where surface winds were strong. When each Snowball finally ended, those biomes were mostly inundated by the meltwater-dominated and rapidly warming lid of a nutrient-rich but depauperate ocean. Some taxa returned to the mountaintops while others restocked the oceans. This ecosystem relocation scenario makes testable predictions. The lineages required for post-Cryogenian biotic radiations should be present in modern polar−alpine biomes. Legacies of polar−alpine ancestry should be found in the genomes of living organisms. Examples of such tests are highlighted herein.

Submesoscale Effects on Formation of the North Pacific Subtropical Mode Water

Abstract The North Pacific subtropical mode water (NPSTMW) is a vertically homogeneous water mass located between the seasonal and permanent thermoclines in the subtropical northwestern Pacific Ocean. It plays a critical role in oceanic heat and carbon storage, significant for understanding climate change. However, coarse-resolution models tend to overestimate NPSTMW formation compared to observations. In this study, we show that the observed NPSTMW formation is well reproduced by a 1/30° submesoscale-permitting Ocean General Circulation Model. Our findings reveal that submesoscale restratification significantly reduces NPSTMW formation. During late winter to early spring—when wintertime convective cooling deepens the mixed layer to its annual maximum—submesoscale effects associated with mixed layer instability (MLI) are mostly energetic. The MLI induces surface buoyancy gain, shallowing the mixed layer, narrowing the outcrop zone, and reducing the ventilation time of NPSTMW. Additionally, submesoscale effects indirectly reduce the NPSTMW formation by modulating the large-scale deep mixed layer pattern in the Kuroshio Extension region. These results underscore the importance of incorporating submesoscale effects for an accurate estimation of the NPSTMW formation, with broader implications for improving climate models. Significance Statement This study evaluates the submesoscale effects on the formation of the North Pacific subtropical mode water (NPSTMW). This is important because the formation and associated strength of NPSTMW plays a significant role in shaping the North Pacific Ocean circulation and regulating the oceanic heat and carbon storage. Our results highlight that explicitly resolving submesoscale processes is essential for accurately simulating mode water. Alternatively, appropriate parameterization of submesoscale effects is needed for climate models, which are often run over hundreds if not thousands of years.

Yanai Waves in the Deep East Indian Ocean Observed with Seismic Ocean Tomography

Abstract Near-surface measurements of meridional velocity suggest that wind forcing excites equatorial waves in the biweekly band in the Indian Ocean. The characteristics of these waves in the deep ocean are poorly constrained, and it is unclear how well models capture the deep variability. In this work, biweekly temperature variations in a few low vertical modes in the deep east Indian Ocean are observed using seismically generated sound waves. These so-called T waves are generated by earthquakes off Sumatra and received by a hydrophone station off Diego Garcia. Changes in their travel times reflect temperature-induced sound speed variations in the intervening ocean. Regression analysis indicates that these variations are caused by westward-propagating Yanai waves. A comparison between T-wave data and model output shows generally good consistency in biweekly variations dominated by the first three vertical modes, although the biweekly variance differs by up to a factor of 2 between the data and the models. A similar degree of discrepancy appears in the comparison between the models and deep mooring measurements. These results highlight the potential of using T-wave data to study biweekly Yanai waves in the deep equatorial ocean and to calibrate numerical simulations of the variability they cause. Significance Statement Biweekly Yanai waves are an important mode of variability in the tropical Indian Ocean, affecting the Indian and Australian–Indonesian monsoons. This study examines biweekly Yanai waves in the east Indian Ocean using sound waves generated by natural repeating earthquakes. The sound waves sample the deep structure of Yanai waves, which has been poorly constrained by previous observations. The seismic data generally show good consistency with output from two ocean general circulation models, although quantitative differences are manifested and can help improve the representation of Yanai waves in numerical models.

A Model Analysis of Circumpolar Deep Water Intrusions on the Continental Shelf Break in Amundsen Sea, Antarctica

AbstractThe ice shelves of the Amundsen Sea are in a phase of rapid melting with intruded Circumpolar Deep Water (CDW) from outside the continental shelf contributing most of the heat. Using a coupled sea ice—ice shelf—ocean general circulation model, the cross‐shelf break heat flux and the mechanism of eastward undercurrent deflection are studied. Model results show higher cross‐shelf break heat transfer during winter months regulated by both the barotropic and baroclinic geostrophic flow. The vorticity budget along the continental shelf break is examined using the depth‐averaged vorticity budget equation based on the model's outputs. Results show that the advection of planetary vorticity (APV) and the joint effect of baroclinicity and relief (JEBAR) dominate the vorticity balance at the CDW intrusion sites on the shelf break, and the JEBAR effect is considered an effective indicator of CDW intrusion. The CDW intrusion is mainly regulated by the southward deflection of the undercurrent on the Amundsen Sea slope. Pre‐deflection, the undercurrent's core lies on the southern edge of the shelf break, enabling it to modulate downstream density transport through its vertical distribution variations. Concurrent increases in undercurrent velocities and vertical extent are captured upstream of intrusion sites, supporting more CDW intrusions per unit time and altering the horizontal density gradient, thereby amplifying the JEBAR effect. Additionally, spectral analysis reveals a semiannual cycle in the JEBAR amplitude and heat flux across the Amundsen Sea slope.

A regional NEMO 4.0 configuration of the subpolar North Atlantic

A regional NEMO 4.0 configuration of the subpolar North Atlantic

Freshwater input from glacier melt outside Greenland alters modeled northern high-latitude ocean circulation

Abstract. As anthropogenic climate change depletes Earth's ice reservoirs, large amounts of fresh water are released into the ocean. Since the ocean has a major influence on Earth's climate, understanding how the ocean changes in response to an increased freshwater input is crucial for understanding ongoing shifts in the climate system. Moreover, to comprehend the evolution of ice–ocean interactions, it is important to investigate if and how changes in the ocean might affect marine-terminating glaciers' stability. Though most attention in this context has been on freshwater input from Greenland, the other Northern Hemisphere glacierized regions are losing ice mass at a combined rate roughly half that of Greenland and should not be neglected. In order to get a first estimate of how glacier mass loss around the Arctic affects the ocean and how potential changes in the ocean circulation might affect marine-terminating glaciers, we conduct one-way coupled experiments with an ocean general circulation model (NEMO-ANHA4) and a glacier evolution model (Open Global Glacier Model; OGGM) for the years 2010 to 2019. We find an increase in the heat content of Baffin Bay due to an enhanced gyre circulation that leads to an increased heat transport through Davis Strait. We also find changes in the subpolar gyre's structure: an increase in density and a decrease in sea surface height in the eastern part and vice versa in the western part. Additionally, we find a decreased heat transport into the Barents Sea due to increased freshwater input from Svalbard and the Russian Arctic. The rerouting of Atlantic water from the Barents Sea Opening through Fram Strait leads to an increased heat transport into the Arctic Ocean and a decrease in sea ice thickness in the Fram Strait area.

Sensitivity of the tropical Atlantic to vertical mixing in two ocean models (ICON-O v2.6.6 and FESOM v2.5)

Abstract. Ocean general circulation models still have large upper-ocean biases, including in tropical sea surface temperature, that are possibly connected to the representation of vertical mixing. In earlier studies, the ocean vertical mixing parameterization has usually been tuned for a specific site or only within a specific model. We present here a systematic comparison of the effects of changes in the vertical mixing scheme in two different global ocean models, ICON-O and FESOM, run at a horizontal resolution of 10 km in the tropical Atlantic. We test two commonly used vertical mixing schemes: the K-profile parameterization (KPP) and the turbulent kinetic energy (TKE) scheme. Additionally, we vary tuning parameters in both schemes and test the addition of Langmuir turbulence in the TKE scheme. We show that the biases of mean sea surface temperature, subsurface temperature, subsurface currents, and mixed layer depth differ more between the two models than between runs with different mixing scheme settings within each model. For ICON-O, there is a larger difference between TKE and KPP than for FESOM. In both models, varying the tuning parameters hardly affects the pattern and magnitude of the mean state biases. For the representation of smaller-scale variability like the diurnal cycle or inertial waves, the choice of the mixing scheme can matter: the diurnally enhanced penetration of equatorial turbulence below the mixed layer is only simulated with TKE, not with KPP. However, tuning of the parameters within the mixing schemes does not lead to large improvements for these processes. We conclude that a substantial part of the upper-ocean tropical Atlantic biases is not sensitive to details of the vertical mixing scheme.

The Weakening and Poleward Migration of the Southeastern Front Associated With the South Indian Countercurrent From 1980 to 2023

AbstractThe Southeast Indian Subantarctic Mode Water (SEISAMW), a thick water layer located between the seasonal and permanent pycnoclines, is important for climate variability in the Southern Ocean. However, its impact on long‐term changes of the Southeastern Front (SEF) associated with the South Indian Countercurrent has not been clearly revealed. By analyzing data from observations, objective products and an eddy resolving ocean general circulation model, the present study shows that the SEISAMW has shifted poleward (0.56° ± 0.12°) and weakened from 1980 to 2023, which causes the poleward migration (0.52° ± 0.12°) and weakening of the SEF in recent 44 years. The SEISAMW changes are associated with a significant shallowing and poleward shifting trend of the austral winter deep mixed layer in the Southeast Indian Ocean, as well as the poleward‐shifted SEISAMW's outcrop lines, all of which are mainly driven by the poleward migrated westerlies in recent decades.

Four Generations of ECMWF Reanalyses: An Overview of the Successes in Modeling Precipitation and Remaining Challenges for Freshwater Budget of Ocean Models

AbstractThis study reviews the progress made in modeling precipitations in four generations of reanalyses from the European Center for Medium‐Range Weather Forecasts, using traditional metrics and a new set of regional metrics. Regional metrics at oceanic basin scales and large land catchment areas over the continents allow for a more comprehensive analysis of the performance of the reanalyses. This leads to the conclusion that significant progress has been made in the past several decades in both the atmospheric model and the assimilation system at the ECMWF, leading to more realistic precipitation. The most recent ERA5 reanalysis outperforms ERA‐Interim and its predecessors by all metrics considered. ERA5 is then used to force a modern ocean general circulation model, and the results show an improvement in terms of the freshwater budget, particularly after the year 2000. However, uncertainties remain about the magnitude and trends of the modeled evaporation.

Marine environment around the Tokara Islands based on CTD observations and ocean general circulation model

Marine environment around the Tokara Islands based on CTD observations and ocean general circulation model

Melancholia states of the Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC) is a much studied component of the climate system, because its suspected multistability is associated with tipping behavior yielding potentially large regional and global climatic impacts. In this paper we investigate the global stability properties of the system using an ocean general circulation model. We construct an unstable AMOC state, i.e., an unstable solution of the flow that resides between the stable regimes of a vigorous and collapsed AMOC. Such a solution, also known as a melancholia or edge state, is a dynamical saddle embedded in the boundary separating the competing basins of attraction. It is physically relevant since it lies on the most probable path of a noise-induced transition between the two stable regimes, and because tipping occurs when one of the attractors and the melancholia state collide. Its properties may thus give hints towards physical mechanisms and predictability of the critical transition. We find that while the AMOC melancholia state as viewed from its upper ocean properties lies between the vigorous and collapsed regimes, it is characterized by an Atlantic deep ocean that is fresher and colder compared to both stable regimes. The melancholia state has higher dynamic enthalpy than either stable state, representing a state of higher potential energy that is in good agreement with the dynamical landscape view on metastability. Published by the American Physical Society 2024

Physical oceanographic factors controlling the ocean circulation-induced magnetic field.

Oceanic tidal constituents and depth-integrated electrical conductivity (ocean conductivity content, or OCC) extracted from electromagnetic (EM) field data are known to have a strong potential for monitoring ocean heat content, which reflects the Earth's energy imbalance. In comparison to ocean tide models, realistic ocean general circulation models have a greater need to be baroclinic; therefore, both OCC and depth-integrated conductivity-weighted velocity ([Formula: see text]) data are required to calculate the ocean circulation-induced magnetic field (OCIMF). Owing to a lack of [Formula: see text] observations, we calculate the OCIMF using an ocean state estimate. There are significant trends in the OCIMF primarily owing to responses in the velocities to external forcings and the warming influence on OCC between 1993 and 2017, particularly in the Southern Ocean. Despite being depth-integrated quantities, OCC and [Formula: see text] (which primarily determine the OCIMF in an idealized EM model) can provide a strong constraint on the baroclinic velocities and ocean mixing parameters when assimilated into an ocean state estimation framework. A hypothetical fleet of full-depth EM-capable floats would therefore help improve the accuracy of the OCIMF computed with an ocean state estimate, which could potentially provide valuable guidance on how to extract the OCIMF from satellite magnetometry observations.This article is part of the theme issue 'Magnetometric remote sensing of Earth and planetary oceans'.

Dissipation Scaled Internal Wave Drag in a Global Heterogeneously Coupled Internal/External Mode Total Water Level Model

AbstractThis study showcases a global, heterogeneously coupled total water level system wherein salinity and temperature outputs from a coarser‐resolution (12 km) ocean general circulation model are used to calculate density‐driven terms within a global, higher‐resolution (2.5 km) depth‐averaged total water level model. We demonstrate that the inclusion of baroclinic forcing in the barotropic model requires modification of the internal wave drag term to prevent excess degradation of tidal results compared to the barotropic model. By scaling the internal tide dissipation by an easy to calculate dissipation ratio, the resulting heterogeneously coupled model has complex root mean square errors (RMSE) of 2.27 cm in the deep ocean and 12.16 cm in shallow waters for the tidal constituent. While this represents a 10%–20% deterioration as compared to the barotropic model, the improvements in total water level prediction more than offset this degradation. Global median RMSE compared to observations of total water levels, 30‐day sea levels, and non‐tidal residuals improve by 1.86 (18.5%), 2.55 (42.5%), and 0.36 (5.3%) cm respectively. The drastic improvement in model performance highlights the importance of including density‐driven effects within global hydrodynamic models and will help to improve the results of both hindcasts and forecasts in modeling extreme and nuisance flooding. With only an 11% increase in model run time compared to the fully barotropic total water level model, this approach paves the way for high resolution coastal water level and flood models to be used alongside climate models, improving operational forecasting of total water levels.

ISOM 1.0: a fully mesoscale-resolving idealized Southern Ocean model and the diversity of multiscale eddy interactions

Abstract. We describe an idealized Southern Ocean model (ISOM 1.0) that contains simplified iconic topographic features in the Southern Ocean and conduct a fully mesoscale-resolving simulation with the horizontal resolution of 2 km, based on the Massachusetts Institute of Technology general circulation model. The model obtains a fully developed and vigorous mesoscale eddying field with a k−3 eddy kinetic energy spectrum and captures the topographic effect on stratification and large-scale flow. To make a natural introduction of large eddy simulation (LES) methods to ocean mesoscale parameterization, we propose the concept of mesoscale ocean direct numerical simulation (MODNS). A qualified MODNS dataset should resolve the first baroclinic deformation radius and ensure that the affected scales by the dissipation schemes are sufficiently smaller than the radius. Such datasets can serve as the benchmark for a priori and a posteriori tests of LES schemes or mesoscale ocean large eddy simulation (MOLES) methods in ocean general circulation models. The 2 km simulation can meet the requirement of MODNS and also capture submesoscale effects. Therefore, its output can be a type of MODNS and provide reliable data support for relevant a priori and a posteriori tests. We demonstrate the diversity of multiscale eddy interactions, validate the crucial role of mesoscale-related strain in submesoscale processes, and uncover the bridge effect of submesoscale processes between mesoscale entities and in the eddy–jet interaction. In addition, we use the model to conduct multipassive tracer experiments and reveal guidelines for the initial settings of passive tracers to delay the homogenization process and ensure the mutual independence of tracers over a long period.

Modeling water availability under climate change scenarios: a systemic approach in the metropolitan area in Morelos, México

The research investigates integrated water management with complex socio-environmental interactions in worsening climate change scenarios. The proposed methodology involves the integration of Atmosphere–Ocean General Circulation Models (AOGCM) with Water Evaluation and Planning models (WEAP) to ensure water availability in the future. Two Shared Socioeconomic Pathways (SSP) integrate environmental, socioeconomic, and political elements under climate change scenarios with greater rainfall variability, extreme droughts, and floods in the metropolitan area of Morelos, located in the Center of México. Water availability in this region, along with population growth, industrialization, service activities, and agriculture, depends on forest conservation in the Ravines System of Northwest Morelos (RSNM). Public policy lacks interdisciplinary socio-environmental development, which prioritizes unsustainable economic growth overexploiting aquifers and polluting rivers. Official data from national and state governments do not reflect water conditions, and aquifer statistics date back decades. The majority of the analyzed models predict a delay in the monsoon, higher temperatures, extreme climate events, depletion of groundwater, and severe water scarcity during the hot months, rendering them unable to meet the increasing demand. This research provides valuable insights into the complex socioeconomic dynamics of a region with future water scarcity, which could be useful for similar conditions in the Global South.

Dynamic projections of extreme sea levels for western Europe based on ocean and wind-wave modelling

Abstract. Extreme sea levels (ESLs) are a major threat for low-lying coastal zones. Climate-change-induced sea level rise (SLR) will increase the frequency of ESLs. In this study, ocean and wind-wave regional simulations are used to produce dynamic projections of ESLs along the western European coastlines. Through a consistent modelling approach, the different contributions to ESLs, such as tides, storm surges, waves, and regionalized mean SLR, as well as most of their non-linear interactions, are included. This study aims at assessing the impact of dynamically simulating future changes in ESL drivers compared to a static approach that does not consider the impact of climate change on ESL distribution. Projected changes in ESLs are analysed using non-stationary extreme value analyses over the whole 1970–2100 period under the SSP5-8.5 and SSP1-2.6 scenarios. The impact of simulating dynamic changes in extremes is found to be statistically significant in the Mediterranean Sea, with differences in the decennial return level of up to +20 % compared to the static approach. This is attributed to the refined mean SLR simulated by the regional ocean general circulation model. In other parts of our region, we observed compensating projected changes between coastal ESL drivers, along with differences in timing among these drivers. This results in future changes in ESLs being primarily driven by mean SLR from the global climate model used as boundary conditions, with coastal contributions having a second-order effect, in line with previous research.

Enhanced eddy activity along the Subantarctic Front under intensified westerly winds

The westerlies in the southern hemisphere have intensified and shifted southward since the middle of the twentieth century. Previous studies have indicated that the expected increase in isopycnal slopes and acceleration of the Antarctic Circumpolar Current (ACC) is considerably weakened by the strengthening of mesoscale eddies and that this “eddy saturation” occurs mainly downstream of the major bottom topographic features such as the Kerguelen Plateau. Such eddy “hotspots” are thus considered to regulate the ACC responses to changes in external forcing. To improve our understanding of the ACC response to intensified winds, a sensitivity study is conducted using an eddy-resolving quasi-global ocean general circulation model named “OFES.” The reference run is driven by a climatological atmospheric forcing and the sensitivity run is driven by artificially intensified climatological westerlies. Our new finding is that the baroclinic energy pathway is enhanced over the Subantarctic Front (SAF) as well as over the hotspots identified by previous studies. A linear stability analysis indicates that the spin-up of the subtropical gyres north of the SAF and the enhanced Ekman upwelling south of the SAF by the intensified wind stress curl increase the vertical shear of zonal velocity along the SAF, enhancing baroclinic instability. We have also performed the same stability analysis comparing the 1985–2018 and 1955–1984 periods of a hindcast run of OFES, confirming the result from the climatological sensitivity study. These results suggest that the SAF is another eddy hotspot when the wind stress curl keeps increasing.

A Study of the Mixed Layer Warming Induced by the Barrier Layer in the Northern Bay of Bengal in 2013

Strong salinity stratification induced by large freshwater fluxes in the northern Bay of Bengal (BOB) results in the formation of a quasi-permanent barrier layer (BL) that covers almost the entire BOB and leads to a unique temperature inversion within the thick BL in winter. In the presence of temperature inversions, the entrainment process at the bottom of the mixed layer (ML) induces warming effects in the ML, but little is known about this. In this paper, we quantify the contribution of the entrainment process to the ML temperature (MLT) in the northern BOB during the winter of 2013 using monthly and daily data from the Ocean General Circulation Model for the Earth Simulator version 2 (OFES2). It is found that the warming effect of the daily entrainment heat flux (EHF), which resolved the high-frequency variations, is 4 orders of magnitude larger than the monthly EHF for most of the wintertime. This significantly enhanced warming effect in daily data offsets up to 87% of the surface cooling induced by net heat flux during wintertime. A further analysis reveals that the larger daily EHF warming effect compared to its monthly counterpart is closely related to the deepened ML, the larger temperature difference within the ML and vertical velocity at the bottom of the ML.

The Sensitivity of the Spatial Pattern of Sea Level Changes to the Depth of Antarctic Meltwater Fluxes

AbstractRegional patterns of sea level rise are affected by a range of factors including glacial melting, which has occurred in recent decades and is projected to increase in the future, perhaps dramatically. Previous modeling studies have typically included fluxes from melting glacial ice only as a surface forcing of the ocean or as an offline addition to the sea surface height fields produced by climate models. However, observational estimates suggest that the majority of the meltwater from the Antarctic Ice Sheet actually enters the ocean at depth through ice shelf basal melt. Here we use simulations with an ocean general circulation model in an idealized configuration. The results show that the simulated global sea level change pattern is sensitive to the depth at which Antarctic meltwater enters the ocean. Further analysis suggests that the response is dictated primarily by the steric response to the depth of the meltwater flux.

Vertical Velocity 3D Estimates from the Linear Vorticity Balance in the North Atlantic Ocean

Abstract Large-scale three-dimensional vertical movements of the world’s oceans play a fundamental role in their functioning, yet they are still overlooked. This study aims to enhance our understanding of the relationship between horizontal and vertical flows as a function of depth to derive some major characteristics of the w field from a relatively well-constrained variable. For this purpose, we analyze an eddy-permitting ocean general circulation model simulation, focusing on the linear vorticity balance (LVB) over the North Atlantic basin. For spatial scales greater than 5°, over more than 50-yr average, the LVB holds with less than 10% error over the whole depth range of most of the subtropical upper tachocline–upper ocean and part of the tropical one, away from western boundary currents and zonal tropical flows. The spatial extent of the balance diminishes in the intermediate ocean, particularly in the western half of the basin. Within the deep tachocline, the balance holds over large portions of the abyssal plains. Based on these results, the β-plane geostrophic vertical velocity is derived from the vertical indefinite integral of the geostrophic LVB from the surface. While its time mean aligns only qualitatively with the model reference w, it accurately captures the interannual variability over most of the basin, even close to eastern boundaries. However, it cannot reproduce the variability in the lower tropics, the western boundary, and the North Atlantic Current system. Our results emphasize the dominance of geostrophy in establishing most of the ocean model vertical flow variability.