Mesoscale eddies dominate the ocean kinetic energy reservoir. However, how and where this energy flows out from the mesoscale remains uncertain. Here, a simplified mesoscale energy budget is used ...where sources due to baroclinic instability are balanced by all the dissipative processes approximated as a linear damping rate. In this simple model, the eddy kinetic energy (EKE) dissipation is computed from a climatological mean field of density and satellite altimeter data, and is proportional to an eddy efficiency parameter α. Assuming an eddy efficiency of α = 0.1, we find a global EKE dissipation rate of 0.66 ± 0.19 TW. The results show an intense dissipation near western boundary currents and in the Antarctic Circumpolar Current, where both large levels of energy and baroclinic conversion occur. The resulting geographical distribution of the dissipation rate brings new insights for closing the ocean kinetic energy budget, as well as constraining future mesoscale parameterizations and associated mixing processes.
Plain Language Summary
The ocean is home to abundant and large swirls from tens to hundreds of kilometers, called “mesoscale eddies.” These eddies contain more momentum than most ocean currents and can thus impact the climate evolution. There are now good reasons to believe the effect of mesoscale eddies is directly related to their strength, and so to their kinetic energy. However, how the energy is removed from these eddies is still unclear mostly due to instrumental and theoretical limitations. In this work, a simplification of the eddy energetic behavior is used to indirectly estimate the dissipation from observations of temperature, salinity and surface currents. Our results confirm intensified dissipation near strong ocean currents and hence constitute a new attempt for the global reconstruction of the eddy kinetic energy dissipation in the world ocean. The work presented here is consistent and complementary to other studies and can help us to understand the ocean energy cycle.
Key Points
Global mesoscale eddy kinetic energy dissipation rate estimated to 0.66 ± 0.19 TW from observation‐based and statistically analyzed data sets
More than 25% of the total dissipation occurs in the western boundary currents and 38% is found in the Antarctic Circumpolar Current
Estimation of the eddy dissipation timescale from observations to inform future parameterization developments
The barotropic vorticity (BV) balance is fundamental when interpreting the ocean gyre circulation. Here we propose an intercomparison of vorticity equations for the depth‐integrated flow applied to ...ocean models. We review four distinct variants of the BV balances, each giving access to diagnostic equations for the depth‐integrated ocean circulation, either meridional, across geostrophic contours or its divergence. We then formulate those balances in the Vorticity Balances in NEMO (VoBiN) diagnostic package aimed at the NEMO ocean platform and more generally C‐grid ocean models. We show that spatial discretization of the equations of motion have profound implications for those vorticity balances. Finally, we diagnose the main balances of a global ocean climate simulation. In all vorticity balances, topographic torques arise from interactions of the flow with slanting topography. We identify significant spurious topographic torques related to the model's C‐grid discretizations, and we suggest ways to address them. In the depth‐integrated and BV balances, bottom vortex stretching and bottom pressure torque drive the flow interaction with topography, respectively. Contrary to Sverdrup theory, the wind stress curl, although dominant in the interior Subtropics, becomes a minor player anywhere significant bottom velocities prevail. The geostrophic contour vorticity balance highlights the limits of barotropic models of the ocean circulation through the so‐called JEBAR term. Finally, the transport divergence vorticity balance stresses the limitations of Ekman plus geostrophic dynamics for the mass balance closure in ocean models. This framework should encourage ocean modellers to diagnose more routinely momentum and vorticity equations.
Plain Language Summary
Ocean gyre theories involve the key role played by the wind variations across latitudes to force an interior flow. However, recent work has put forward the role played by bottom topography as a guide, or an obstacle, to the gyre circulation. The general framework employed in those theories is the so‐called barotropic vorticity equation involving the balancing of the spin induced by the Earth rotation in an ocean in motion. This work proposes a review on oceanic vorticity balances that synthesizes the informations that they provide about the ocean circulation. We then apply this framework to an ocean model used for climate projections. Our analysis confirms the key role played by interactions with topography in driving the gyre circulation. The ocean bottom topography is a geological constant, so that it constitutes a long‐lasting constraint for the circulation. Finally, we also stress the large effect of the ocean circulation formulation in the computer programs that make up an ocean model and should be designed carefully.
Key Points
The large‐scale ocean circulation includes four distinct vorticity balances for the depth‐integrated flow
Those vorticity balances are discretized in the VoBiN diagnostic module designed for C‐grid ocean models such as the NEMO platform
The barotropic vorticity equation of a global NEMO climate simulation is dominated by Sverdrup and topographic balances
For more than five decades, the Mediterranean Sea has been identified as a region of so‐called thermohaline circulation, namely, of basin‐scale overturning driven by surface heat and freshwater ...exchanges. The commonly accepted view is that of an interaction of zonal and meridional conveyor belts that sink at intermediate or deep convection sites. However, the connection between convection and sinking in the overturning circulation remains unclear. Here we use a multidecadal eddy‐permitting numerical simulation and glider transport measurements to diagnose the location and physical drivers of this sinking. We find that most of the net sinking occurs within 50 km of the boundary, away from open sea convection sites. Vorticity dynamics provides the physical rationale for this sinking near topography: only dissipation at the boundary is able to balance the vortex stretching induced by any net sinking, which is hence prevented in the open ocean. These findings corroborate previous idealized studies and conceptually replace the historical offshore conveyor belts by boundary sinking rings. They challenge the respective roles of convection and sinking in shaping the oceanic overturning circulation and confirm the key role of boundary currents in ventilating the interior ocean.
Plain Language Summary
The oceanic thermohaline or overturning circulation is a global circulation that ventilates the deep ocean, that is, the bulk of the global oceanic volume. It has been historically represented as a so‐called conveyor belt that sinks at deep convection sites. Those areas are known to undergo intense vertical exchanges as a result of surface cooling and to determine the physical properties of deep waters. However, because of the Earth's rotation, the ocean can hardly sink far from the coasts, which questions the commonly accepted equivalence between convection and sinking. In this study, we focus on the Mediterranean Sea which displays an overturning circulation and we use both a numerical model and observations to address where and why this sinking occurs. We find that indeed, little to no sinking takes place at convection sites, whereas boundary currents undergo intense sinking within 50 km of the coast. Our physical analysis confirms that it is due to the Earth's rotation prohibiting any significant sinking away from the coast. Hence, we propose to view the thermohaline circulation as sinking rings of boundary currents that deepen along their path rather than conveyor belts that sink offshore.
Key Points
Because of the Earth's rotation, the sinking of the overturning circulation occurs near the boundaries, away from open sea convection sites
An estimation of sinking is derived from the observed deepening and barotropization of the main northwestern Mediterranean boundary current
We update the conceptual view of the Mediterranean overturning circulation from offshore conveyor belts to boundary sinking rings
The Strait of Gibraltar is a narrow and shallow channel that controls the Mediterranean Sea thermohaline and biogeochemical balances. Strong tidal currents significantly modulate exchanges across ...this strait and induce an intense vertical mixing, impacting both the Mediterranean Sea and the Atlantic Ocean on a climatic scale. However, the turbulent processes controlling the tidal mixing location, timing, and magnitude remain unclear. To fill this gap, we investigate tidal mixing at the Strait of Gibraltar in yearly twin tidal and non-tidal simulations from a regional configuration of the three-dimensional numerical model MITgcm, using a high spatial resolution around the Strait of Gibraltar (1/200°, 100 vertical levels). More specifically, we investigate the model turbulence closure scheme, based on a turbulent kinetic energy budget, and illustrate that vertical buoyancy fluxes should be preferred to diapycnal diffusivities as mixing indicators. In agreement with previous literature, we find that tides strongly intensify vertical mixing and motions within the Strait of Gibraltar. We then demonstrate that tidal mixing relies on two main ingredients: a sustained vertical shear of horizontal velocities and a local weakening of stratification. In the Mediterranean layer, the former drives diapycnal mixing near the seafloor and the latter in shallower areas above the prominent sills of the strait. We also evidence the frequent but irregular occurrences of static instabilities in the vicinity of these sills. In the Atlantic layer, both vertical shear and stratification are involved in diapycnal mixing that develops along the trail of the eastward internal bore released at the Camarinal sill. At high frequency, the local weakening of stratification results from convergence and divergence patterns in the vicinity of the Camarinal and Espartel sills, feeding recirculation cells between the Atlantic and Mediterranean layers. In addition, we highlight that diapycnal mixing mainly develops during the westward tidal phase in the Mediterranean layer and the eastward tidal phase in the Atlantic layer. We conclude by proposing a revised conceptual view of tidal mixing at the Strait of Gibraltar, where tidally-induced recirculation cells play an instrumental role in transforming the exchanged water masses. Overall, this study emphasizes the relevance of a realistic representation of both tides and abrupt topography to simulate the exchanges through the Strait of Gibraltar and argues for the use of a specific tidal mixing parameterization otherwise.
•Tidal mixing mechanisms are investigated in a highresolution simulation of the Strait of Gibraltar.•Tidal convergence and divergence patterns, that feed recirculation cells, play an instrumental role in enhanced mixing at the Strait of Gibraltar.•Tides drive diapycnal mixing by enhancing vertical shear and weakening vertical stratification.•Accurate representation of both tides and abrupt topography is key for representing exchanges through the Strait of Gibraltar.
Abstract
This work aims to clarify the relation between the Atlantic meridional overturning circulation (AMOC) and the thermal wind. We derive a new and generic dynamical AMOC decomposition that ...expresses the thermal wind transport as a simple vertical integral function of eastern minus western boundary densities. This allows us to express density anomalies at any depth as a geostrophic transport in Sverdrups (1 Sv ≡ 10
6
m
3
s
−1
) per meter and to predict that density anomalies around the depth of maximum overturning induce most AMOC transport. We then apply this formalism to identify the dynamical drivers of the centennial AMOC variability in the CNRM-CM6 climate model. The dynamical reconstruction and specifically the thermal wind component explain over 80% of the low-frequency AMOC variance at all latitudes, which is therefore almost exclusively driven by density anomalies at both zonal boundaries. This transport variability is dominated by density anomalies between depths of 500 and 1500 m, in agreement with theoretical predictions. At those depths, southward-propagating western boundary temperature anomalies induce the centennial geostrophic AMOC transport variability in the North Atlantic. They are originated along the western boundary of the subpolar gyre through the Labrador Sea deep convection and the Davis Strait overflow.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This paper describes the main characteristics of CNRM‐CM6‐1, the fully coupled atmosphere‐ocean general circulation model of sixth generation jointly developed by Centre National de Recherches ...Météorologiques (CNRM) and Cerfacs for the sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6). The paper provides a description of each component of CNRM‐CM6‐1, including the coupling method and the new online output software. We emphasize where model's components have been updated with respect to the former model version, CNRM‐CM5.1. In particular, we highlight major improvements in the representation of atmospheric and land processes. A particular attention has also been devoted to mass and energy conservation in the simulated climate system to limit long‐term drifts. The climate simulated by CNRM‐CM6‐1 is then evaluated using CMIP6 historical and Diagnostic, Evaluation and Characterization of Klima (DECK) experiments in comparison with CMIP5 CNRM‐CM5.1 equivalent experiments. Overall, the mean surface biases are of similar magnitude but with different spatial patterns. Deep ocean biases are generally reduced, whereas sea ice is too thin in the Arctic. Although the simulated climate variability remains roughly consistent with CNRM‐CM5.1, its sensitivity to rising CO2 has increased: the equilibrium climate sensitivity is 4.9 K, which is now close to the upper bound of the range estimated from CMIP5 models.
Key Points
Description of CNRM‐CM6‐1 model components, their coupling, and tuning procedures are described
Historical simulations and DECK experiments are assessed
Preindustrial simulation is stable and mean climate and variability in historical runs is realistic
This study introduces CNRM‐ESM2‐1, the Earth system (ES) model of second generation developed by CNRM‐CERFACS for the sixth phase of the Coupled Model Intercomparison Project (CMIP6). CNRM‐ESM2‐1 ...offers a higher model complexity than the Atmosphere‐Ocean General Circulation Model CNRM‐CM6‐1 by adding interactive ES components such as carbon cycle, aerosols, and atmospheric chemistry. As both models share the same code, physical parameterizations, and grid resolution, they offer a fully traceable framework to investigate how far the represented ES processes impact the model performance over present‐day, response to external forcing and future climate projections. Using a large variety of CMIP6 experiments, we show that represented ES processes impact more prominently the model response to external forcing than the model performance over present‐day. Both models display comparable performance at replicating modern observations although the mean climate of CNRM‐ESM2‐1 is slightly warmer than that of CNRM‐CM6‐1. This difference arises from land cover‐aerosol interactions where the use of different soil vegetation distributions between both models impacts the rate of dust emissions. This interaction results in a smaller aerosol burden in CNRM‐ESM2‐1 than in CNRM‐CM6‐1, leading to a different surface radiative budget and climate. Greater differences are found when comparing the model response to external forcing and future climate projections. Represented ES processes damp future warming by up to 10% in CNRM‐ESM2‐1 with respect to CNRM‐CM6‐1. The representation of land vegetation and the CO2‐water‐stomatal feedback between both models explain about 60% of this difference. The remainder is driven by other ES feedbacks such as the natural aerosol feedback.
Key Points
This study introduces CNRM‐ESM2‐1 and describes its set‐up for CMIP6
Represented Earth system processes further impact the model response to external forcing than the model performance over present‐day
Represented Earth system processes damp future warming by up to 10%
Observing, modelling and understanding the climate-scale variability of the deep water formation (DWF) in the North-Western Mediterranean Sea remains today very challenging. In this study, we first ...characterize the interannual variability of this phenomenon by a thorough reanalysis of observations in order to establish reference time series. These quantitative indicators include 31 observed years for the yearly maximum mixed layer depth over the period 1980–2013 and a detailed multi-indicator description of the period 2007–2013. Then a 1980–2013 hindcast simulation is performed with a fully-coupled regional climate system model including the high-resolution representation of the regional atmosphere, ocean, land-surface and rivers. The simulation reproduces quantitatively well the mean behaviour and the large interannual variability of the DWF phenomenon. The model shows convection deeper than 1000 m in 2/3 of the modelled winters, a mean DWF rate equal to 0.35 Sv with maximum values of 1.7 (resp. 1.6) Sv in 2013 (resp. 2005). Using the model results, the winter-integrated buoyancy loss over the Gulf of Lions is identified as the primary driving factor of the DWF interannual variability and explains, alone, around 50 % of its variance. It is itself explained by the occurrence of few stormy days during winter. At daily scale, the Atlantic ridge weather regime is identified as favourable to strong buoyancy losses and therefore DWF, whereas the positive phase of the North Atlantic oscillation is unfavourable. The driving role of the vertical stratification in autumn, a measure of the water column inhibition to mixing, has also been analyzed. Combining both driving factors allows to explain more than 70 % of the interannual variance of the phenomenon and in particular the occurrence of the five strongest convective years of the model (1981, 1999, 2005, 2009, 2013). The model simulates qualitatively well the trends in the deep waters (warming, saltening, increase in the dense water volume, increase in the bottom water density) despite an underestimation of the salinity and density trends. These deep trends come from a heat and salt accumulation during the 1980s and the 1990s in the surface and intermediate layers of the Gulf of Lions before being transferred stepwise towards the deep layers when very convective years occur in 1999 and later. The salinity increase in the near Atlantic Ocean surface layers seems to be the external forcing that finally leads to these deep trends. In the future, our results may allow to better understand the behaviour of the DWF phenomenon in Mediterranean Sea simulations in hindcast, forecast, reanalysis or future climate change scenario modes. The robustness of the obtained results must be however confirmed in multi-model studies.
Chaotic intrinsic variability is a fundamental driver of the oceanic variability. Its understanding is key to interpret observations, evaluate numerical models, and predict the future ocean and ...climate. Here we study intrinsic variability of deep convection in the northwestern Mediterranean Sea using an ensemble eddy‐resolving hindcast simulation over the period 1979–2013. We find that the variability of deep convection is mostly forced but also, to a considerable extent, intrinsic. The intrinsic variability can dominate the total convection variability locally and over a single winter. It also makes up a significant fraction of its interannual variability but has only modest impacts on the long‐term mean state. We find that the occurrence of deep convection is random 18% of years at the basin scale, and 29% locally at the LION observational site. Spatially, the intrinsic variability is highest far from the continental shelf. We relate this pattern to baroclinic instability theory that takes bottom stabilization into account.
Key Points
The chronology of deep convection is largely random due to the Intrinsic Ocean Variability arising from a mesoscale‐eddying ocean
The spatial patterns of intrinsic variability can be explained by the effect of bottom topography on baroclinic instability
Our results support the paradigm change from deterministic to probabilistic oceanography
The summer of 2022 was memorable and record-breaking, ranking as the second hottest summer in France since 1900, with a seasonal surface air temperature average of 22.7 ∘C. In particular, France ...experienced multiple record-breaking heatwaves during the meteorological summer. As the main heat reservoir of the Earth system, the oceans are at the forefront of events of this magnitude which enhance oceanic disturbances such as marine heatwaves (MHWs). In this study, we investigate the sea surface temperature (SST) of French maritime basins using remotely sensed measurements to track the response of surface waters to the atmospheric heatwaves and determine the intensity of such feedback. Beyond the direct relationship between SSTs and surface air temperatures, we explore the leading atmospheric parameters affecting the upper-layer ocean heat budget. Despite some gaps in data availability, the SSTs measured during the meteorological summer of 2022 were record-breaking, the mean SST was between 1.3 and 2.6 ∘C above the long-term average (1982–2011), and the studied areas experienced between 4 and 22 d where the basin-averaged SSTs exceeded the maximum recorded basin-averaged SSTs from 1982 to 2011. We found a significant SST response during heatwave periods with maximum temperatures measured locally at 30.8 ∘C in the north-western Mediterranean Sea. Our results show that in August 2022 (31 July to 13 August), France experienced above-average surface solar radiation correlated with below-average total cloud cover and negative wind speed anomalies. Our attribution analysis based on a simplified mixed-layer heat budget highlights the critical role of ocean–atmosphere fluxes in initiating abnormally warm SSTs, while ocean mixing plays a crucial role in the cessation of such periods. We find that the 2 m temperatures and specific humidity that are consistently linked to the advection of warm and moist air masses are key variables across all the studied regions. Our results reveal that the influence of wind on heatwaves is variable and of secondary importance. Moreover, we observe that the incident solar radiation has a significant effect only on the Bay of Biscay (BB) and the English Channel (EC) areas. Our study findings are consistent with previous research and demonstrate the vulnerability of the Mediterranean Sea to the increasing frequency of extreme weather events resulting from climate change. Furthermore, our investigation reveals that the recurring heatwave episodes during the summer of 2022 had an undeniable impact on all the surveyed maritime areas in France. Our study therefore provides valuable insights into the complex mechanisms underlying the ocean–atmosphere interaction and demonstrates the need for an efficient and sustainable operational system combining polar-orbiting and geostationary satellites to monitor the alterations that threaten the oceans in the context of climate change.
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