| 1. |
- Ballarotta, Maxime, 1984-, et al.
(författare)
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The residual circulation of the Southern Ocean : Which spatio-temporal scales are needed?
- 2013
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Ingår i: Ocean Modelling. - : Elsevier BV. - 1463-5003 .- 1463-5011. ; 64, s. 46-55
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Tidskriftsartikel (refereegranskat)abstract
- The Southern Ocean circulation consists of a complicated mixture of processes and phenomena that arise at different time and spatial scales which need to be parametrized in the state-of-the-art climate models. The temporal and spatial scales that give rise to the present-day residual mean circulation are here inves- tigated by calculating the Meridional Overturning Circulation (MOC) in density coordinates from an eddy-permitting global model. The region sensitive to the temporal decomposition is located between 38°S and 63°S, associated with the eddy-induced transport. The ‘‘Bolus’’ component of the residual circu- lation corresponds to the eddy-induced transport. It is dominated by timescales between 1 month and 1 year. The temporal behavior of the transient eddies is examined in splitting the ‘‘Bolus’’ component into a ‘‘Seasonal’’, an ‘‘Eddy’’ and an ‘‘Inter-monthly’’ component, respectively representing the correlation between density and velocity fluctuations due to the average seasonal cycle, due to mesoscale eddies and due to large-scale motion on timescales longer than one month that is not due to the seasonal cycle. The ‘‘Seasonal’’ bolus cell is important at all latitudes near the surface. The ‘‘Eddy’’ bolus cell is dominant in the thermocline between 50°S and 35°S and over the whole ocean depth at the latitude of the Drake Passage. The ‘‘Inter-monthly’’ bolus cell is important in all density classes and is maximal in the Brazil– Malvinas Confluence and the Agulhas Return Current. The spatial decomposition indicates that a large part of the Eulerian mean circulation is recovered for spatial scales larger than 11.25°, implying that small-scale meanders in the Antarctic Circumpolar Current (ACC), near the Subantarctic and Polar Fronts, and near the Subtropical Front are important in the compensation of the Eulerian mean flow.
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| 2. |
- Brodeau, Laurent, et al.
(författare)
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An ERA40-based atmospheric forcing for global ocean circulation models
- 2010
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Ingår i: Ocean Modelling. - : Elsevier BV. - 1463-5003 .- 1463-5011. ; 31:3-4, s. 88-104
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Tidskriftsartikel (refereegranskat)abstract
- We develop, calibrate and test a dataset intended to drive global ocean hindcasts simulations of the last five decades. This dataset provides surface meteorological variables needed to estimate air-sea fluxes and is built from 6-hourly surface atmospheric state variables of ERA40. We first compare the raw fields of ERA40 to the CORE.v1 dataset of Large and Yeager (2004), used here as a reference, and discuss our choice to use daily radiative fluxes and monthly precipitation products extracted from satellite data rather than their ERA40 counterparts. Both datasets lead to excessively high global imbalances of heat and freshwater fluxes when tested with a prescribed climatological sea surface temperature. After identifying unrealistic time discontinuities (induced by changes in the nature of assimilated observations) and obvious global and regional biases in ERA40 fields (by comparison to high quality observations), we propose a set of corrections. Tropical surface air humidity is decreased from 1979 onward, representation of Arctic surface air temperature is improved using recent observations and the wind is globally increased. These corrections lead to a significant decrease of the excessive positive global imbalance of heat. Radiation and precipitation fields are then submitted to a small adjustment (in zonal mean) that yields a near-zero global imbalance of heat and freshwater. A set of 47-year-long simulations is carried out with the coarse-resolution (2° × 2°) version of the NEMO OGCM to assess the sensitivity of the model to the proposed corrections. Model results show that each of the proposed correction contributes to improve the representation of central features of the global ocean circulation.
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| 3. |
- Döös, Kristofer, et al.
(författare)
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Dispersion of surface drifters and model-simulated trajectories
- 2011
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Ingår i: Ocean Modelling. - : Elsevier BV. - 1463-5003 .- 1463-5011. ; 39:3-4, s. 301-310
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Tidskriftsartikel (refereegranskat)abstract
- From a data set encompassing the years 1990-2008 pairs of surface drifters with maximum initial separations of 5, 10 and 25 km have been identified. Model trajectories have been calculated using the same initial positions and times as the selected pairs of surface drifters. The model trajectories are based on the TRACMASS trajectory code and driven by the ocean general circulation model NEMO. The trajectories are calculated off-line, i.e. with the stored velocity fields from the circulation model. The sensitivity of the trajectory simulations to the frequency of the stored velocity fields was tested for periods of 3 and 6 h as well as 5 days. The relative dispersion of the surface-drifter and model trajectories has been compared, where the latter was found to be too low compared to the relative dispersion of the drifters. Two low-order trajectory sub-grid parameterisations were tested and successfully tuned so that the total amplitude of the relative dispersion of the model trajectories is similar to that associated with the drifter trajectories. These parameterisations are, however, too simple for a correct simulation of Lagrangian properties such as the correlation time scales and the variance of the eddy kinetic energy. The importance of model-grid resolution is quantified by comparing the relative dispersion from an eddy-permitting and a coarse-resolution model, respectively. The dispersion rate is halved with the coarse grid. The consequences of the two-dimensionality of the trajectories is evaluated by comparing the results obtained with the 2D and the Lagrangian 3D trajectories. This shows that the relative dispersion is 15% stronger when the trajectories are freely advected with the 3D velocity field.
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| 4. |
- Hieronymus, Magnus, 1982-, et al.
(författare)
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The budgets of heat and salinity in NEMO
- 2013
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Ingår i: Ocean Modelling. - : Elsevier BV. - 1463-5003 .- 1463-5011. ; 67, s. 28-38
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Tidskriftsartikel (refereegranskat)abstract
- The near steady state heat and salinity budgets under surfaces of constant depth are examined in the Nucleus for European Modelling of the Ocean (NEMO) model. It is seen that the heat fluxes in NEMO are difficult to reconcile with the idea of a deep ocean in advection-diffusion balance. Some reasons for this are that the resolved heat advection is downward above 2000 m, and that geothermal heating is, in fact, a major heat source in the deeper parts of the domain. It is also seen that isoneutral diffusion gives a very large contribution to the budgets and that the fluxes from isoneutral diffusion is in general upward. It is explained how the sign of these fluxes depends on the stratification. The heat budget for the upper 100 m of the ocean is seen to be dominated by penetrative shortwave radiation, which is so influential that we would have a mixed layer of considerable thickness even in the absence of other sources of turbulent mixing. Penetrative shortwave radiation is therefore a considerable source of potential energy.
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| 5. |
- Ibragimov, Ranis, et al.
(författare)
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Effects of rotation on self-resonant internal gravity waves in the ocean
- 2010
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Ingår i: Ocean Modelling. - : Elsevier. - 1463-5003 .- 1463-5011. ; 31:3-4
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Tidskriftsartikel (refereegranskat)abstract
- The Resonant Triad Model (RTM) developed in (Ibragimov, 2007), is used to study the Thorpe’s problem (Thorpe, 1997) on the existence of self-resonant internal waves, i.e., the waves for which a resonant interaction occurs at second order between the incident and reflected internal waves off slopes. The RTM represents the extension of the McComas and Bretherton’s three wave hydrostatic model (McComas and Bretherton, 1977) which ignores the effects of the earth’s rotation to the case of the non-hydrostatic analytical model involving arbitrarily large number of rotating internal waves with frequencies spanning the range of possible frequencies, i.e., between the maximum of the buoyancy frequency (vertical motion) and a minimum of the inertial frequency (horizontal motion). The present analysis is based on classification of resonant interactions into the sum, middle and difference interaction classes. It is shown in this paper that there exists a certain value of latitude, which is classified as the singular latitude, at which the coalescence of the middle and difference interaction classes occurs. Such coalescence, which apparently had passed unnoticed before, can be used to study the Thorpe’s problem on the existence of selfresonant waves. In particular, it is shown that the value of the bottom slope at which the second-order frequency and wave number components of the incident and reflected waves satisfy the internal wave dispersion relation can be approximated by two latitude-dependent parameters in the limiting case when latitude approaches its singular value. Since the existence of a such singular latitude is generic for resonant triad interactions, a question on application of the RTM to the modeling of enhanced mixing in the vicinity of ridges in the ocean arises.
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| 6. |
- Tao, Y., et al.
(författare)
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A simulation modeling approach to hydrothermal plumes and its comparison to analytical models
- 2013
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Ingår i: Ocean Modelling. - : Elsevier BV. - 1463-5003 .- 1463-5011. ; 61, s. 68-80
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Tidskriftsartikel (refereegranskat)abstract
- We study the dynamics of hydrothermal plumes with the 3D time-dependent, Eulerian, adaptive mesh refinement code GERRIS, which solves the equations of viscous, incompressible hydrodynamics. We have implemented a new module into Gerris that treats buoyancy-driven turbulence by means of a subgrid mode. Our model is validated in numerical experiment and applied to the dynamics of a rising plume. First we simulate hydrothermal plumes in a static environment and compare our results to the widely used integral models (MTT or Briggs' model). The entrainment coefficient that we deduce from simulations falls into the range of the experimentally determined values. We also investigate the ratio between the level of the neutral-buoyancy layer and the maximum plume height. This ratio is frequently used to estimate plume heat flux via the measured level of neutral buoyancy. Although the ratio is only moderately (less than 10%) higher than the one predicted by the integral model, heat flux estimations can be substantially different. Finally, we explore the importance of background currents. We find that the simulated trajectories agree with integral models in the rising stage but the subsequent oscillations around the neutral-buoyancy layer are damped much more quickly and the level of the neutral buoyancy is also higher, same as the calm environment cases. By simulating the oscillation of a plume with suppressed transported turbulence and find a stronger oscillation than the original simulation, we suggest that a significant fraction of the difference between our model and the integral model can be explained by the absence of the turbulent transport of the latter.
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