| 1. |
- Aldama Campino, Aitor, 1989-
(författare)
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Atmospheric and oceanic circulation from a thermodynamic perspective
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The climate system is continuously transporting and exchanging heat, freshwater, carbon and other tracers in different spatio-temporal scales. Therefore, analysing the system from a thermodynamic or biogeochemical framework is highly convenient. In this thesis the interaction between the ocean and the atmospheric circulation is analysed using thermodynamical and biogeochemical coordinates. Due to the dimensionality of the climate system stream functions are used to reduce this complexity and facilitate the understanding of the different processes that take place. The first half of this thesis, focuses on the interaction between the atmospheric and the ocean circulation from a thermodynamic perspective. We introduce the hydrothermohaline stream function which combines the atmospheric circulation in humidity-potential temperature (hydrothermal) space and the ocean circulation in salinity-temperature coordinates (thermohaline). A scale factor of 7.1 is proposed to link humidity and salinity coordinates. Future scenarios are showing an increase of humidity in the atmosphere due to the increase of temperatures which results in a widening of the hydrothermal stream function along the humidity coordinate. In a similar way, the ocean circulation in the thermohaline space expands along the salinity coordinate. The link between salinity and humidity changes is strongest at net evaporation regions where the gain of water vapour in the atmosphere results in a salinification in the ocean. In addition, the ocean circulation in latitude-carbon space is investigated. By doing so, we are able to distinguish the roles of different water masses and circulation pathways for ocean carbon. We find that the surface waters in the subtropical gyres are the main drivers of the meridional carbon transport in the ocean. By separating the carbon in its different constituents we show that the carbon transported by the majority of the water masses is a result of the solubility pump. The contribution of the biological pump is predominant in the deep Pacific Ocean. The effects of the Mediterranean Overflow Waters on the North Atlantic are discussed in the final part of the thesis.
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| 2. |
- Ballarotta, Maxime, 1984-, et al.
(författare)
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A Last Glacial Maximum world-ocean simulation at eddy-permitting resolution – Part 1: Experimental design and basic evaluation
- 2013
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Ingår i: Climate of the Past Discussions. - : Copernicus GmbH. - 1814-9340 .- 1814-9359. ; 9, s. 297-328
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Tidskriftsartikel (refereegranskat)abstract
- Most state-of-the-art climate models include a coarsely resolved oceanic compo- nent, which has difficulties in capturing detailed dynamics, and therefore eddy- permitting/eddy-resolving simulations have been developed to reproduce the observed World Ocean. In this study, an eddy-permitting numerical experiment is conducted to simulate the global ocean state for a period of the Last Glacial Maximum (LGM, ∼26500 to 19000yr ago) and to investigate the improvements due to taking into account these higher spatial scales. The ocean general circulation model is forced by a 49-yr sample of LGM atmospheric fields constructed from a quasi-equilibrated climate-model simulation. The initial state and the bottom boundary condition conform to the Paleoclimate Modelling Intercomparison Project (PMIP) recommendations. Be- fore evaluating the model efficiency in representing the paleo-proxy reconstruction of the surface state, the LGM experiment is in this first part of the investigation, compared with a present-day eddy-permitting hindcast simulation as well as with the available PMIP results. It is shown that the LGM eddy-permitting simulation is consistent with the quasi-equilibrated climate-model simulation, but large discrepancies are found with the PMIP model analyses, probably due to the different equilibration states. The strongest meridional gradients of the sea-surface temperature are located near 40° N and S, this due to particularly large North-Atlantic and Southern-Ocean sea-ice covers. These also modify the locations of the convection sites (where deep-water forms) and most of the LGM Conveyor Belt circulation consequently takes place in a thinner layer than today. Despite some discrepancies with other LGM simulations, a glacial state is captured and the eddy-permitting simulation undertaken here yielded a useful set of data for comparisons with paleo-proxy reconstructions.
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| 3. |
- Ballarotta, Maxime, 1984-, et al.
(författare)
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A Last Glacial Maximum World-Ocean simulation at eddy-permitting resolution – Part 2: Confronting the paleo-proxy data
- 2013
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Ingår i: Climate of the Past Discussions. - : Copernicus GmbH. - 1814-9340 .- 1814-9359. ; 9, s. 329-350
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Tidskriftsartikel (refereegranskat)abstract
- Previous investigations concerning the design of an eddy-permitting LGM oceanic sim- ulation are here extended with focus on whether this type of simulation is capable of improving the numerical results with regard to the available paleo-proxy reconstructions. Consequently, an eddy-permitting and two coarse-grid simulations of the same LGM period are confronted with a dataset from the Multiproxy Approach for the Recon- struction of the Glacial Ocean Sea Surface Temperatures (MARGO SSTs) and a num- ber of sea-ice reconstructions. From a statistical analysis it was found that the eddy- permitting simulation does not significantly improve the SST representation with regard to the paleo-reconstructions. The western boundary currents are better resolved in the high-resolution experiment than in the coarse simulations, but, although these more detailed SST structures yield a locally improved consistency between modelled pre- dictions and proxies, they do not contribute significantly to the global statistical score. As in the majority of the PMIP2 simulations, the modelled sea-ice conditions are still inconsistent with the paleo-reconstructions, probably due to the choice of the model equilibrium.
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| 4. |
- Ballarotta, Maxime, et al.
(författare)
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Last Glacial Maximum world ocean simulations at eddy-permitting and coarse resolutions : do eddies contribute to a better consistency between models and palaeoproxies?
- 2013
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 9:6, s. 2669-2686
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Tidskriftsartikel (refereegranskat)abstract
- Most state-of-the-art climate models include a coarsely resolved oceanic component, which hardly captures detailed dynamics, whereas eddy-permitting and eddy-resolving simulations are developed to reproduce the observed ocean. In this study, an eddy-permitting and a coarse resolution numerical experiment are conducted to simulate the global ocean state for the period of the Last Glacial Maximum (LGM, similar to 26 500 to 19 000 yr ago) and to investigate the improvements due to taking into account the smaller spatial scales. The ocean state from each simulation is confronted with a data set from the Multiproxy Approach for the Reconstruction of the Glacial Ocean (MARGO) sea surface temperatures (SSTs), some reconstructions of the palaeo-circulations and a number of sea-ice reconstructions. The western boundary currents and the Southern Ocean dynamics are better resolved in the high-resolution experiment than in the coarse simulation, but, although these more detailed SST structures yield a locally improved consistency between model predictions and proxies, they do not contribute significantly to the global statistical score. The SSTs in the tropical coastal upwelling zones are also not significantly improved by the eddy-permitting regime. The models perform in the mid-latitudes but as in the majority of the Paleo-climate Modelling Intercomparison Project simulations, the modelled sea-ice conditions are inconsistent with the palaeo-reconstructions. The effects of observation locations on the comparison between observed and simulated SST suggest that more sediment cores may be required to draw reliable conclusions about the improvements introduced by the high resolution model for reproducing the global SSTs. One has to be careful with the interpretation of the deep ocean state which has not reached statistical equilibrium in our simulations. However, the results indicate that the meridional overturning circulations are different between the two regimes, suggesting that the model parametrizations might also play a key role for simulating past climate states.
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| 5. |
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| 7. |
- Jönsson, Bror, 1972-
(författare)
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Some Concepts of Estuarine Modeling
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- If an estuarine system is to be investigated using an oceanographic modeling approach, a decision must be made whether to use a simple and robust framework based on e.g. mass-balance considerations, or if a more advanced process-resolving three-dimensional (3-D) numerical model are necessary. Although the former are straightforward to apply, certain fundamental constraints must be fulfilled. 3-D modeling, even though requiring significant efforts to implement, generates an abundance of highly resolved data in time and space, which may lead to problems when attempting to specify the "representative state" of the system, a common goal in estuarine studies.In this thesis, different types of models suitable for investigating estuarine systems have been utilized in various settings. A mass-balance model was applied to investigate potential changes of water fluxes and salinities due to the restoration of a mangrove estuary in northern Colombia. Seiches, i.e. standing waves, in the Baltic Sea were simulated using a 2-D shallow-water model which showed that the dominating harmonic oscillation originates from a fjord seiche in the Gulf of Finland rather than being global. Another study pertaining to the Gulf of Finland used velocity-fields from a 3-D numerical model together with Lagrangian-trajectory analyses to investigate the mixing dynamics. The results showed that water from the Baltic proper is mixed with that from the river Neva over a limited zone in the inner parts of the Gulf. Lagrangian-trajectory analysis was finally also used as a tool to compare mass-balance and 3-D model results from the Gulf of Riga and the Bay of Gdansk, highlighting when and where each method is applicable.From the present thesis it can be concluded that the above described estuarine-modeling approaches not only require different levels of effort for their implementation, but also yield results of varying quality. If oceanographic aspects are to be taken into account within Integrated Coastal Zone Managment, which most likely should be the case, it is therefore important to decide as early as possible in the planning process which model to use, since this choice ultimately determines how much information about the physical processes characterizing the system the model can be expected to provide.
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| 8. |
- Jönsson, Bror, et al.
(författare)
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Standing waves in the Gulf of Finland and their relationship to the basin-wide Baltic seiches
- 2008
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Ingår i: Journal of Geophysical Research: Oceans. ; 113, s. C03004-
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Tidskriftsartikel (refereegranskat)abstract
- A linear shallow-water model was used to study different harmonic oscillations in the Baltic Sea. The model was initialized using a linear sea-surface slope from east to west, and was hereafter run without forcing. In our results, we could identify three different local oscillatory modes: one in the Gulf of Finland, with the two distinct periods 23 and 27 hours, one in the Danish Belt Sea, with a less distinct period in the range 23-27 hours, and one in the Gulf of Riga, with the period 17 hours. The most pronounced mode is that in the Gulf of Finland. No clear indications of basin-wide seiches in the Baltic could be found from our simulations. These results were further corroborated by a frequency analysis of sea-level observations from the Baltic. This shows an amplification of the K1 and O1 tidal modes in the Gulf of Finland, but not of the M2 and S2 modes. No such amplification was seen in the rest of the Baltic Sea. On the basis of our model simulations, we propose that sea-level oscillations of the Baltic be regarded as a ensemble of weakly coupled local oscillators. Each oscillator corresponds to a ''fjord mode" or "harbour mode" in a particular bay or sub-basin. These are not proper eigenmodes since their energy gradually leaks out to the rest of the Baltic Sea, resulting in radiation damping. Nevertheless, their resonance may in fact be sharper than that of the proper basin-wide eigenmodes.
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| 9. |
- Mårtensson, Sebastian, 1981-
(författare)
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Ridged sea ice modelling in climate applications
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work aims to increase our understanding of the nature of large scale features of sea ice from a dynamics point of view.Sea ice plays an important part in the exchange of heat and humidity between sea and air and thus is an important component of the climate system. Its physical presence also directly impacts the various forms of life such as diatoms, polar bears and humans alike.The dynamics of sea ice affect both weather and climate, through the large scale drift in the Arctic from the Siberian coast towards Fram Strait, through creation of cracks in the ice called leads or polynyas, and through ridging and other mechanical deformations of ice floes.In this work, we have focused on modelling of ridged ice for a number of reasons. Direct observations of the internal ice state is very difficult to perform and in general, observations of sea ice are either sparse or of limited information density. Ridged ice can be seen as the memory of high ice stress events, giving us a view on these highly dynamic events. Ridging is of major importance for the ice thickness distribution, as the thickest ice can only be formed through mechanical processes. Further, ridged ice is of direct interest for anyone conducting shipping through seasonal or perennial ice covered seas as it can form impenetrable barriers or in extreme even cases crush a ship caught within the ice pack.To this end, a multi-category sea ice model, the HELsinki Multi category Ice model (HELMI), was implemented into the Rossby Centre Ocean model (RCO). HELMI has explicit formulations for ridged and rafted ice, as well as sub-grid scale ice thickness distribution (a feature shared with other multi category models) and an ice strength based on energetics. These features give RCO better representation of sub-grid scale physics and gives us the possibility to study the deformed ice in detail.In paper I we look at the change in behaviour in the Arctic as the ice becomes more mobile, leading to a slight increase in modelled ridged ice volume in the central Arctic, despite a general trend of a decreasing ice cover.Paper II takes us to the Baltic Sea and the possibilities of modelling ridge ice concentration with a statistical model.In Paper III we investigate how the diminishing ice cover in future scenarios affects the biological activity in the Baltic Sea.Finally Paper IV investigates how the ice stress and the internal ice force can be interpreted in terms of ice compression on the ship scale.
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