1.
Aldama Campino, Aitor, 1989-
(författare)
Atmospheric and oceanic circulation from a thermodynamic perspective
2019
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.
2.
Berglund, Sara, 1990-
(författare)
Tracing pathways in the ocean circulation : A temperature and salinity perspective
2021
Doktorsavhandling (övrigt vetenskapligt/konstnärligt) abstract
The ocean circulation plays an essential role in our climate system. It redistributes heat, salt, carbon and other tracers across the globe, making the climate of Earth more moderate. This thesis targets density differences that are driving the ocean circulation. These differences are caused by changes in temperature and salinity. The analysis is based on the usage of Lagrangian trajectories simulated with velocity fields from an Earth System Model. The Lagrangian approach opens up for the possibility to follow specific water paths and water masses. The results herein provide a new insight to specific circulation patterns in the ocean, and which regions that play an important role in controlling temperature and salinity changes.In the first two articles, the Lagrangian divergence is introduced. It shows the geographical distribution of heat and salt changes of a simulated water mass. Using this, we are able to show that the northward flowing water in the Atlantic Ocean cools and freshens in the North Atlantic Subtropical Gyre, the Gulf Stream and the North Atlantic Current. Similarly, we show that the water flowing from the Drake Passage, following the Antarctic Circumpolar Current (ACC) and moving northwards into the Atlantic, Pacific and Indian oceans, transforms from cold and fresh to warm and saline. This warming and salinification are a result of, not only air-sea fluxes, but also interior mixing.In the third study, we show that 70% of the water flowing northwards as part of the Atlantic Meridional Overturning Circulation circuits the North Atlantic Subtropical Gyre at least once before it continues northwards. In the gyre, the water spirals downwards as it gets denser, due to a combination of air-sea fluxes and interior mixing. These results bring a new perspective on the Subtropical Gyre's role to the circulation patterns of the Atlantic Meridional Overturning Circulation.In the last part of this thesis, the circulation in the North Atlantic Ocean is traced into four different pathways. The pathways are visualised geographically together with their change in temperature, salinity and density. With this, we are able to show that the northward flowing water in the Atlantic Ocean exchanges heat and salt with the colder and fresher waters circulating the Subpolar Gyre.
3.
Dey, Dipanjan, et al.
(författare)
A complete view of the atmospheric hydrologic cycle
Annan publikation (övrigt vetenskapligt/konstnärligt) abstract
The global atmospheric water transport from the evaporation to the precipitation regions has beentraced using Lagrangian trajectories. A matrix has been constructed by selecting various groupof trajectories based on their starting (evaporation) and ending (precipitation) positions to show the connectivity of the atmospheric water transport within and between the three major ocean basins and the global landmass. The analysis reveals that a major portion of the evaporated water precipitates back into the same region, namely 67% for the Indian, 64% for the Atlantic, 85% for the Pacific Ocean and 72% for the global landmass. The evaporation from the subtropical regions of the Indian, Atlantic and Pacific Oceans is found to be the primary source of atmospheric water for precipitation over the Intertropical Convergence Zone (ITCZ) in the corresponding basins. The evaporated waters from the subtropical and western Indian Ocean were traced as the source for precipitation over the South Asian and Eastern African landmass, while Atlantic Ocean waters are responsible for rainfall over North Asia and Western Africa. Atlantic storm tracks were identified as the carrier of atmospheric water that precipitates over Europe, while the Pacific storm tracks were responsible for North American, eastern Asian and Australian precipitation. The bulk of South and Central American precipitation is found to have its source in the tropical Atlantic Ocean. The recycling of evapotranspirated water from land is pronounced over the western coast of South America, Northeastern Asia, Canada and Greenland. The ocean-to-land and land-to-ocean water transport through the atmosphere was computed to be 2×109 kg/s and 1×109 kg/s, respectively.The difference between them (net ocean-to-land transport), i.e. 1×109 kg/s, is transported to land. This net transport is approximately the same as found in previous Eulerian estimates
4.
Dey, Dipanjan, 1991-
(författare)
Tracing water transport pathways in the coupled ocean-atmosphere system
2021
Doktorsavhandling (övrigt vetenskapligt/konstnärligt) abstract
Water is the most precious substance on the Earth and thus it is important to know how it moves around and is recycled.This knowledge will be useful to formulate educated future strategies about water usage in society and also to understand the near-surface salinity contrasts in the world ocean.In this thesis, water movement around the globe, known also as the hydrological or water cycle, have been traced regardless of in which phase the water is.Water is always on the move through space and in time and thus to reduce the dimensionality and complexity of the system, a stream-function approach was applied to gain a better understanding of the processes that govern the water cycle.The present work provides a unique picture of the complete hydrological cycle and how the water circulation in the atmosphere and ocean is connected at the surface by evaporation and precipitation.In addition, emphasize was given to track and understand the atmospheric part of the water circulation, which was accomplished using an Eulerian and a novel Lagrangian framework. The first half of the thesis was focused on making use of a water-mass conservation equation for the atmosphere derived by computing the rate of change of the water-mass content inside a grid box and water transport through its faces. The vertical water transport calculated from this conservation equation thus not only consist of vertical advection of the water vapor, but also includes the evaporation and precipitation.This Eulerian methodology and the overturning stream function diagnostic was then used to present average atmospheric water circulation pathways along with their oceanic counterparts in a meridional-vertical coordinate system. Six coupled atmosphere-ocean water cells were discovered, which show the redistribution of the freshwater in the climate system.A warmer-climate scenario indicates a strengthening of these cells, which implies that the wet regions will be wetter and dry regions will get drier.In addition, atmospheric water transport from the Atlantic to the Pacific Ocean was computed from a Lagrangian perspective.The results shows that westerly winds, which prevail in the mid-latitudes and flow across Afro-Eurasia, actually contribute about 60% of the total Atlantic-to-Pacific atmospheric water transport, significantly more than previously thought.In the latter half of the thesis, the origin and its variability responsible for the South Asian Summer monsoon precipitation was traced using Lagrangian atmospheric water trajectories.The Central and South Indian Ocean was found to be the main contributor to the precipitation and its variability over South Asia during the monsoon months.A complete view of the atmospheric hydrological cycle was finally achieved by tracing the global atmospheric water transport from the evaporation to the precipitation regions using Lagrangian trajectories. A matrix was constructed by sorting trajectories based on their starting (evaporation) and ending (precipitation) positions to show the atmospheric water transport connectivity within and between the three major ocean basins and the global landmass. In addition, a simplified schematic of the annual mean atmospheric water transports between global ocean and land was provided based on Eulerian and Lagrangian perspectives.This schematic reflects the advantage of using a Lagrangian framework, from which ocean-to-ocean, ocean-to-land, land-to-land and land-to-ocean atmospheric water transport could be and was calculated. The ocean-to-land and land-to-ocean water transport through the atmosphere was computed to be 2×109 kg/s and 1×109 kg/s respectively, a result which is not possible to achieve by using an Eulerian perspective.
5.
Zarroug, Moundheur, 1978-
(författare)
Asymptotic methods applied to some oceanography-related problems
2010
Doktorsavhandling (övrigt vetenskapligt/konstnärligt) abstract
In this thesis a number of issues related to oceanographic problems have been dealt with on the basis of applying asymptotic methods. The first study focused on the tidal generation of internal waves, a process which is quantifed by the conversion rates. These have traditionally been calculated by using the WKB approximation. However, the systematic imprecision of this theory for the lowest modes as well as turbulence at the seabed level affect the results. To handle these anomalies we introduced another asymptotic technique, homogenization theory, which led to signifcant improvements, especially for the lowest modes. The second study dealt with the dynamical aspects of a nonlinear oscillator which can be interpreted as a variant of the classical two-box models used in oceanography. The system is constituted by two connected vessels containing a fluid characterised by a nonlinear equation of state and a large volume differences between the vessels is prescribed. It is recognised that the system, when performing relaxation oscillations, exhibits almost-discontinuous jumps between the two branches of the slow manifold of the problem. The lowest-order analysis yielded reasonable correspondence with the numerical results. The third study is an extension of the lowest-order approximation of the relaxation oscillations undertaken in the previous paper. A Mandelstam condition is imposed on the system by assuming that the total heat content of the system is conserved during the discontinuous jumps. In the fourth study an asymptotic analysis is carried out to examine the oscillatory behaviour of the thermal oscillator. It is found that the analytically determined corrections to the zeroth-order analysis yield overall satisfying results even for comparatively large values of the vessel-volume ratio.
6.
Ballarotta, Maxime, et al.
(författare)
Last Glacial Maximum world ocean simulations at eddy-permitting and coarse resolutions : do eddies contribute to a better consistency between models and palaeoproxies?
2013
Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 9:6, s. 2669-2686
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.
7.
Berglund, Sara, et al.
(författare)
The Downward Spiralling Nature of the North Atlantic Subtropical Gyre
Annan publikation (övrigt vetenskapligt/konstnärligt) abstract
The Atlantic Meridional Overturning Circulation (AMOC) regulates the heat distribution and climate of Earth. Here we identify a new feature of the circulation within the North Atlantic Subtropical Gyre that is associated with the northward flowing component of the AMOC.We find that 70% of the water that flows northwards as part of the AMOC circulates the Gyre at least once before it can continue northwards.These circuits are needed to achieve an increase of density and depth through a combination of air-sea interaction and interior mixing processes, before water can escape the latitudes of the Gyre and join the northern upper branch of the AMOC.This points towards an important role of the Gyre circulations in determining the strength and variability of the AMOC and the northward heat transport.Understanding this newly identified role of the North Atlantic Subtropical Gyre is needed to properly represent future changes of the AMOC.
8.
Corell, Hanna, 1977, et al.
(författare)
Difference in Particle Transport Between Two Coastal Areas in the Baltic Sea Investigated with High-Resolution Trajectory Modeling
2013
Ingår i: Ambio. - Netherlands : Springer. - 0044-7447 .- 1654-7209. ; 42:4, s. SI 455-463
Tidskriftsartikel (refereegranskat) abstract
A particle-tracking model based on high-resolution ocean flow data was used to investigate particle residence times and spatial distribution of settling sediment for two geo-morphologically different Swedish coastal areas. The study was a part of a safety assessment for the location of a future nuclear-waste repository, and information about the particle-transport patterns can contribute to predictions of the fate of a possible leakage. It is also, to our knowledge, the first time particle-transport differences between two coastal areas have been quantified in this manner. In Forsmark, a funnel-shaped bay shielded by a number of islands, the average residence time for clay particles was 5 times longer than in the modeled part of Simpevarp, which is open to the Baltic Sea. In Forsmark, < 10 % of the released particles left the domain compared to 60-80 % in Simpevarp. These site-specific differences will increase over time with the differences in land uplift between the areas.
9.
Berglund, Sara, 1990-, et al.
(författare)
North Atlantic Ocean Circulation and Related Exchange of Heat and Salt Between Water Masses
2023
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 50:13
Tidskriftsartikel (refereegranskat) abstract
The meridional transport of mass, heat, and salt in the North Atlantic Ocean is often described for separate regions and parts, but rarely are all components of the circulation followed at once. Lagrangian trajectories have here been used to divide the North Atlantic Ocean circulation into four different pathways. In the boundary between the Subpolar and Subtropical Gyres, we show that the northward flowing waters exchange heat and salt with the water originating from the subpolar regions. This subsurface water mass exchange takes place in the first 1,000 m and is a key piece of the puzzle of how the Atlantic Meridional Overturning Circulation transports heat and salt. Between 30 & DEG; and 60 & DEG;N the northward flowing water loses 8.8 Gg/s salt to the Subpolar Gyre and an equivalent loss of only 1.7 Gg/s to the atmosphere due to the net fresh water influx.
10.
Dey, Dipanjan, et al.
(författare)
The coupled ocean–atmosphere hydrologic cycle
2019
Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 71:1, s. 1-11
Tidskriftsartikel (refereegranskat) abstract
The freshwater cycle has in the present study been traced as one integrated process in the coupled ocean–atmosphere system for both present and possible future climates simulated with an Earth-System Model. A method based on water-mass conservation was used in order to calculate mass fluxes of water from regions of evaporation to regions of precipitation. These fluxes include not only advection of moisture by the winds but also the vertical water-mass transport due to precipitation forming hence a mass-conserved 3D water-mass transport field. Six atmospheric hydrological cells were revealed, which cross the sea surface, where they join the oceanic overturning circulation. These atmospheric water cells can be interpreted as an extension of the oceanic overturning circulation, since the otherwise open ocean streamlines at the surface continue into the atmosphere due to evaporation and back into the ocean due to precipitation. Although these atmospheric water cells are related to the usual air cells, they are only half part of the coupled water cells and located differently. The future-climate scenario shows that the mid- and high-latitude atmospheric water-mass cells will transport more moisture towards the poles as well as increase of the northward cross-Equatorial atmospheric water-mass transport.
