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  • 1.
    Aldama Campino, Aitor
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Multidecadal variability of the Mediterranean Overflow Water in the North AtlanticManuscript (preprint) (Other academic)
    Abstract [en]

    The Mediterranean overflow water is one of the most important intermediate--depth water masses in the North Atlantic. This water mass, formed in the Mediterranean Sea, produces a saline and warm water tongue at a depth of 1000 m that spreads out from the Strait of Gibraltar and fills a large area of the North Atlantic basin. The production of  this dense water is a result of the excess of evaporation over precipitation and river runoff. A pre-industrial simulation with the earth system model EC-Earth is used to investigate the overflow water. The multidecadal variability of the outflow is analysed by examining the modelled volume and salt transports through the Strait of Gibraltar as well as different atmospheric patterns (such as the wind pattern and the net freshwater fluxes). The salinity evolution in the main core of the outflow in the mid Atlantic is also taken into account.  \ald{The leading empirical orthogonal functions for the modeled salinity 1000 m coincided with the modeled distribution of outflow water}. The associated principal component showed a multidecadal variability of the salinity field. The variability of the net salt transport through the Strait of Gibraltar showed a similar behaviour where the Atlantic--Mediterranean system manifested two clear states. One of these is when the Mediterranean imports salt from the Atlantic and the other is where salt export to the Atlantic predominates. \ald{This result indicates that the Mediterranean Sea acts as a storage of salt alternating between the two states.

  • 2.
    Aldama Campino, Aitor
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    The effects of global warming on the coupled Ocean-Atmosphere Hydrothermohaline circulationManuscript (preprint) (Other academic)
    Abstract [en]

    Global warming will have an impact on the hydrological cycle affecting both the atmospheric and oceanic circulation. In this study we analyse these impacts from a thermodynamic perspective using streamfunctions defined in general thermodynamic coordinates. Both the atmospheric and oceanic circulation showed a weakening of the circulation in a future scenario but an expansion in both humidity and salinity directions. The Clausius-Clapeyron relationship is hence here extended to not only to give a relationship between air temperature and moisture but also with the sea-surface salinity. As a consequence, not only the atmospheric hydrothermal circulation, but also the oceanic thermohaline circulation, will follow the Clausius-Clapeyron relationship as climate warms up. This results in a direct relationship between the increase of atmospheric moisture and an increase of the ocean salinity as a consequence of the changes in the freshwater forcing at the sea surface.

  • 3.
    Aldama Campino, Aitor
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Fransner, Filippa
    Ödalen, Malin
    Stockholm University, Faculty of Science, Department of Meteorology .
    Groeskamp, Sjoerd
    Yool, Andrew
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Meridional Ocean Carbon TransportManuscript (preprint) (Other academic)
    Abstract [en]

    The ocean's ability to take up and store CO$_{2}$ is a key factor for understanding past and future climate variability. However, qualitative and quantitative understanding of surface-to-interior pathways, and how the ocean circulation affects the CO$_2$ uptake, is limited. Consequently, how changes in ocean circulation may influence carbon uptake and storage and therefore the future climate remains ambiguous.Here we quantify the roles played by ocean circulation and various water masses in the meridional redistribution of carbon.We do so by calculating stream functions defined in Dissolved Inorganic Carbon (DIC) and latitude coordinates, using output from a coupled biogeochemical-physical model. By further separating DIC into components originating from the solubility pump and a residual including the biological pump, air-sea disequilibrium and anthropogenic CO$_2$, we are able to distinguish the dominant pathways of how carbon enters particular water masses.With this new tool, we show that the largest meridional carbon transport occurs in a pole-to-equator transport in the subtropical gyres in the upper ocean. We are able to show that this pole-to-equator DIC transport, and the Atlantic Meridional Overturning Circulation (AMOC) related DIC transport, are mainly driven by the solubility pump. By contrast, the DIC transport associated with deep circulation, including that in Antarctic Bottom Water and Pacific Deep Water, is mostly driven by the biological pump. As these two pumps, as well as ocean circulation, are widely expected to be impacted by anthropogenic changes, these findings have implications for the future role of the ocean as a climate-buffering carbon reservoir.

  • 4.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brandefelt, Jenny
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    A Last Glacial Maximum world-ocean simulation at eddy-permitting resolution – Part 1: Experimental design and basic evaluation2013In: Climate of the Past Discussions, ISSN 1814-9340, E-ISSN 1814-9359, Vol. 9, p. 297-328Article in journal (Refereed)
    Abstract [en]

    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. 

  • 5.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Drijfhout, Sybren
    Kuhlbrodt, Till
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    The residual circulation of the Southern Ocean: Which spatio-temporal scales are needed?2013In: Ocean Modelling, ISSN 1463-5003, E-ISSN 1463-5011, Vol. 64, p. 46-55Article in journal (Refereed)
    Abstract [en]

    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. 

  • 6.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brandefelt, Jenny
    A Last Glacial Maximum World-Ocean simulation at eddy-permitting resolution – Part 2: Confronting the paleo-proxy data2013In: Climate of the Past Discussions, ISSN 1814-9340, E-ISSN 1814-9359, Vol. 9, p. 329-350Article in journal (Refereed)
    Abstract [en]

    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. 

  • 7.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Falahat, Saeed
    Stockholm University, Faculty of Science, Department of Meteorology .
    The vigorous large-scale ocean circulations during the Last Glacial MaximumManuscript (preprint) (Other academic)
    Abstract [en]

    The representation of the ocean thermohaline circulation (THC) under glacial and interglacial climate conditions is investigated using a new global thermohaline stream function. Consequently, the interglacial and glacial THCs are compared from two experiments based on an ocean general circulation model forced at the surface by conditions representing the present-day and the period of the Last Glacial Maximum (LGM, ≈ 21kyr ago). It is shown  that the LGM THC is amplified by the salinity/density contrast between the Atlantic and the Pacific basins, as well as in the abyss due to larger salinity gradients. Even though the circuit along the Conveyor Belt loop is not drastically changed, the water mass transformations can regionally differ between the two periods. Additionally, the LGM Conveyor Belt Cell is more  isolated from the abyss and its turnover time is between two and three times shorter than in the present-day simulation, suggesting vigorous large-scale circulation. 

  • 8.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Falahat, Saeed
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the glacial and interglacial thermohaline circulation and the associated transports of heat and freshwater2014In: Ocean Science, ISSN 1812-0784, E-ISSN 1812-0792, Vol. 10, no 6, p. 907-921Article in journal (Refereed)
    Abstract [en]

    The thermohaline circulation (THC) and the oceanic heat and freshwater transports are essential for understanding the global climate system. Streamfunctions are widely used in oceanography to represent the THC and estimate the transport of heat and freshwater. In the present study, the regional and global changes of the THC, the transports of heat and freshwater and the timescale of the circulation between the Last Glacial Maximum (LGM, ≈ 21 kyr ago) and the present-day climate are explored using an Ocean General Circulation Model and streamfunctions projected in various coordinate systems. We found that the LGM tropical circulation is about 10% stronger than under modern conditions due to stronger wind stress. Consequently, the maximum tropical transport of heat is about 20% larger during the LGM. In the North Atlantic basin, the large sea-ice extent during the LGM constrains the Gulf Stream to propagate in a more zonal direction, reducing the transport of heat towards high latitudes by almost 50% and reorganising the freshwater transport. The strength of the Atlantic Meridional Overturning Circulation depends strongly on the coordinate system. It varies between 9 and 16 Sv during the LGM, and between 12 to 19 Sv for the present day. Similar to paleo-proxy reconstructions, a large intrusion of saline Antarctic Bottom Water takes place into the Northern Hemisphere basins and squeezes most of the Conveyor Belt circulation into a shallower part of the ocean. These different haline regimes between the glacial and interglacial period are illustrated by the streamfunctions in latitude–salinity coordinates and thermohaline coordinates. From these diagnostics, we found that the LGM Conveyor Belt circulation is driven by an enhanced salinity contrast between the Atlantic and the Pacific basin. The LGM abyssal circulation lifts and makes the Conveyor Belt cell deviate from the abyssal region, resulting in a ventilated upper layer above a deep stagnant layer, and an Atlantic circulation more isolated from the Pacific. An estimate of the timescale of the circulation reveals a sluggish abyssal circulation during the LGM, and a Conveyor Belt circulation that is more vigorous due to the combination of a stronger wind stress and a shortened circulation route.

  • 9.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Laurent, Brodeau
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jenny, Brandefelt
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Last Glacial Maximum world ocean simulations at eddy-permitting and coarse resolutions: do eddies contribute to a better consistency between models and palaeoproxies?2013In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, no 6, p. 2669-2686Article in journal (Refereed)
    Abstract [en]

    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, ~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 sig- nificantly 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 simula-tions. 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. 

  • 10.
    Berglund, Sara
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lagrangian tracing of the water-mass transformations in the Atlantic Ocean2017In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 69, article id 1306311Article in journal (Refereed)
    Abstract [en]

    The thermohaline stream function has previously been used to describe the ocean circulation in temperature and salinity space. In the present study, the Lagrangian thermohaline stream function is introduced and computed for northward flowing water masses in the Atlantic Ocean, using Lagrangian trajectories. The stream function shows the water-mass transformations in the Atlantic Ocean, where warm and saline water is converted to cold and fresh as it flows from 17 degrees S to 58 degrees N. By analysing the Lagrangian divergence of heat and salt flux, the conversion of temperature is found to take place in the Gulf Stream, the upper flank of the North Atlantic subtropical gyre and in the North Atlantic Drift, whereas the conversion of salinity rather occurs over a narrower band in the same regions. Thus, conversions of temperature and salinity as shown by the Lagrangian thermohaline stream function are confined to the same regions in the domain. The study of a specific, representative trajectory shows that, in the absence of air-sea interactions, a mixing process leads to the conversion of temperature and salinity from warm and saline to cold and fresh, and that this process is confined to the North Atlantic subtropical gyre. However, to define and to understand this process, further investigation is needed.

  • 11.
    Corell, Hanna
    et al.
    Univ Gothenburg, Dept Biol & Environm Sci, Gothenburg, Sweden.
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Difference in Particle Transport Between Two Coastal Areas in the Baltic Sea Investigated with High-Resolution Trajectory Modeling2013In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 42, no 4, p. SI 455-463Article in journal (Refereed)
    Abstract [en]

    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.

  • 12.
    Corell, Hanna
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Moksnes, Per-Olav
    Engqvist, Anders
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jonsson, Per R.
    Depth distribution of larvae critically affects their dispersal and the efficiency of marine protected areas2012In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 467, p. 29-+Article in journal (Refereed)
    Abstract [en]

    This study aims to improve estimates of dispersal by including information on larval traits, and in particular to explore how larval depth distribution affects connectivity and MPA (marine protected area) functionality in the Baltic Sea. A field survey showed that both invertebrates and fish differed in their larval depth distribution, ranging from surface waters to >100 m. A biophysical model of larval dispersal in the Baltic Sea showed that decreased depth distribution increased average dispersal distance 2.5-fold, decreased coastal retention and local recruitment, and substantially increased connectivity. Together with pelagic larval duration (PLD), depth distribution explained 80% of total variation in dispersal distance, whereas spawning season, and geographic and annual variations in circulation had only marginal effects. Median dispersal distances varied between 8 and 46 km, with 10% of simulated trajectories dispersing 30 to 160 km depending on drift depth and PLD. In the Baltic Sea, the majority of shallow Natura 2000 MPAs are <8 km in diameter. In the present study, only 1 of the 11 assessed larval taxa would have a recruitment >10% within MPAs of this size. Connectivity between MPAs was expected to be low for most larval trait combinations. Our simulations and the empirical data suggest that the MPA size within the Natura 2000 system is considerably below what is required for local recruitment of most sessile invertebrates and sedentary fish. Future designs of MPA networks would benefit from spatially explicit biophysical models that consider dispersal and connectivity for complex circulation patterns and informed larval traits.

  • 13.
    Corell, Hanna
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Broström, Göran
    Norwegian Meteorological Institute, Oslo, Norway.
    Wind sensitivity of the inter-ocean heat exchange2009In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 61, no 5, p. 635-653Article in journal (Refereed)
    Abstract [en]

    An idealised two-basin model is used to investigate the impact of the wind field on the heat exchange between the ocean basins. The scalar potential of the divergent component of the horizontal heat flux is computed, which gives a 'coarse-grained' image of the surface heat flux that captures the large-scale structure of the horizontal heat transport. Further the non-divergent component is examined, as well as the meridional heat transport and the temperature–latitude overturning stream function. A sensitivity analysis examines the heat transport response to changes in wind stress at different latitudes. The results are compared with results from an eddy-permitting global circulation model. The westerly wind stress over the Southern Ocean has two effects: a local reduction of the surface heat loss in response to the equatorward surface Ekman drift, and a global re-routing of the heat export from the Indo-Pacific. Without wind forcing, the Indo-Pacific heat export is released to the atmosphere in the Southern Ocean, and the net heat transport in the southern Atlantic is southward. With wind forcing, the Indo-Pacific export enters the Atlantic through the Aghulas and is released in the Northern Hemisphere. The easterlies enhance the poleward heat transport in both basins.

  • 14.
    Dey, Dipanjan
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    The coupled ocean–atmosphere hydrologic cycle2019In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 71, no 1, p. 1-11Article in journal (Refereed)
    Abstract [en]

    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.

  • 15.
    Döös, Kristofer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Engqvist, Anders
    Assessment of water exchange between a discharge region and the open sea: A comparison of different methodological concepts2007In: Estuarine coastal and shelf science, ISSN 0272-7714, Vol. 74, no 4, p. 709-721Article in journal (Refereed)
  • 16.
    Döös, Kristofer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Engqvist, Anders
    Book review of Lagrangian Analysis and Prediction of Coastal and Ocean Dynamics2008In: Marin Geophysical Researches, Vol. 29, no 2, p. 159-Article, book review (Other (popular science, discussion, etc.))
  • 17.
    Döös, Kristofer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jönsson, Bror
    Kjellsson, Joakim
    Evaluation of oceanic and atmospheric trajectory schemes in the TRACMASS trajectory model v6.02017In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 10, no 4, p. 1733-1749Article in journal (Refereed)
    Abstract [en]

    Three different trajectory schemes for oceanic and atmospheric general circulation models are compared in two different experiments. The theories of the trajectory schemes are presented showing the differential equations they solve and why they are mass conserving. One scheme assumes that the velocity fields are stationary for set intervals of time between saved model outputs and solves the trajectory path from a differential equation only as a function of space, i.e. stepwise stationary. The second scheme is a special case of the stepwise-stationary scheme, where velocities are assumed constant between general circulation model (GCM) outputs; it uses hence a fixed GCM time step. The third scheme uses a continuous linear interpolation of the fields in time and solves the trajectory path from a differential equation as a function of both space and time, i.e. a time-dependent scheme. The trajectory schemes are tested offline, i.e. using the already integrated and stored velocity fields from a GCM. The first comparison of the schemes uses trajectories calculated using the velocity fields from a high-resolution ocean general circulation model in the Agulhas region. The second comparison uses trajectories calculated using the wind fields from an atmospheric reanalysis. The study shows that using the time-dependent scheme over the stepwise-stationary scheme greatly improves accuracy with only a small increase in computational time. It is also found that with decreasing time steps the stepwise-stationary scheme becomes increasingly more accurate but at increased computational cost. The time-dependent scheme is therefore preferred over the stepwise-stationary scheme. However, when averaging over large ensembles of trajectories, the two schemes are comparable, as intrinsic variability dominates over numerical errors. The fixed GCM time step scheme is found to be less accurate than the stepwisestationary scheme, even when considering averages over large ensembles.

  • 18.
    Döös, Kristofer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kjellsson, Joakim
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jönsson, Bror Fredrik
    Department of Geosciences, Princeton University.
    TRACMASS—A Lagrangian Trajectory Model2013In: Preventive Methods for Coastal Protection: Towards the Use of Ocean Dynamics for Pollution Control / [ed] Soomere, Tarmo; Quak, Ewald, Springer, 2013, p. 225-249Chapter in book (Refereed)
    Abstract [en]

    A detailed description of the Lagrangian trajectory model TRACMASS is presented. The theory behind the original scheme for steady state velocities is derived for rectangular and curvilinear grids with different vertical coordinates for the oceanic and atmospheric circulation models. Two different ways to integrate the trajectories in time in TRACMASS are presented. These different time schemes are compared by simulating inertial oscillations, which show that both schemes are sufficiently accurate not to deviate from the analytical solution.The TRACMASS are exact solutions to differential equations and can hence be integrated both forward and backward with unique solutions. Two low-order trajectory subgrid parameterizations, which are available in TRACMASS, are explained. They both enable an increase of the Lagrangian dispersion, but are, however, too simple to simulate some of the Lagrangian properties that are desirable. The mass conservation properties of TRACMASS are shown to make it possible to follow the water or air masses both forward and backward in time, which also opens up for all sorts of calculations of water/air mass exchanges as well as Lagrangian stream functions.

  • 19.
    Döös, Kristofer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kjellsson, Joakim
    Zika, Jan
    Laliberte, Frederic
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Aldama Campino, Aitor
    Stockholm University, Faculty of Science, Department of Meteorology .
    The Coupled Ocean-Atmosphere Hydrothermohaline Circulation2017In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 30, no 2, p. 631-647Article in journal (Refereed)
    Abstract [en]

    The thermohaline circulation of the ocean is compared to the hydrothermal circulation of the atmosphere. The oceanic thermohaline circulation is expressed in potential temperature-absolute salinity space and comprises a tropical cell, a conveyor belt cell, and a polar cell, whereas the atmospheric hydrothermal circulation is expressed in potential temperature-specific humidity space and unifies the tropical Hadley and Walker cells as well as the midlatitude eddies into a single, global circulation. The oceanic thermohaline streamfunction makes it possible to analyze and quantify the entire World Ocean conversion rate between cold-warm and fresh-saline waters in one single representation. Its atmospheric analog, the hydrothermal streamfunction, instead captures the conversion rate between cold-warm and dry-humid air in one single representation. It is shown that the ocean thermohaline and the atmospheric hydrothermal cells are connected by the exchange of heat and freshwater through the sea surface. The two circulations are compared on the same diagramby scaling the axes such that the latent heat energy required to move an air parcel on the moisture axis is equivalent to that needed to move a water parcel on the salinity axis. Such a comparison leads the authors to propose that the Clausius-Clapeyron relationship guides both the moist branch of the atmospheric hydrothermal circulation and the warming branches of the tropical and conveyor belt cells of the oceanic thermohaline circulation.

  • 20.
    Döös, Kristofer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Meier, H E Markus
    Döscher, Ralf
    The Baltic haline conveyor belt or the overturning circulation and mixing in the Baltic.2004In: Ambio, ISSN 0044-7447, Vol. 33, no 4-5, p. 261-6Article in journal (Refereed)
  • 21.
    Döös, Kristofer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ballarotta, Maxime
    Stockholm University, Faculty of Science, Department of Meteorology .
    The world ocean thermohaline circulation2012In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 42, no 9, p. 1445-1460Article in journal (Refereed)
    Abstract [en]

    A new global streamfunction is presented and denoted the thermohaline streamfunction. This is defined as the volume transport in terms of temperature and salinity (hence no spatial variables). The streamfunction is used to analyze and quantify the entire World Ocean conversion rate between cold/warm and fresh/saline waters. It captures two main cells of the global thermohaline circulation, one corresponding to the conveyor belt and one corresponding to the shallow tropical circulation. The definition of a thermohaline streamfunction also enables a new method of estimating the turnover time as well as the heat and freshwater transports of the conveyor belt. The overturning time of the conveyor belt is estimated to be between 1000 and 2000 yr, depending on the choice of stream layer. The heat and freshwater transports of these two large thermohaline cells have been calculated by integrating the thermohaline streamfunction over the salinity or temperature, yielding a maximum heat transport of the conveyor belt of 1.2 PW over the 34.2-PSU salinity surface and a freshwater transport of 0.8 Sv (1 Sv = 10(6) m(3) s(-1)) over the 9 degrees C isotherm. This is a measure of the net interocean exchange of heat between the Atlantic and Indo-Pacific due to the thermohaline circulation.

  • 22.
    Döös, Kristofer
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology.
    Coward, Andrew C.
    Lagrangian decomposition of the Deacon Cell2008In: Journal of Geophysical Research: Oceans, Vol. 113, p. C07028-Article in journal (Refereed)
    Abstract [en]

    The meridional overturning cells in the Southern Ocean are decomposed by Lagrangian tracing using velocity and density fields simulated with an ocean general circulation model. Particular emphasis is given to the Deacon Cell. The flow is divided into four major components: 1) water circling around Antarctica in the Antarctic Circumpolar Current (ACC), 2) water leaving the ACC towards the north into the three world oceans, 3) water coming from the north and joining the ACC, mainly consisting of North Atlantic Deep Water (NADW) and 4) inter-ocean exchange between the three world oceans without circling around Antarctica. The Deacon Cell has an amplitude of 20 Sv, of which 6 Sv can be explained by the the east-west tilt of the ACC, 5 Sv by the east-west tilt of the sub-tropical gyre, and the remaining 9 Sv by the differences of the slope and depth of the southward transport of NADW and its return flow as less dense water. The diabatic or cross-isopycnal Deacon Cell is only 2 Sv.

  • 23.
    Engqvist, Anders
    et al.
    Stockholm University, Faculty of Science, Department of Systems Ecology.
    Döös, Kristofer
    Department of Meteorology.
    Andrejev, O
    Modeling water exchange and contaminant transport through a Baltic coastal region2006In: Ambio, ISSN 0044-7447, Vol. 35, no 8, p. 435-447Article in journal (Refereed)
  • 24.
    Hieronymus, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hallberg, Robert
    Oceanic Overturning and Heat Transport: The Role of Background Diffusivity2019In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, no 3, p. 701-716Article in journal (Refereed)
    Abstract [en]

    The role of oceanic background diapycnal diffusion for the equilibrium climate state is investigated in the global coupled climate model CM2G. Special emphasis is put on the oceanic meridional overturning and heat transport. Six runs with the model, differing only by their value of the background diffusivity, are run to steady state and the statistically steady integrations are compared. The diffusivity changes have large-scale impacts on many aspects of the climate system. Two examples are the volume-mean potential temperature, which increases by 3.6 degrees C between the least and most diffusive runs, and the Antarctic sea ice extent, which decreases rapidly as the diffusivity increases. The overturning scaling with diffusivity is found to agree rather well with classical theoretical results for the upper but not for the lower cell. An alternative empirical scaling with the mixing energy is found to give good results for both cells. The oceanic meridional heat transport increases strongly with the diffusivity, an increase that can only partly be explained by increases in the meridional overturning. The increasing poleward oceanic heat transport is accompanied by a decrease in its atmospheric counterpart, which keeps the increase in the planetary energy transport small compared to that in the ocean.

  • 25. Hordoir, Robinson
    et al.
    Axell, Lars
    Höglund, Anders
    Dieterich, Christian
    Fransner, Filippa
    Stockholm University, Faculty of Science, Department of Meteorology . Bergen University, Norway; Bjerknes Centre for Climate Research, Norway.
    Gröger, Matthias
    Liu, Ye
    Pemberton, Per
    Schimanke, Semjon
    Andersson, Helen
    Ljungemyr, Patrik
    Nygren, Petter
    Falahat, Saeed
    Nord, Adam
    Jönsson, Anette
    Lake, Iréne
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hieronymus, Magnus
    Dietze, Heiner
    Löptien, Ulrike
    Kuznetsov, Ivan
    Westerlund, Antti
    Tuomi, Laura
    Haapala, Jari
    Nemo-Nordic 1.0: a NEMO-based ocean model for the Baltic and North seas - research and operational applications2019In: Geoscientific Model Development, ISSN 1991-959X, E-ISSN 1991-9603, Vol. 12, no 1, p. 363-386Article in journal (Refereed)
    Abstract [en]

    We present Nemo-Nordic, a Baltic and North Sea model based on the NEMO ocean engine. Surrounded by highly industrialized countries, the Baltic and North seas and their assets associated with shipping, fishing and tourism are vulnerable to anthropogenic pressure and climate change. Ocean models providing reliable forecasts and enabling climatic studies are important tools for the shipping infrastructure and to get a better understanding of the effects of climate change on the marine ecosystems. Nemo-Nordic is intended to be a tool for both short-term and long-term simulations and to be used for ocean forecasting as well as process and climatic studies. Here, the scientific and technical choices within Nemo-Nordic are introduced, and the reasons behind the design of the model and its domain and the inclusion of the two seas are explained. The model's ability to represent barotropic and baroclinic dynamics, as well as the vertical structure of the water column, is presented. Biases are shown and discussed. The short-term capabilities of the model are presented, especially its capabilities to represent sea level on an hourly timescale with a high degree of accuracy. We also show that the model can represent longer timescales, with a focus on the major Baltic inflows and the variability in deep-water salinity in the Baltic Sea.

  • 26. Hordoir, Robinson
    et al.
    Axell, Lars
    Höglund, Anders
    Dieterich, Christian
    Fransner, Filippa
    Stockholm University, Faculty of Science, Department of Meteorology .
    Gröger, Matthias
    Liu, Ye
    Pemberton, Per
    Schimanke, Semjon
    Andersson, Helen
    Ljungemyr, Patrik
    Nygren, Petter
    Falahat, Saeed
    Nord, Adam
    Jönsson, Anette
    Lake, Iréne
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hieronymus, Magnus
    Stockholm University, Faculty of Science, Department of Meteorology .
    Dietze, Heiner
    Löptien, Ulrike
    Kuznetsov, Ivan
    Westerlund, Antti
    Tuomi, Laura
    Haapala, Jari
    Nemo-Nordic: A NEMO based ocean model for Baltic & North Seas, research and operational applicationsIn: Geoscientific Model Development Discussions, ISSN 1991-9611, E-ISSN 1991-962XArticle in journal (Refereed)
  • 27. Jacobi, Martin Nilsson
    et al.
    Andre, Carl
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jonsson, Per R.
    Identification of subpopulations from connectivity matrices2012In: Ecography, ISSN 0906-7590, E-ISSN 1600-0587, Vol. 35, no 11, p. 1004-1016Article in journal (Refereed)
    Abstract [en]

    Dispersal on the landscape/seascape scale may lead to complex spatial population structure with non-synchronous demography and genetic divergence. In this study we present a novel approach to identify subpopulations and dispersal barriers based on estimates of dispersal probabilities on the landscape scale. A theoretical framework is presented where the landscape connectivity matrix is analyzed for clusters as a signature of partially isolated subpopulations. Identification of subpopulations is formulated as a minimization problem with a tuneable penalty term that makes it possible to generate population subdivisions with varying degree of dispersal restrictions. We show that this approach produces superior results compared to alternative standard methods. We apply this theory to a dataset of modeled dispersal probabilities for a sessile marine invertebrate with free-swimming larvae in the Baltic Sea. For a range of critical connectivities we produce a hierarchical partitioning into subpopulations spanning dispersal probabilities that are typical for both genetic divergence and demographic independence. The mapping of subpopulations suggests that the Baltic Sea includes a fine-scale (100600 km) mosaic of invisible dispersal barriers. An analysis of the present network of marine protected areas reveal that protection is very unevenly distributed among the suggested subpopulations. Our approach can be used to assess the location and strength of dispersal barriers in the landscape, and identify conservation units when extensive genotyping is prohibitively costly to cover necessary spatial and temporal scales, e.g. in spatial management of marine populations.

  • 28.
    Jönsson, Bror
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology.
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology.
    Standing waves in the Gulf of Finland and their relationship to the basin-wide Baltic seiches2008In: Journal of Geophysical Research: Oceans, Vol. 113, p. C03004-Article in journal (Refereed)
    Abstract [en]

    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.

  • 29.
    Jönsson, Bror
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology.
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology.
    Baltic sub-basin turnover times examined using the Rossby Centre Ocean model.2004In: Ambio, ISSN 0044-7447, Vol. 33, no 4-5, p. 257-60Article in journal (Refereed)
  • 30.
    Kjellsson, Joakim
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lagrangian decomposition of the Hadley and Ferrel cells2012In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, p. L15807-Article in journal (Refereed)
    Abstract [en]

    The meridional overturning circulation of the atmosphere between 45°S and 45°N is decomposed using simulated 3D Lagrangian trajectories for calculating the Lagrangian overturning streamfunctions. This permits an analysis of meridional mass transports which otherwise cancel in time-averaged zonal-mean Eulerian streamfunctions. Overturning circulations inferred from trajectories of no net meridional transport are qualitatively similar to the Eulerian mean, but yield half the Hadley cell amplitude, and twice that of the Ferrel cell. Cross-equatorial transports of some 130 Sv result in two cells that account for the remainder of the Hadley cells. The overturning in midlatiudes is partly cancelled by large (≈150 Sv) meridional transports approximately following isentropes. Changes and implications for various coordinate systems, e.g., isentropic, are also discussed.

  • 31.
    Kjellsson, Joakim
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Surface drifters and model trajectories in the Baltic sea2012In: Boreal environment research, ISSN 1239-6095, E-ISSN 1797-2469, Vol. 17, no 6, p. 447-459Article in journal (Refereed)
    Abstract [en]

    Results from recent deployments of surface drifters in the Baltic Sea are presented. For the first time ever, the realism of model-generated trajectories was assessed by a statistical com- parison with trajectories of SVP drifters. The absolute dispersion (i.e. the distance from the initial point as a function of time) was found to be somewhat underestimated by the model trajectories. A severe underestimation of the relative dispersion (pair separation) was also noted, which may, to some extent, be due to the limited resolution of the model. However, the relative dispersion was also found to be very dependent on the initial separation of the model trajectory pairs. After filtering the inertial oscillations, a good agreement of the veloc- ity auto-correlations between the drifters and model trajectories was found. A discussion on the impact of these results on future trajectory modelling in the Baltic Sea is also provided. 

  • 32.
    Kjellsson, Joakim
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Laliberté, Frédéric B.
    Zika, Jan
    The Atmospheric General Circulation in Thermodynamical Coordinates2014In: Journal of Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 71, no 3, p. 916-928Article in journal (Refereed)
    Abstract [en]

    The zonal and meridional components of the atmospheric general circulation are used to define a global thermodynamic stream function in dry static energy versus latent heat coordinates. Diabatic motions in the tropical circulations and fluxes driven by midlatitude eddies are found to form a single, global thermodynamic cycle. Calculations based on the ERA-Interim reanalysis dataset indicate that the cycle has a peak transport of 428 Sv (Sv = 109 kg s−1). The thermodynamic cycle encapsulates a globally interconnected heat and water cycle comprising ascent of moist air where latent heat is converted into dry static energy, radiative cooling where dry air loses dry static energy, and a moistening branch where air is warmed and moistened. It approximately follows a tropical moist adiabat and is bounded by the Clausius-Clapeyron relationship for near-surface air. The variability of the atmospheric general circulation is related to ENSO events using reanalysis data from recent years (1979-2009) and historical simulations from the EC-Earth coupled climate model (1850-2005). The thermodynamic cycle in both EC-Earth and ERA-Interim widens and weakens with positive ENSO phases and narrows and strengthens during negative ENSO phases with a high correlation coefficient. Weakening in amplitude suggests a reduction in moist convection in the tropics, while widening suggests an increase in mean tropical near-surface moist static energy.

  • 33.
    Kjellsson, Joakim
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Soomere, Tarmo
    Evaluation and Tuning of Model Trajectories and Spreading Rates in the Baltic Sea Using Surface Drifter Observations2013In: Preventive Methods for Coastal Protection: Towards the Use of Ocean Dynamics for Pollution Control / [ed] Tarmo Soomere, Ewald Quak, Springer, 2013, p. 251-281Chapter in book (Refereed)
    Abstract [en]

    Results from experiments with surface drifters in the Baltic Sea in 2010–2011 are presented and discussed. In a first experiment, 12 SVP-B (Surface Velocity Program, with Barometer) drifters with a drogue at 12–18 m depth were deployed in the Baltic Sea. In a second experiment, shallow drifters extending to a depth of 1.5 m were deployed in the Gulf of Finland. Results from the SVP-B drifter experiment are compared to results from a regional ocean model and a trajectory code. Differences between the observed SVP-B drifters and simulated drifters are found for absolute dispersion (i.e., squared displacement from initial position) and relative dispersion (i.e., squared distance between two initially paired drifters). The former is somewhat underestimated since the simulated currents are neither as fast nor as variable as those observed. The latter is underestimated both due to the above-mentioned reasons and due to the resolution of the ocean model.

    For the shallower drifters, spreading in the upper 1–2 m of the Gulf of Finland is investigated. The spreading rate is about 200 m/day for separations <0.5 km, 500 m/day for separations below 1 km and in the range of 0.5–3 km/day for separations in the range of 1–4 km. The spreading rate does not follow Richardson’s law. The initial spreading, up to a distance of about d=100–150 m, is governed by the power law dt 0.27 whereas for larger separations the distance increases as dt2.5.

  • 34. Laliberte, F.
    et al.
    Zika, J.
    Mudryk, L.
    Kushner, P. J.
    Kjellsson, J.
    Doos, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Constrained work output of the moist atmospheric heat engine in a warming climate2015In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 347, no 6221, p. 540-543Article in journal (Refereed)
    Abstract [en]

    Incoming and outgoing solar radiation couple with heat exchange at Earth's surface to drive weather patterns that redistribute heat and moisture around the globe, creating an atmospheric heat engine. Here, we investigate the engine's work output using thermodynamic diagrams computed from reanalyzed observations and from a climate model simulation with anthropogenic forcing. We show that the work output is always less than that of an equivalent Carnot cycle and that it is constrained by the power necessary to maintain the hydrological cycle. In the climate simulation, the hydrological cycle increases more rapidly than the equivalent Carnot cycle. We conclude that the intensification of the hydrological cycle in warmer climates might limit the heat engine's ability to generate work.

  • 35. Nilsson, Jenny A. U.
    et al.
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ruti, Paolo M.
    Artale, Vincenzo
    Coward, Andrew
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Observed and Modeled Global Ocean Turbulence Regimes as Deduced from Surface Trajectory Data2013In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 43, no 11, p. 2249-2269Article in journal (Refereed)
    Abstract [en]

    A large-scale tool for systematic analyses of the dispersal and turbulent properties of ocean currents and the subsequent separation of dynamical regimes according to the prevailing trajectories taxonomy in a certain area was proposed by Rupolo. In the present study, this methodology has been extended to the analysis of model trajectories obtained by analytical computations of the particle advection equation using the Lagrangian open-source software package Tracing the Water Masses of the North Atlantic and the Mediterranean (TRACMASS), and intercomparisons have been made between the surface velocity fields from three different configurations of the global Nucleus for European Modelling of the Ocean (NEMO) ocean/sea ice general circulation model. Lagrangian time scales of the observed and synthetic trajectory datasets have been calculated by means of inverse Lagrangian stochastic modeling, and the influence of the model field spatial and temporal resolution on the analyses has been investigated. In global-scale ocean modeling, compromises are frequently made in terms of grid resolution and time averaging of the output fields because high-resolution data require considerable amounts of storage space. Here, the implications of such approximations on the modeled velocity fields and, consequently, on the particle dispersion, have been assessed through validation against observed drifter tracks. This study aims, moreover, to shed some light on the relatively unknown turbulent properties of near-surface ocean dynamics and their representation in numerical models globally and in a number of key regions. These results could be of interest for other studies within the field of turbulent eddy diffusion parameterization in ocean models or ocean circulation studies involving long-term coarse-grid model experiments.

  • 36.
    Nycander, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology.
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology.
    Broström, Göran
    Stockholm University, Faculty of Science, Department of Meteorology.
    Thermodynamic analysis of ocean circulation2007In: Journal of Physical Oceanography, Vol. 37, p. 2038-2052Article in journal (Refereed)
    Abstract [en]

    Calculating a stream function as function of depth and density is proposed as a new way of analysing the thermodynamic character of the overturning circulation in the global ocean. The sign of an overturning cell in this stream function directly shows whether it is driven mechanically by large-scale wind stress, or ''thermally'' by heat conduction and small scale mixing. It is also shown that the integral of this stream function gives the thermodynamic work performed by the fluid. The analysis is also valid for the Boussinesq equations, although formally there is no thermodynamic work in an incompressible fluid. The proposed method is applied both to an idealized coarse-resolution three-dimensional numerical ocean model, and to the realistic high-resolution OCCAM model. It is shown that the overturning circulation in OCCAM between 200 m and 1000 m depth is dominated by a thermally indirect cell of 24 Sv, forced by Ekman pumping. In the densenst and deepest waters there is a thermally direct cell of 18 Sv, which requires a forcing by around 100 GW of parameterized small-scale mixing.

  • 37. Soomere, Tarmo
    et al.
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Lehmann, Andreas
    Meier, H. E. Markus
    Murawski, Jens
    Myrberg, Kai
    Stanev, Emil
    The Potential of Current- and Wind-Driven Transport for Environmental Management of the Baltic Sea2014In: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 43, no 1, p. 94-104Article in journal (Refereed)
    Abstract [en]

    The ever increasing impact of the marine industry and transport on vulnerable sea areas puts the marine environment under exceptional pressure and calls for inspired methods for mitigating the impact of the related risks. We describe a method for preventive reduction of remote environmental risks caused by the shipping and maritime industry that are transported by surface currents and wind impact to the coasts. This method is based on characterizing systematically the damaging potential of the offshore areas in terms of potential transport to vulnerable regions of an oil spill or other pollution that has occurred in a particular area. The resulting maps of probabilities of pollution to be transported to the nearshore and the time it takes for the pollution to reach the nearshore are used to design environmentally optimized fairways for the Gulf of Finland, Baltic Proper, and south-western Baltic Sea.

  • 38.
    Thompson, Bijoy
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Ventilation of the Miocene Arctic Ocean: An idealized model study2010In: Paleoceanography, ISSN 0883-8305, E-ISSN 1944-9186, Vol. 25, p. PA4216-Article in journal (Refereed)
    Abstract [en]

    A model study of an idealized early Miocene Arctic Ocean has been undertaken. The work is motivated by the first drill core retrieved from the Lomonosov Ridge in the central Arctic Ocean, which suggests a transition from anoxic to oxic condition during the early Miocene, a feature presumably related to the opening of the Fram Strait. Here, the ventilation in a semienclosed basin, connected with the ocean through a strait with a sill, is examined using an ocean circulation model that includes a passive age tracer. In particular, we investigate how the ventilation depends on strait geometry, freshwater influx, and surface wind stress. We find that the turnover time, characterizing the bulk ventilation rate, is primarily controlled by the strait width and the wind stress. Generally, the oldest water in the basin is encountered near the sill depth, but wind forcing displaces the oldest water downward. For narrow straits, the turnover time gives an upper bound on the mean age of the basin water. The results have implications when translating local oxygen conditions, recorded in the sediment sequence from the Lomonosov Ridge, to basin-scale circulation patterns. Further, the results indicate that the early Miocene Arctic Ocean became well ventilated when the Fram Strait reached a width of about 100 km.

  • 39.
    Thompson, Bijoy
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Estimating ventilation time scales using overturning stream functions2014In: Ocean Dynamics, ISSN 1616-7341, E-ISSN 1616-7228, Vol. 64, no 6, p. 797-807Article in journal (Refereed)
    Abstract [en]

    A simple method for estimating ventilation time scales from overturning stream functions is proposed. The stream function may be computed using either geometric coordinates or a generalized vertical coordinate, such as potential density (salinity in our study). The method is tested with a three-dimensional circulation model describing an idealized semi-enclosed ocean basin ventilated through a narrow strait over a sill, and the result is compared to age estimates obtained from a passive numerical age tracer. The best result is obtained when using the stream function in salinity coordinates. In this case, the reservoir-averaged advection time obtained from the overturning stream function in salinity coordinates agrees rather well with the mean age of the age tracer, and the corresponding maximum ages agree very well.

  • 40. van Sebille, Erik
    et al.
    Griffies, Stephen M.
    Abernathey, Ryan
    Adams, Thomas P.
    Berloff, Pavel
    Biastoch, Arne
    Blanke, Bruno
    Chassignet, Eric P.
    Cheng, Yu
    Cotter, Colin J.
    Deleersnijder, Eric
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Drake, Henri F.
    Drijfhout, Sybren
    Gary, Stefan F.
    Heemink, Arnold W.
    Kjellsson, Joakim
    Koszalka, Inga Monika
    Lange, Michael
    Lique, Camille
    MacGilchrist, Graeme A.
    Marsh, Robert
    Adame, C. Gabriela Mayorga
    McAdam, Ronan
    Nencioli, Francesco
    Paris, Claire B.
    Piggott, Matthew D.
    Polton, Jeff A.
    Ruehs, Siren
    Shah, Syed H. A. M.
    Thomas, Matthew D.
    Wang, Jinbo
    Wolfram, Phillip J.
    Zanna, Laure
    Zika, Jan D.
    Lagrangian ocean analysis: Fundamentals and practices2018In: Ocean Modelling, ISSN 1463-5003, E-ISSN 1463-5011, Vol. 121, p. 49-75Article, review/survey (Refereed)
    Abstract [en]

    Lagrangian analysis is a powerful way to analyse the output of ocean circulation models and other ocean velocity data such as from altimetry. In the Lagrangian approach, large sets of virtual particles are integrated within the three-dimensional, time-evolving velocity fields. Over several decades, a variety of tools and methods for this purpose have emerged. Here, we review the state of the art in the field of Lagrangian analysis of ocean velocity data, starting from a fundamental kinematic framework and with a focus on large-scale open ocean applications. Beyond the use of explicit velocity fields, we consider the influence of unresolved physics and dynamics on particle trajectories. We comprehensively list and discuss the tools currently available for tracking virtual particles. We then showcase some of the innovative applications of trajectory data, and conclude with some open questions and an outlook. The overall goal of this review paper is to reconcile some of the different techniques and methods in Lagrangian ocean analysis, while recognising the rich diversity of codes that have and continue to emerge, and the challenges of the coming age of petascale computing.

  • 41. Wen, Qin
    et al.
    Yao, Jie
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Yang, Haijun
    Decoding Hosing and Heating Effects on Global Temperature and Meridional Circulations in a Warming Climate2018In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 31, no 23, p. 9605-9623Article in journal (Refereed)
    Abstract [en]

    The global temperature changes under global warming result from two effects: one is the pure radiative heating effect caused by a change in greenhouse gases, and the other is the freshwater effect related to changes in precipitation, evaporation, and sea ice. The two effects are separated in a coupled climate model through sensitivity experiments in this study. It is indicated that freshwater change has a significant cooling effect that can mitigate the global surface warming by as much as similar to 30%. Two significant regional cooling centers occur: one in the subpolar Atlantic and one in the Southern Ocean. The subpolar Atlantic cooling, also known as the warming hole, is triggered by sea ice melting and the southward cold-water advection from the Arctic Ocean, and is sustained by the weakened Atlantic meridional overturning circulation. The Southern Ocean surface cooling is triggered by sea ice melting along the Antarctic and is maintained by the enhanced northward Ekman flow. In these two regions, the effect of freshwater flux change dominates over that of radiation flux change, controlling the sea surface temperature change in the warming climate. The freshwater flux change also results in the Bjerknes compensation, with the atmosphere heat transport change compensating the ocean heat transport change by about 80% during the transient stage of global warming. In terms of global temperature and Earth's energy balance, the freshwater change plays a stabilizing role in a warming climate.

  • 42.
    Zarroug, Moundheur
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Energetics of tidally generated internal waves2010In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 62, no 1, p. 71-79Article in journal (Refereed)
    Abstract [en]

    The generation of internal waves from barotropic tides can be quantified in terms of the conversion rates. These have often been obtained by applying the WKB approximation, which yields an expression for the conversion rates which is proportional to the seabed buoyancy frequency NB . For small values of NB or strong variations of the buoyancy profile N (z), this gives unreliable results. Using homogenization theory it is here shown that the conversion rate instead depends on the value of N (z) averaged over a vertical region at the bottom of the same magnitude as the vertical length-scale of the internal wave, which for the lowest modes is of the same order as the entire ocean depth. This gives a substantially larger conversion rate. 

1 - 42 of 42
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