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  • 1. Bahrami, Fariba
    et al.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology.
    Existence of energy minimizing vortices attached to a flat-top seamount2007In: Nonlinear Analysis: Real World Applications, ISSN 1468-1218, Vol. 8, 288-294 p.Article in journal (Refereed)
    Abstract [en]

    The existence of an energy minimizer relative to a class of

    rearrangements of a given function is proved. The minimizers are stationary and stable solutions of the two-dimensional barotropic vorticity equation, governing the evolution of geophysical flow over a surface of variable height. The theorem proved implies the existence of a family of stable anticyclonic vortices with cyclonic potential vorticity over a seamount, and a corresponding family of cyclonic vortices with anticyclonic potential vorticity over a localized depression. The seamount is described by a characteristic function (corresponding to a flat top) with arbitrary shape.

  • 2.
    Bahrami, Fariba
    et al.
    University of Tabriz, Dept. of Mathematics.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Alikhani, Robab
    University of Tabriz, Dept. of Mathematics.
    Existence of energy maximizing vortices in a three-dimensionalquasigeostrophic shear flow with bounded height2010In: Nonlinear Analysis, ISSN 1468-1218, Vol. 11, no 3, 1589-1599 p.Article in journal (Refereed)
    Abstract [en]

    The existence of an energy maximizer relative to a class of rearrangements of agiven function is proved. The maximizers are stationary and stable solutions of thequasigeostrophic equation, which governs the time evolution of large-scale threedimensionalgeophysical flow in a vertically bounded domain. The background flow isunidirectional, with linear horizontal shear. The theorem proved implies the existence of afamily of stationary and stable vortices that rotate in the same direction as the backgroundshear. It extends an earlier theorem by Burton and Nycander, which is valid for a verticallyunbounded domain.

  • 3. Bahrami, Fariba
    et al.
    Taghvafard, Hadi
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mohammadi, Abbasali
    Stability investigation for steady solutions of the barotropic vorticity equation in R-22013In: Communications in nonlinear science & numerical simulation, ISSN 1007-5704, Vol. 18, no 3, 541-546 p.Article in journal (Refereed)
    Abstract [en]

    In this paper, we investigate the p-norm stability for vortices of geophysical flows over a surface of variable height that are strict maximizers of the kinetic energy relative to all isovortical flows. In this note, stability means nonlinear stability in the p-norm on the space of vorticity.

  • 4.
    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. 

  • 5.
    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, 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.

  • 6.
    Claesson, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Combined effect of global warming and increased CO2-concentration on vegetation growth in water-limited conditions2013In: Ecological Modelling, ISSN 0304-3800, Vol. 256, 23-30 p.Article in journal (Refereed)
    Abstract [en]

    The most severe impact of climate change on vegetation growth and agriculture is likely to occur under water-limited conditions. Under such conditions the plants optimize the inward flux of CO2 and the outward flux of water vapor (the transpiration) by regulating the size of the stomatal openings. Higher temperature increases water loss through transpiration, forcing the plants to diminish the stomatal openings, which decreases photosynthesis. This is counteracted by higher CO2 concentration, which allows plants to maintain the inward flux of CO2 through the smaller openings. These two counteracting effects, combined with the change in precipitation, determine the net change of biological productivity. Here, a vegetation sensitivity approximation (VSA) is introduced, in order to understand and estimate the combined effect of changed temperature, CO2 and precipitation to first order. The VSA is based on the physical laws of gas flux through the stomatal openings, and is only valid under water-limited conditions. It assumes that the temperature depends logarithmically on the CO2 concentration with a given climate sensitivity. Precipitation is included by assuming that it is proportional to the transpiration. This is reasonable underwater-limited conditions, when transpiration is often a large fraction of the precipitation. The VSA is compared to simulations with the dynamic vegetation model LPJ. The agreement is reasonable, and the deviations can be understood by comparison with Koppen's definition of arid climate: in an arid climate growth increases more according to LPJ than according to the VSA, and in non-arid conditions the reverse is true. Both the VSA and the LPJ simulations generally show increased growth with increasing CO2 levels and the resulting temperature increase, assuming precipitation to be unchanged. Thus, in this case the negative temperature effect is more than compensated by the positive effect of CO2.

  • 7.
    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, Vol. 42, no 9, 1445-1460 p.Article 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.

  • 8.
    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, C07028- p.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.

  • 9.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the Generation of Bottom - Trapped Internal Tides2015In: Journal of Physical Oceanography, ISSN 0022-3670, Vol. 45, no 2, 526-545 p.Article in journal (Refereed)
    Abstract [en]

    The interaction of the barotropic tide with bottom topography when the tidal frequency omega is smaller than the Coriolis frequency f is examined. The resulting waves are called bottom-trapped internal tides. The energy density associated with these waves is computed using linear wave theory and vertical normal-mode decomposition in an ocean of finite depth. The global calculation of the modal energy density is performed for the semidiurnal M-2 tidal constituent and the two major diurnal tidal constituents K-1 and O-1. An observationally based decay time scale of 3 days is then used to transform the energy density to energy flux in units of watts per square meter. The globally integrated energy fluxes are found to be 1.99 and 1.43GW for the K-1 and O-1 tidal constituents, respectively. For the M-2 tidal constituent, it is found to be 1.15 GW. The Pacific Ocean is found to be the most energetic basin for the bottom-trapped diurnal tides. Two regional estimates of the bottom-trapped energy flux are given for the Kuril Islands and the Arctic Ocean, in which the bottom-trapped waves play a role for the tidally induced vertical mixing. The results of this study can be incorporated into ocean general circulation models and coupled climate models to improve the parameterization of the vertical mixing induced by breaking of the internal tides.

  • 10.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    On the generation of bottom-trapped internal tidesManuscript (preprint) (Other academic)
  • 11.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Thurnherr, Andreas M.
    Hibiya, Toshiyuki
    Comparison of calculated energy flux of internal tides with microstructure measurements2014In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 66, 23240- p.Article in journal (Refereed)
    Abstract [en]

    Vertical mixing caused by breaking of internal tides plays a major role in maintaining the deep-ocean stratification. This study compares observations of dissipation from microstructure measurements to calculations of the vertical energy flux from barotropic to internal tides, taking into account the temporal variation due to the spring-neap tidal cycle. The dissipation data originate from two surveys in the Brazil Basin Tracer Release Experiment (BBTRE), and one over the LArval Dispersal along the Deep East Pacific Rise (LADDER3), supplemented with a few stations above the North-Atlantic Ridge (GRAVILUCK) and in the western Pacific (IZU). A good correlation is found between logarithmic values of energy flux and local dissipation in BBTRE, suggesting that the theory is able to predict energy fluxes. For the LADDER3, the local dissipation is much smaller than the calculated energy flux, which is very likely due to the different topographic features of BBTRE and LADDER3. The East Pacific Rise consists of a few isolated seamounts, so that most of the internal wave energy can radiate away from the generation site, whereas the Brazil Basin is characterised by extended rough bathymetry, leading to a more local dissipation. The results from all four field surveys support the general conclusion that the fraction of the internal-tide energy flux that is dissipated locally is very different in different regions.

  • 12.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Thurnherr, Andreas M.
    Hibiya, Toshiyuki
    Comparison of calculated energy flux of internal tides with microstructure measurementsManuscript (preprint) (Other academic)
  • 13.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zarroug, Moundheur
    Stockholm University, Faculty of Science, Department of Meteorology .
    Global Calculation of Tidal Energy Conversion into Vertical Normal Modes2014In: Journal of Physical Oceanography, ISSN 0022-3670, Vol. 44, no 12, 3225-3244 p.Article in journal (Refereed)
    Abstract [en]

    A direct calculation of the tidal generation of internal waves over the global ocean is presented. The calculation is based on a semianalytical model, assuming that the internal tide characteristic slope exceeds the bathymetric slope (subcritical slope) and the bathymetric height is small relative to the vertical scale of the wave, as well as that the horizontal tidal excursion is smaller than the horizontal topographic scale. The calculation is performed for the M-2 tidal constituent. In contrast to previous similar computations, the internal tide is projected onto vertical eigenmodes, which gives two advantages. First, the vertical density profile and the finite ocean depth are taken into account in a fully consistent way, in contrast to earlier work based on the WKB approximation. Nevertheless, the WKB-based total global conversion follows closely that obtained using the eigenmode decomposition in each of the latitudinal and vertical distributions. Second, the information about the distribution of the conversion energy over different vertical modes is valuable, since the lowest modes can propagate over long distances, while high modes are more likely to dissipate locally, near the generation site. It is found that the difference between the vertical distributions of the tidal conversion into the vertical modes is smaller for the case of very deep ocean than the shallow-ocean depth. The results of the present work pave the way for future work on the vertical and horizontal distribution of the mixing caused by internal tides.

  • 14.
    Falahat, Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zarroug, Moundheur
    Stockholm University, Faculty of Science, Department of Meteorology .
    Global calculation of tidal energy conversion rate into vertical normal modesManuscript (preprint) (Other academic)
  • 15.
    Fransner, Filippa
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute. Stockholm University, Faculty of Science, Department of Geological Sciences.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute. Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Meier, H. E. Markus
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Hordoir, Robinson
    Gustafsson, Erik
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Deutsch, Barbara
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute. Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Tracing terrestrial DOC in the Baltic Sea - a 3-D model study2016In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 30, no 2, 134-148 p.Article in journal (Refereed)
    Abstract [en]

    The fate of terrestrial organic matter brought to the coastal seas by rivers, and its role in the global carbon cycle, are still not very well known. Here the degradation rate of terrestrial dissolved organic carbon (DOCter) is studied in the Baltic Sea, a subarctic semi-enclosed sea, by releasing it as a tracer in a 3-D circulation model and applying linear decay constants. A good agreement with available observational data is obtained by parameterizing the degradation in two rather different ways; one by applying a decay time on the order of 10 years to the whole pool of DOCter, and one by dividing the DOCter into one refractory pool and one pool subject to a decay time on the order of 1 year. The choice of parameterization has a significant effect on where in the Baltic Sea the removal takes place, which can be of importance when modeling the full carbon cycle and the CO2 exchange with the atmosphere. In both cases the biogeochemical decay operates on time scales less than the water residence time. Therefore only a minor fraction of the DOCter reaches the North Sea, whereas approximately 80% is removed by internal sinks within the Baltic Sea. This further implies that DOCter mineralization is an important link in land-sea-atmosphere cycling of carbon in coastal- and shelf seas that are heavily influenced by riverine DOC.

  • 16. Green, J. A. Mattias
    et al.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    A comparison of tidal conversion parameterizations for tidal models2013In: Journal of Physical Oceanography, ISSN 0022-3670, Vol. 43, no 1, 104-119 p.Article in journal (Refereed)
    Abstract [en]

    The conversion of barotropic to baroclinic tidal energy in the global abyssal ocean is calculated using three different formulations. The calculations are done both offline, that is, using externally given tidal currents to estimate the energy conversion, and online, that is, by using the formulations to parameterize linear wave drag in a prognostic tidal model. All three schemes produce globally integrated offline dissipation rates beneath 500-m depth of similar to 0.6-0.8TW for the M-2 constituent, but the spatial structures vary significantly between the parameterizations. Detailed investigations of the energy transfer in local areas confirm the global results: there are large differences between the schemes, although the horizontally integrated conversion rates are similar. The online simulations are evaluated by comparing the sea surface elevation with data from the TOPEX/Poseidon database, and the error is then significantly lower when using the parameterization provided by Nycander than with the other two parameterizations examined.

  • 17. Hassler, John
    et al.
    Krusell, Per
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Climate policy2015In: Economic Policy: A European Forum, ISSN 0266-4658, E-ISSN 1468-0327, Vol. 31, no 87, 501-+ p.Article in journal (Refereed)
    Abstract [en]

    This paper makes suggestions for climate policy and defends them based on recent research in economics and the natural sciences. In summary: (i) the optimal carbon tax is rather modest; (ii) the key climate threat is coal; (iii) a carbon tax is to be preferred over a quantity-based system; (iv) the optimal tax on carbon does not appreciably harm growth; (v) subsidies to green technology are beneficial for the climate only to the extent that they make green technology outcompete coal; and (vi) a carbon tax is politically feasible.

  • 18.
    Heifetz, Eyal
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Tel Aviv University, Israel.
    Mak, J.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Umurhan, O. M.
    Interacting vorticity waves as an instability mechanism for magnetohydrodynamic shear instabilities2015In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 767, 199-225 p.Article in journal (Refereed)
    Abstract [en]

    The interacting vorticity wave formalism for shear flow instabilities is extended here to the magnetohydrodynamic (MUD) setting, to provide a mechanistic description for stabilising and destabilising shear instabilities by the presence of a background magnetic field. The interpretation relies on local vorticity anomalies inducing a non-local velocity field, resulting in action at a distance. It is shown here that the waves supported by the system are able to propagate vorticity via the Lorentz force, and waves may interact. The existence of instability then rests upon whether the choice of basic state allows for phase locking and constructive interference of the vorticity waves via mutual interaction. To substantiate this claim, we solve the instability problem of two representative basic states, one where a background magnetic field stabilises an unstable flow and the other where the field destabilises a stable flow, and perform relevant analyses to show how this mechanism operates in MHD.

  • 19.
    Hieronymus, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Walin, Gösta
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Exhaling Alkalinity: The potential for sulfate reduction and pyrite deposition to alter the ocean-atmosphere carbon balance during oceanic anoxic eventsManuscript (preprint) (Other academic)
  • 20.
    Hieronymus, Magnus
    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 .
    Water Mass Transformation in Salinity Temperature Space2014In: Journal of Physical Oceanography, ISSN 0022-3670, Vol. 44, no 9, 2547-2568 p.Article in journal (Refereed)
    Abstract [en]

    This article presents a new framework for studying water mass transformations in salinity temperature space that can, with equal ease, be applied to study water mass transformation in spaces defined by any two conservative tracers. It is shown how the flow across isothermal and isohaline surfaces in the ocean can be quantified from knowledge of the nonadvective fluxes of heat and salt. It is also shown how these cross-isothermal and cross-isohaline flows can be used to form a continuity equation in salinity temperature space. These flows are then quantified in a state-of-the-art ocean model. Two major transformation cells are found: a tropical cell driven primarily by surface fluxes and dianeutral diffusion and a conveyor belt cell where isoneutral diffusion is also important. Both cells are similar to cells found in earlier work on the thermohaline streamfunction. A key benefit with this framework over a streamfunction approach is that transformation due to different diabatic processes can be studied individually. The distributions of volume and surface area in S-T space are found to be useful for determining how transformations due to these different processes affect the water masses in the model. The surface area distribution shows that the water mass transformations due to surface fluxes tend to be directed away from S-T regions that occupy large areas at the sea surface.

  • 21. Hieronymus, Magnus
    et al.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Finding the Minimum Potential Energy State by Adiabatic Parcel Rearrangements with a Nonlinear Equation of State: An Exact Solution in Polynomial Time2015In: Journal of Physical Oceanography, ISSN 0022-3670, Vol. 45, no 7, 1843-1857 p.Article in journal (Refereed)
    Abstract [en]

    The problem of finding the state of minimum potential energy through the rearrangement of water parcels with a nonlinear equation of state is discussed in the context of a combinatorial optimization problem. It is found that the problem is identical to a classical optimization problem called the linear assignment problem. This problem belongs to a problem class known as P, a class of problems that have known efficient solutions. This is very fortunate since this study's problem has been suggested to be an asymmetric traveling salesman problem. A problem that belongs to a class called NP-hard, for which no efficient solutions are known. The difference between the linear assignment problem and the traveling salesman problem is discussed and made clear by looking at the different constraints used for the two problems. It is also shown how the rearrangement of water parcels that minimizes the potential energy can be found in polynomial time using the so-called Hungarian algorithm. The Hungarian algorithm is then applied to a simplified ocean stratification, and the result is compared to a few different approximate solutions to the minimization problem. It is found that the improved accuracy over the approximate methods comes at a high computational cost. Last, the algorithm is applied to a realistic ocean stratification using a technique that splits the minimization problem into smaller bits.

  • 22.
    Hieronymus, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    The budgets of heat and salinity in NEMO2013In: Ocean Modelling, ISSN 1463-5003, Vol. 67, 28-38 p.Article in journal (Refereed)
    Abstract [en]

    The near steady state heat and salinity budgets under surfaces of constant depth are examined in the Nucleus for European Modelling of the Ocean (NEMO) model. It is seen that the heat fluxes in NEMO are difficult to reconcile with the idea of a deep ocean in advection-diffusion balance. Some reasons for this are that the resolved heat advection is downward above 2000 m, and that geothermal heating is, in fact, a major heat source in the deeper parts of the domain. It is also seen that isoneutral diffusion gives a very large contribution to the budgets and that the fluxes from isoneutral diffusion is in general upward. It is explained how the sign of these fluxes depends on the stratification. The heat budget for the upper 100 m of the ocean is seen to be dominated by penetrative shortwave radiation, which is so influential that we would have a mixed layer of considerable thickness even in the absence of other sources of turbulent mixing. Penetrative shortwave radiation is therefore a considerable source of potential energy.

  • 23.
    Hieronymus, Magnus
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    The buoyancy budget with a nonlinear equation of state2013In: Journal of Physical Oceanography, ISSN 0022-3670, Vol. 43, no 1, 176-186 p.Article in journal (Refereed)
    Abstract [en]

    The nonlinear equation of state of seawater introduces a sink or source of buoyancy when water parcels of unequal salinities and temperatures are mixed. This article contains quantitative estimates of these nonlinear effects on the buoyancy budget of the global ocean. It is shown that the interior buoyancy sink can be determined from surface buoyancy fluxes. These surface buoyancy fluxes are calculated using two surface heat flux climatologies, one based on in situ measurements and the other on a reanalysis, in both cases using a nonlinear equation of state. It is also found that the buoyancy budget in the ocean general circulation model Nucleus for European Modeling of the Ocean (NEMO) is in good agreement with the buoyancy budgets based on the heat flux climatologies. Moreover, an examination of the vertically resolved buoyancy budget in NEMO shows that in large parts of the ocean the nonlinear buoyancy sink gives the largest contribution to this budget.

  • 24.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Marin geovetenskap.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Marin geovetenskap.
    Rudels, Bert
    Nycander, Jonas
    Department of Meteorology. Oceanografi.
    Frank, Martin
    Mayer, Larry
    Jokat, Wilfried
    Sangiorgi, Fransesca
    O'Regan, Matt
    Brinkhuis, Henk
    King, John
    Moran, Kathryn
    The Early Miocene Onset of a Ventilated Circulation Regime in the Arctic Ocean2007In: Nature, Vol. 447, no 7147, 986-990 p.Article in journal (Refereed)
    Abstract [en]

    Deep-water formation in the northern North Atlantic Ocean and the Arctic Ocean is a key driver of the global thermohaline circulation and hence also of global climate. Deciphering the history of the circulation regime in the Arctic Ocean has long been prevented by the lack of data from cores of Cenozoic sediments from the Arctic’s deep-sea floor. Similarly, the timing of the opening of a connection between the northern North Atlantic and the Arctic Ocean, permitting deep-water exchange, has been poorly constrained. This situation changed when the first drill cores were recovered from the central Arctic Ocean. Here we use these cores to show that the transition from poorly oxygenated to fully oxygenated (‘ventilated’) conditions in the Arctic Ocean occurred during the later part of early Miocene times. We attribute this pronounced change in ventilation regime to the opening of the Fram Strait. A palaeo-geographic and palaeo-bathymetric reconstruction of the Arctic Ocean, together with a physical oceanographic analysis of the evolving strait and sill conditions in the Fram Strait, suggests that the Arctic Ocean went from an oxygenpoor ‘lake stage’, to a transitional ‘estuarine sea’ phase with variable ventilation, and finally to the fully ventilated ‘ocean’ phase 17.5 Myr ago. The timing of this palaeo-oceanographic change coincides with the onset of the middle Miocene climatic optimum, although it remains unclear if there is a causal relationship between these two events.

  • 25.
    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, C03004- p.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.

  • 26.
    Magnusson, Linus
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Källén, Erland
    Stockholm University, Faculty of Science, Department of Meteorology.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology.
    Initial state perturbations in ensemble forecasting2008In: Nonlinear processes in geophysics, ISSN 1023-5809, Vol. 15, no 5, 751-759 p.Article in journal (Refereed)
    Abstract [en]

    Due to the chaotic nature of atmospheric dynamics, numerical weather prediction systems are sensitive to errors in the initial conditions. To estimate the forecast uncertainty, forecast centres produce ensemble forecasts based on perturbed initial conditions. How to optimally perturb the initial conditions remains an open question and different methods are in use. One is the singular vector (SV) method, adapted by ECMWF, and another is the breeding vector (BV) method (previously used by NCEP). In this study we compare the two methods with a modified version of breeding vectors in a low-order dynamical system (Lorenz-63). We calculate the Empirical Orthogonal Functions (EOF) of the subspace spanned by the breeding vectors to obtain an orthogonal set of initial perturbations for the model. We will also use Normal Mode perturbations. Evaluating the results, we focus on the fastest growth of a perturbation. The results show a large improvement for the BV-EOF perturbations compared to the non-orthogonalised BV. The BV-EOF technique also shows a larger perturbation growth than the SVs of this system, except for short time-scales. The highest growth rate is found for the second BV-EOF for the long-time scale. The differences between orthogonal and non-orthogonal breeding vectors are also investigated using the ECMWF IFS-model. These results confirm the results from the Loernz-63 model regarding the dependency on orthogonalisation

  • 27.
    Magnusson, Linus
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Källén, Erland
    Stockholm University, Faculty of Science, Department of Meteorology .
    Flow-dependent versus flow-independent initial perturbations for ensemble forecasting2009In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, Vol. 61A, no 2, 194-209 p.Article in journal (Refereed)
  • 28. Melet, Angelique
    et al.
    Nikurashin, Maxim
    Muller, Caroline
    Falahat, Saeed
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Timko, Patrick G.
    Arbic, Brian K.
    Goff, John A.
    Internal tide generation by abyssal hills using analytical theory2013In: Journal of Geophysical Research - Oceans and Atmospheres, ISSN 0148-022A, Vol. 118, no 11, 6303-6318 p.Article in journal (Refereed)
    Abstract [en]

    Internal tide driven mixing plays a key role in sustaining the deep ocean stratification and meridional overturning circulation. Internal tides can be generated by topographic horizontal scales ranging from hundreds of meters to tens of kilometers. State of the art topographic products barely resolve scales smaller than ∼10 km in the deep ocean. On these scales abyssal hills dominate ocean floor roughness. The impact of abyssal hill roughness on internal-tide generation is evaluated in this study. The conversion of M2 barotropic to baroclinic tidal energy is calculated based on linear wave theory both in real and spectral space using the Shuttle Radar Topography Mission SRTM30_PLUS bathymetric product at 1/120° resolution with and without the addition of synthetic abyssal hill roughness. Internal tide generation by abyssal hills integrates to 0.1 TW globally or 0.03 TW when the energy flux is empirically corrected for supercritical slope (i.e., ∼10% of the energy flux due to larger topographic scales resolved in standard products in both cases). The abyssal hill driven energy conversion is dominated by mid-ocean ridges, where abyssal hill roughness is large. Focusing on two regions located over the Mid-Atlantic Ridge and the East Pacific Rise, it is shown that regionally linear theory predicts an increase of the energy flux due to abyssal hills of up to 100% or 60% when an empirical correction for supercritical slopes is attempted. Therefore, abyssal hills, unresolved in state of the art topographic products, can have a strong impact on internal tide generation, especially over mid-ocean ridges.

  • 29. Nycander, J
    et al.
    Rudels, B
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Oceanographic implications of the Fram Strait opening in Early Miocene2007In: First Conference on Arctic Palaeoclimate and its Extremes (APEX): The Royal Swedish Academy of Sciences, 2007, March 28-29 p.Conference paper (Other academic)
  • 30.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Energy Conversion, Mixing Energy, and Neutral Surfaces with a Nonlinear Equation of State2011In: Journal of Physical Oceanography, ISSN 0022-3670, Vol. 41, no 1, 28-41 p.Article in journal (Refereed)
    Abstract [en]

    A local neutral plane is defined so that a water parcel that is displaced adiabatically a small distance along the plane continues to have the same density as the surrounding water. Since such a displacement does not change the density field or the gravitational potential energy, it is generally assumed that it does not produce a restoring buoyancy force. However, it is here shown that because of the nonlinear character of the equation of state (in particular the thermobaric effect) such a neutral displacement is accompanied by a conversion between internal energy E and gravitational potential energy U, and an equal conversion between U and kinetic energy K. While there is thus no net change of U. K does change. This implies that a force is in fact required for the displacement. It is further shown that displacements that are orthogonal to a vector P do not induce conversion between U and K, and therefore do not require a force. Analogously to neutral surfaces, which are defined to be approximately orthogonal to the dianeutral vector N. one may define P surfaces to be approximately orthogonal to P. These P surfaces are intermediate between neutral surfaces and surfaces of constant sigma(0) (potential density reference to the surface). If the equation of state is linear, there exists a well-known expression for the mixing energy in terms of the diapycnal flow. This expression is here generalized for a general nonlinear equation of state. The generalized expression involves the velocity component along P. Since P is not orthogonal to neutral surfaces, this means that stationary flow along neutral surfaces in general requires mixing energy.

  • 31.
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Horizontal convection with a non-linear equation of state: generalization of a theorem of Paparella and Young2010In: Tellus. Series A, Dynamic meteorology and oceanography, ISSN 0280-6495, E-ISSN 1600-0870, Vol. 62, no 2, 134-137 p.Article in journal (Refereed)
    Abstract [en]

    An upper bound is derived for the total dissipation rate in an ocean forced exclusively by surface fluxes of heat and freshwater, assuming a non-linear equation of state. This generalizes the upper bound found by Paparella and Young, which is valid for a flow forced by an imposed temperature distribution at the surface and a linear equation of state. Like this previous result, the present one shows that the dissipation rate vanishes in the limit of vanishing molecular diffusivity of temperature and salinity, if the range of temperatures and salinities occurring in the fluid is regarded as given. A numerical evaluation for realistic ocean parameters shows that the upper bound is two orders of magnitude smaller than present estimates of the energy transformation involved in the deep ocean circulation. This supports the conclusion that mechanical forcing by winds and tides is necessary to sustain the deep ocean circulation.

  • 32.
    Nycander, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Hieronymus, Magnus
    Roquet, Fabien
    Stockholm University, Faculty of Science, Department of Meteorology .
    The nonlinear equation of state of sea water and the global water mass distribution2015In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 18, 7714-7721 p.Article in journal (Refereed)
    Abstract [en]

    The role of nonlinearities of the equation of state (EOS) of seawater for the distribution of water masses in the global ocean is examined through simulations with an ocean general circulation model with various manipulated versions of the EOS. A simulation with a strongly simplified EOS, which contains only two nonlinear terms, still produces a realistic water mass distribution, demonstrating that these two nonlinearities are indeed the essential ones. Further simulations show that each of these two nonlinear terms affects a specific aspect of the water mass distribution: the cabbeling term is crucial for the formation of Antarctic Intermediate Water and the thermobaric term for the layering of North Atlantic Deep Water and Antarctic Bottom Water.

  • 33.
    Nycander, Jonas
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology.
    Hogg, Andrew
    Frankcombe, Leela
    Open boundary conditions for nonlinear channel flow2008In: Ocean Modelling, Vol. 24, 108-121 p.Article in journal (Refereed)
    Abstract [en]

    Open boundary conditions are derived for the one-dimensional nonrotating two-layer shallow-water equations. The conditions are based on characteristics of the external and internal modes. It is possible to find exact nonlinear characteristic conditions for the external mode, as well as approximate nonlinear conditions for the internal mode. These conditions can also be linearised by Taylor expansion; the approximate linear conditions are similar to those used in several previous studies. Both of the nonlinear and linearised conditions perform well, indicating that either the nonlinear or linearised conditions may potentially be extended to the more general case of multi-layer flows.

  • 34.
    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, 2038-2052 p.Article 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.

  • 35.
    Roquet, Fabien
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Madec, Gurvan
    Brodeau, Laurent
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Defining a Simplified Yet "Realistic" Equation of State for Seawater2015In: Journal of Physical Oceanography, ISSN 0022-3670, E-ISSN 1520-0485, Vol. 45, no 10, 2564-2579 p.Article in journal (Refereed)
    Abstract [en]

    There is a growing realization that the nonlinear nature of the equation of state has a deep impact on the global ocean circulation; however, the understanding of the global effects of these nonlinearities remains elusive. This is partly because of the complicated formulation of the seawater equation of state making it difficult to handle in theoretical studies. In this paper, a hierarchy of polynomial equations of state of increasing complexity, optimal in a least squares sense, is presented. These different simplified equations of state are then used to simulate the ocean circulation in a global 2 degrees-resolution configuration. Comparisons between simulated ocean circulations confirm that nonlinear effects are of major importance, in particular influencing the circulation through determination of the static stability below the mixed layer, thus controlling rates of exchange between the atmosphere and the ocean interior. It is found that a simple polynomial equation of state, with a quadratic term in temperature (for cabbeling), a temperature-pressure product term (for thermobaricity), and a linear term in salinity, that is, only four tuning parameters, is enough to simulate a reasonably realistic global circulation. The best simulation is obtained when the simplified equation of state is forced to have an accurate thermal expansion coefficient near the freezing point, highlighting the importance of polar regions for the global stratification. It is argued that this simplified equation of state will be of great value for theoretical studies and pedagogical purposes.

  • 36.
    Thompson, Bijoy
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    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 .
    Doos, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    A model study of the first ventilated regime of the Arctic Ocean during the early Miocene2012In: Polar Research, ISSN 0800-0395, Vol. 31, 10859- p.Article in journal (Refereed)
    Abstract [en]

    The tectonic opening of Fram Strait during the Neogene was a significant geological event that transferred the Arctic Ocean from a poorly ventilated enclosed basin, with weak exchange with the North Atlantic, to a fully ventilated ocean stage''. Previous tectonic and physical oceanographic analyses suggest that the early Miocene Fram Strait was likely several times narrower and less than half as deep as the present-day 400 km wide and 2550 m deep strait. Here we use an ocean general circulation model with a passive age tracer included to further address the effect of the Fram Strait opening on the early Miocene Arctic Ocean circulation. The model tracer age exhibits strong spatial gradient between the two major Arctic Ocean deep basins: the Eurasian and Amerasian basins. There is a two-layer stratification and the exchange flow through Fram Strait shows a bi-layer structure with a low salinity outflow from the Arctic confined to a relatively thin upper layer and a saline inflow from the North Atlantic below. Our study suggests that although Fram Strait was significantly narrower and shallower during early Miocene, and the ventilation mechanism quite different in our model, the estimated ventilation rates are comparable to the chemical tracer estimates in the present-day Arctic Ocean. Since we achieved ventilation of the Arctic Ocean with a prescribed Fram Strait width of 100 km and sill depth of 1000 m, ventilation may have preceded the timing of a full ocean depth connection between the Arctic Ocean and North Atlantic established through seafloor spreading and the development of the Lena Trough.

  • 37.
    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, Vol. 25, PA4216- p.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.

  • 38.
    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, 797-807 p.Article 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.

  • 39. Turnewitsch, Robert
    et al.
    Falahat, Saeed
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Dale, Andrew
    Scott, Robert B.
    University of Texas Austin, USA.
    Furnival, Darran
    Deep-sea fluid and sediment dynamics-Influence of hill- to seamount-scale seafloor topography2013In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 127, 203-241 p.Article in journal (Refereed)
    Abstract [en]

    Deep-sea sediments play a central role in a wide range of subject areas. A number of important controls on the formation of sedimentary deposits have been studied. However, to date, the impact of submarine landscape geometry as a possible control has received comparatively little attention. This seems to be particularly true for intermediate-scale topographic features such as abyssal hills, knolls and seamounts that can be found in many regions of the global seafloor: recent estimates suggest that in the deep open oceans, away from continental margins, there might be as many as similar to 25 x 10(6) abyssal hills, knolls and seamounts. Despite this large number very little is known about how they influence environmental complexity and patchiness, biogeochemical fluxes and the formation of sedimentary records. This paper reviews the currently known types of fluid-flow interactions with abyssal hills, knolls and seamounts that could potentially influence the way sediments are formed. The main types of relevant flow components are: quasi-steady to eddying background flow; internal lee and near-inertial waves; barotropic and baroclinic tides; and seamount-trapped waves. Previous studies looking into systematic links between fluid dynamics and sediments at hills, knolls and seamounts are reviewed. Finally, a case study is presented which aims to combine our current knowledge and investigate whether a given combination of recent fluid-flow components leaves a detectable imprint in the recent sediments on and around a short seamount. The main conclusions and implications are as follows. (1) Topographically generated flow-field geometries that are composed of a number of different prevailing fluid-flow components can be reflected and detected in properties of the underlying sediments. (2) Tidal and other higher-frequency (lee-wave, near-inertial) components of deep-ocean currents can be essential for locally driving total current velocities across threshold values for non-deposition/erosion/resuspension of freshly deposited deep-sea sediments. Moreover, there is evidence suggesting that not only maximum current speeds but also intensities of higher-frequency (tidal and/or (near-)inertial) current-direction variability might control sediment dynamics and sediment formation. This relativises the view that current speed is the main, or even only, controlling factor for sediment dynamics and sediment formation. (3) When it comes to the reconstruction of paleo-flows, these findings imply that certain sedimentary records may well reveal more about variability in the higher-frequency flow components than about variability in the basin-scale net flow component that often is the focus of paleoceanographic studies. (4) Single-core paleo-records from hill-, seamount- or similarly controlled sediment deposits may be biased due to the asymmetry of flow fields around these topographic features. To arrive at unbiased paleo-records for non-fluid-dynamic parameters, the influence of the flow-field geometry would have to be removed from the record first (5) It seems the mechanistic understanding of hill- and seamount-related flow/topography interactions and their links to sediment dynamics is approaching a level that may (a) facilitate improved interpretation of topographically controlled sedimentary paleo-records, (b) help fill in the knowledge gap that exists for functional deep-sea biodiversity at intermediate space scales, and (c) improve predictive capabilities for exploration of economically relevant iron-manganese (Fe-Mn) crusts on seamounts.

  • 40. Turnewitsch, Robert
    et al.
    Reyss, Jean-Louis
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology.
    Waniek, Joanna
    Lampitt, Richard
    Internal tides and sediment dynamics in the deep sea: Evidence from radioactive 234Th/238U disequilibria2008In: Deep-Sea Research. Part I: Oceanographic Research Papers, ISSN 0967-0637, Vol. 55, no 12, 1727-1747 p.Article in journal (Refereed)
    Abstract [en]

    Residual flow, barotropic tides and internal (baroclinic) tides interact in a number of ways with kilometer-scale seafloor topography such as abyssal hills and seamounts. Because of their likely impact on vertical mixing such interactions are potentially important for ocean circulation and the mechanisms and the geometry of these interactions are a matter of ongoing studies. In addition, very little is known about how these interactions are reflected in the sedimentary record. This multi-year study investigates if flow / topography interactions are reflected in distributional patterns of the natural short-lived (half life: 24.1 d) particulate-matter tracer 234Th relative to its conservative (non-particle-reactive) and very long-lived parent nuclide 238U. The sampling sites were downstream of, or surrounded by, fields of short seamounts and, therefore, very likely to be influenced by nearby flow / topography interactions. Between about 200 m and 1000 m above the seafloor at the sampling sites recurrent ‘fossil’ disequilibria were detected. ‘Fossil’ disequilibria are defined by clearly detectable 234Th/238U disequilibria (total 234Th radioactivity < 238U radioactivity, indicating a history of intense particulate 234Th scavenging and particulate-matter settling from the sampled parcel of water) and conspicuously low particle-associated 234Th activities. ‘Fossil’ disequilibria were centered around levels in the water column which correspond to the average height of the short seamounts near the sampling sites. This suggests the ‘fossil’ disequilibria are formed on the seamount slopes. Moreover, the magnitude of the ‘fossil’ disequilibria suggests that the slopes of the short seamounts in the study region are characterized by particularly vigorous fluid dynamics. Since ‘fossil’ disequilibria already occurred at ~ O(1 – 10 km) away from the seamount slopes it is likely that these vigorous fluid dynamics rapidly decay away from the slopes on scales of O(1 ? 10 km). These conclusions are supported by the horizontal distribution and magnitude of the modeled total (barotropic + baroclinic) tidal current velocities of the predominating tidal M2 constituent: On (near?)critical seamount slopes baroclinic tides lead to localized [~ O(1 km)] increases of the overall tidal current velocity by a factor of ~ 2, thereby pushing the total current velocity well above the threshold for sediment erosion. The results of this and a previous study (Turnewitsch et al. 2004, Earth and Planetary Science Letters 222, 1023-1036) show that kilometer-scale flow / topography interactions leave a marine geochemical imprint which may help to develop sediment proxies for the reconstruction of past changes of fluid dynamics in the deep sea, particularly the internal wave field. Sites with (near?)critical seafloor slopes are the most promising sites to study paleo-changes of internal tides. To reconstruct the paleo-flow of residual currents and barotropic tides, sediments covering isolated topography of comparable breadth and width, and with sub-critical slopes, seem to be more appropriate. For the sediment-based reconstruction of paleo-parameters other than physical oceanographic ones kilometer-scale topography should be avoided altogether.Residual flow, barotropic tides and internal (baroclinic) tides interact in a number of ways with kilometer-scale seafloor topography such as abyssal hills and seamounts. Because of their likely impact on vertical mixing such interactions are potentially important for ocean circulation and the mechanisms and the geometry of these interactions are a matter of ongoing studies. In addition, very little is known about how these interactions are reflected in the sedimentary record. This multi-year study investigates if flow / topography interactions are reflected in distributional patterns of the natural short-lived (half life: 24.1 d) particulate-matter tracer 234Th relative to its conservative (non-particle-reactive) and very long-lived parent nuclide 238U. The sampling sites were downstream of, or surrounded by, fields of short seamounts and, therefore, very likely to be influenced by nearby flow / topography interactions. Between about 200 m and 1000 m above the seafloor at the sampling sites recurrent ‘fossil’ disequilibria were detected. ‘Fossil’ disequilibria are defined by clearly detectable 234Th/238U disequilibria (total 234Th radioactivity < 238U radioactivity, indicating a history of intense particulate 234Th scavenging and particulate-matter settling from the sampled parcel of water) and conspicuously low particle-associated 234Th activities. ‘Fossil’ disequilibria were centered around levels in the water column which correspond to the average height of the short seamounts near the sampling sites. This suggests the ‘fossil’ disequilibria are formed on the seamount slopes. Moreover, the magnitude of the ‘fossil’ disequilibria suggests that the slopes of the short seamounts in the study region are characterized by particularly vigorous fluid dynamics. Since ‘fossil’ disequilibria already occurred at ~ O(1 – 10 km) away from the seamount slopes it is likely that these vigorous fluid dynamics rapidly decay away from the slopes on scales of O(1 ? 10 km). These conclusions are supported by the horizontal distribution and magnitude of the modeled total (barotropic + baroclinic) tidal current velocities of the predominating tidal M2 constituent: On (near?)critical seamount slopes baroclinic tides lead to localized [~ O(1 km)] increases of the overall tidal current velocity by a factor of ~ 2, thereby pushing the total current velocity well above the threshold for sediment erosion. The results of this and a previous study (Turnewitsch et al. 2004, Earth and Planetary Science Letters 222, 1023-1036) show that kilometer-scale flow / topography interactions leave a marine geochemical imprint which may help to develop sediment proxies for the reconstruction of past changes of fluid dynamics in the deep sea, particularly the internal wave field. Sites with (near?)critical seafloor slopes are the most promising sites to study paleo-changes of internal tides. To reconstruct the paleo-flow of residual currents and barotropic tides, sediments covering isolated topography of comparable breadth and width, and with sub-critical slopes, seem to be more appropriate. For the sediment-based reconstruction of paleo-parameters other than physical oceanographic ones kilometer-scale topography should be avoided altogether.

  • 41. Walin, G.
    et al.
    Hieronymus, Jenny
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Source-related variables for the description of the oceanic carbon system2014In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, Vol. 15, no 9, 3675-3687 p.Article in journal (Refereed)
    Abstract [en]

    The oceanic carbon system is commonly described in terms of the two state variables total carbon, DIC, and alkalinity, Alk. Here we suggest the use of alternative source adapted state variables, Acidic Carbon, AC and Basic Carbon, BC, defined by and related to (DIC, Alk) with a simple linear transformation. (AC, BC) can be interpreted as representing respectively the supply to the system of carbon dioxide and dissolved carbonate, keeping in mind that supply of hydrogen ions acts to transform from basic carbon to acidic carbon. Accordingly these variables tell us how much carbon dioxide or dissolved carbonate we actually have in the water, despite the fact that the major part of the carbon resides in bicarbonate ions. We claim that using these source-related variables as a compliment to the traditional variables, offers a number of advantages in the formulation of continuity equations, as well as in the interpretation of observations and modeling results. The traditional definition of alkalinity is related to a measuring procedure rather than to the supply of material to the system. Here we demonstrate that alkalinity, though defined in the traditional way, may be interpreted in terms of sources and sinks acting on the system. In the case of ocean water this amounts to twice the supply of dissolved carbonate minus the net supply of free hydrogen ions. We argue that this interpretation is a useful complement to the traditional definition. Every process that affects the state of the carbon system may be quantified in terms of supply of carbon dioxide, F-a, carbonate ions, F-b, or hydrogen ions, E.

  • 42. Walin, Gösta
    et al.
    Hieronymus, Jenny
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nycander, Jonas
    Stockholm University, Faculty of Science, Department of Meteorology .
    Source related variables for the description of the oceanic carbon systemManuscript (preprint) (Other academic)
  • 43.
    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, 71-79 p.Article 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 - 43 of 43
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