| 1. |
- Chafik, Léon, et al.
(författare)
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Discovery of an unrecognized pathway carrying overflow waters toward the Faroe Bank Channel
- 2020
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- The dense overflow waters of the Nordic Seas are an integral link and important diagnostic for the stability of the Atlantic Meridional Overturning Circulation (AMOC). The pathways feeding the overflow remain, however, poorly resolved. Here we use multiple observational platforms and an eddy-resolving ocean model to identify an unrecognized deep flow toward the Faroe Bank Channel. We demonstrate that anticyclonic wind forcing in the Nordic Seas via its regulation of the basin circulation plays a key role in activating an unrecognized overflow path from the Norwegian slope - at which times the overflow is anomalously strong. We further establish that, regardless of upstream pathways, the overflows are mostly carried by a deep jet banked against the eastern slope of the Faroe-Shetland Channel, contrary to previous thinking. This deep flow is thus the primary conduit of overflow water feeding the lower branch of the AMOC via the Faroe Bank Channel. The authors show that overflow waters flowing toward the Faroe Bank Channel can take a previously unidentified path to the Faroe-Shetland Channel where it joins an unrecognized deep-reaching jet located along its eastern rather than its western boundary.
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| 2. |
- Döös, Kristofer, et al.
(författare)
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Evaluation of oceanic and atmospheric trajectory schemes in the TRACMASS trajectory model v6.0
- 2017
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Ingår i: Geoscientific Model Development. - : Copernicus GmbH. - 1991-959X .- 1991-9603. ; 10:4, s. 1733-1749
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Döös, Kristofer, et al.
(författare)
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The Coupled Ocean-Atmosphere Hydrothermohaline Circulation
- 2017
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 30:2, s. 631-647
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Döös, Kristofer, et al.
(författare)
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TRACMASS—A Lagrangian Trajectory Model
- 2013
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Ingår i: Preventive Methods for Coastal Protection. - Heidelberg : Springer. - 9783319004402 - 9783319004396 ; , s. 225-249
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Bokkapitel (refereegranskat)abstract
- 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.
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| 5. |
- Kjellsson, Joakim, 1986-
(författare)
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Atmospheric & Oceanic Applications of Eulerian and Lagrangian Transport Modelling
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents several ways to understand transports of air and water masses in the atmosphere and ocean, and the transports of energy that they imply. It presents work using various kinds of observations as well as computer simulations of the atmosphere and oceans. One of the main focuses is to identify similarities and differences between models and observations, as well as between different models.The first half of the thesis applies Lagrangian methods to study flows in the atmosphere and oceans. Part of the work focuses on understanding how particles follow the currents in the Baltic Sea and how they disperse. It is suggested that the commonly used regional ocean model for the Baltic Sea, RCO, underestimates the transport and the dispersion of the particles, which can have consequences for studies of e.g. biogeochemistry as well as for operational use. A similar methodology is used to study how particles are transported between the tropics and mid-latitudes by the large-scale atmospheric circulation. It is found that the mass transport associated with northbound and southbound particles can cancel in the zonally averaged circulation, and we propose that the degree of cancellation depends on the method of averaging.The latter half of the thesis focuses on Eulerian stream functions and specifically a thermodynamic stream function that combines the zonal and meridional circulations of the atmosphere into a single circulation. The results are used to study the inter-annual variability of the intensity and thermodynamic properties of the global atmospheric circulation. A significant correlation to ENSO variability is found both in reanalysis and the EC-Earth coupled climate model. It is also shown that a set of models from the CMIP5 project show a slowdown of the atmospheric circulation as a result of global warming and associated changes in near-surface moisture content and upper-level radiative cooling.
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| 6. |
- Kjellsson, Joakim, 1986-, et al.
(författare)
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Evaluation and Tuning of Model Trajectories and Spreading Rates in the Baltic Sea Using Surface Drifter Observations
- 2013
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Ingår i: Preventive Methods for Coastal Protection. - Heidelberg : Springer. - 9783319004396 - 9783319004402 ; , s. 251-281
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Bokkapitel (refereegranskat)abstract
- 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 d∼t 0.27 whereas for larger separations the distance increases as d∼t2.5.
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| 7. |
- Kjellsson, Joakim, 1986-, et al.
(författare)
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Lagrangian decomposition of the Hadley and Ferrel cells
- 2012
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 39, s. L15807-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
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| 9. |
- Kjellsson, Joakim, 1986-, et al.
(författare)
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Surface drifters and model trajectories in the Baltic sea
- 2012
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Ingår i: Boreal environment research. - 1239-6095 .- 1797-2469. ; 17:6, s. 447-459
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Kjellsson, Joakim, 1986-, et al.
(författare)
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The Atmospheric General Circulation in Thermodynamical Coordinates
- 2014
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 71:3, s. 916-928
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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