Kristofer Döös

The Deacon Cell and the Other Meridional Cells of the Southern Ocean

Abstract The meridional circulation cells of the Southern Ocean are investigated using the results from a fine-resolution primitive equation model. Zonal integration along depth levels shows the classical series of meridional cells but integration along density layers shows a number of differences, including the virtual disappearance of the Deacon cell. To investigate the differences, the meridional transport is calculated as a function of both density and depth. The results show that the Deacon cell is associated with systematic changes in the depth of density surfaces between the western boundary current region off South America and the return flow in the interior of the ocean. Water flowing on each density surface produces a meridional cell with a vertical excursion of a few hundred meters. Thew cells combine, without water crossing density surfaces, to produce a single integrated Deacon cell extending from the surface to below 2000 m. The results also show that, at each latitude, water on each of the ...

Interocean exchange of water masses

A new method for calculating water mass transport between different ocean basins from the velocity fields obtained by numerical models is presented. The method is applied to the velocity field of the Southern Ocean simulated by a primitive equation model (fine resolution Antarctic model). With this method it is possible to judge whether a water mass has been ventilated or not, to estimate how many times it has circled around Antarctica, and to calculate the time it has spent in the Southern Ocean. Calculations have also been undertaken revealing to what extent the changes of temperature, salinity, and density have been caused by mixing and by ventilation. Two major ways to redistribute the water through the Southern Ocean are identified. The first one redistributes 53% of the water and involves an un ventilated direct exchange between the oceans, the second one redistributes 33% by going around Antarctica. It is found that, on average, the water mass makes six circuits before the water is ventilated and subsequently driven to the north by the Ekman transport. A heat transport study is carried out for the Atlantic, showing that the northward heat transport into the Atlantic comes 85% from the Indian Ocean and the rest from the Drake Passage.

Tasman leakage: A new route in the global ocean conveyor belt

The existence of a new route that draws relatively cold waters from the Pacific Ocean to the North Atlantic via the Tasman outflow is presented. The new route materialises with comparable magnitude and characteristics in three independent numerical realisations of the global ocean circulation. Its realism is supported by hydrographic data interpolated via an inverse model. The “Tasman leakage” constitutes a sizeable component of the upper branch of the global conveyor belt and represents an extension to the prevailing views that hitherto emphasised the routes via the Drake Passage and the Indonesian Throughflow [ Gordon, 1986 ].

Calculating Lagrangian Trajectories Using Time-Dependent Velocity Fields

Abstract The authors report on the development of an efficient and accurate tool for computing Lagrangian trajectories using time-varying velocities. By linearly interpolating the velocities both in space and time one can obtain analytical expressions for the trajectory inside each model grid box. In combination with a numerical determination of the transit times through each grid box, these expressions allow for the implementation of expedient algorithms for offline analyses of large datasets resulting from general circulation models. The authors demonstrate the efficacy of this approach on a time-varying two-dimensional model gyre.

The Baltic Haline Conveyor Belt or The Overturning Circulation and Mixing in the Baltic

A study of the water-mass circulation of the Baltic has been undertaken by making use of a three dimensional Baltic Sea model simulation. The saline water from the North Atlantic is traced through the Danish Sounds into the Baltic where it upwells and mixes with the fresh water inflow from the rivers forming a Baltic haline conveyor belt. The mixing of the saline water from the Great Belt and Oresund with the fresh water is investigated making use of overturning stream functions and Lagrangian trajectories. The overturning stream function was calculated as a function of four different vertical coordinates (depth, salinity, temperature and density) in order to understand the path of the water and where it upwells and mixes. Evidence of a fictive depth overturning cell similar to the Deacon Cell in the Southern Ocean was found in the Baltic proper corresponding to the gyre circulation around Gotland, which vanishes when the overturning stream function is projected on density layers. A Lagrangian trajectory study was performed to obtain a better view of the circulation and mixing of the saline and fresh waters. The residence time of the water masses in the Baltic is calculated to be 26-29 years and the Lagrangian dispersion reaches basin saturation after 5 years.

A Global Diagnostic of Interocean Mass Transfers

An objective and quantitative estimate of all mean annual interocean mass transfers together with a picture of the associated mean pathways is presented. The global ocean circulation transfers mass, heat, and salinity between the various ocean subbasins on timescales that are likely to interact with the evolution of climate regimes. As an effort to consolidate our knowledge of the present global ocean climatological state, the output of a complex and realistic ocean model is analyzed with newly developed Lagrangian techniques to produce a global circulation scheme that helps and completes our physical understanding of the three-dimensional ocean circulation.

Lagrangian decomposition of the Deacon Cell

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 toward 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) interocean 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 subtropical 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.

Thermodynamic analysis of ocean circulation

Abstract Calculating a streamfunction as function of depth and density is proposed as a new way of analyzing the thermodynamic character of the overturning circulation in the global ocean. The sign of an overturning cell in this streamfunction 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 streamfunction 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 Ocean Circulation and Climate Advanced Model (OCCAM). It is shown that the overturning circulation in OCCAM between the 200- and 1000-m depth is dominated by a thermally indirect cell of 24 Sverdrups (1 Sv ≡ 106 m3 s−1), forced by Ekman pumping. In th...

Impact of eddy-induced transport on the Lagrangian structure of the upper branch of the thermohaline circulation

The effect of the eddy-induced transport (EIT) on the Lagrangian structure of the upper branch of the thermohaline circulation is investigated. The Lagrangian pathways, transport, and flow characteristics such as the large-scale chaotic mixing are examined in the OCCAM global, eddy-permitting ocean general circulation model. The motions of water masses are traced employing Lagrangian trajectories. These are computed using both the time-averaged Eulerian velocity and a velocity field that contains the EIT. In all aspects of the flow investigated the neglect of the EIT leads to severely biased results. Below the mixed layer divergences of eddy mass fluxes nearly cancel those of the mean flow. As a result, diapycnal motion is reduced by the EIT. In the surface layer, the EIT counteracts the Ekman flow. This compensation is found to hold both locally and nearly everywhere in the basin. Typically, the surface layer EIT reduces the Ekman transport by 50{percnt}. Both reduced diapycnal motion and compensation of the Ekman flow prolong the circulation in wind-driven gyres and counteract dispersion of particles into the interior. Subsequently, the distribution of Lagrangian transport times becomes more peaked at shorter timescales and the transport times between sections decrease. At longer timescales the functional time dependence of the distribution is significantly changed. The spreading of particles and water masses without the EIT is governed by the {ldquo}wrong{rdquo} physics The fact that the EIT makes the flow more aligned along isopycnals, and subsequently more quasi two-dimensional, implies reduced chaotic mixing

Constrained work output of the moist atmospheric heat engine in a warming climate

Because the rain falls and the wind blows Global warming is expected to intensify the hydrological cycle, but it might also make the atmosphere less energetic. Laliberté et al. modeled the atmosphere as a classical heat engine in order to evaluate how much energy it contains and how much work it can do (see the Perspective by Pauluis). They then used a global climate model to project how that might change as climate warms. Although the hydrological cycle may increase in intensity, it does so at the expense of its ability to do work, such as powering large-scale atmospheric circulation or fueling more very intense storms. Science, this issue p. 540; see also p. 475 A more intense hydrological cycle in a warmer world might make atmospheric circulation less energetic. [Also see Perspective by Pauluis] 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.