No abstract

The possibility that a decreased equator-to-Pole surface density difference could imply stronger rather than weaker thermohaline circulation (THC) is explored theoretically as well as with the aid of numerical simulations. The idea builds on the classical thermocline scaling, stating that the THC should increase with density difference as well as with vertical diffusivity. To explore possible changes in vertical diffusivity that would follow a change in the oceanic density difference, simple models of internal wave mixing are considered. For reasonable assumptions concerning the energy supply to vertical mixing, the overall diffusivity tends to increase with decreasing density difference. This enhancement of the vertical diffusivity acts to deepen the thermocline, an effect that can cause the THC to amplify despite that the surface density difference is reduced. This remarkable state of affairs is illustrated with simulations from a one-hemisphere ocean circulation model. In the simulations, two stratification-dependent diffusivity representations are investigated, which both imply that a weaker density difference will be associated with a stronger THC. The more common mixing representation, where the diffusivity is taken to be fixed, yields the opposite and well-known result: a weaker density difference will be associated with a weaker THC.

International audienc

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 ϵ 10 6 m 3 s Ϫ1 ), forced by Ekman pumping. In the densest 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.

Plastic preproduction pellets are found in environmental samples all over the world and their presence is often linked to spills during production and transportation. To better understand how these pellets end up in the environment we assessed the release of plastic pellets from a polyethylene production site in a case study area on the Swedish west coast. The case study encompasses; field measurements to evaluate the level of pollution and pathways, models and drifters to investigate the potential spread and a revision of the legal framework and the company permits. This case study show that millions of pellets are released from the production site annually but also that there are national and international legal frameworks that if implemented could help prevent these spills. Bearing in mind the negative effects observed by plastic pollution there is an urgent need to increase the responsibility and accountability of these spills.