Reduced heat transfer in saltwater by a magnetic field: do oceans have a "geomagnetic brake"?
Geophysical and Astrophysical Fluid Dynamics
Seawater is the major heat transporter in our global environment, covering more than two-thirds of the surface of the earth. With an average salinity of ∼3.5%, it is a moderate electric conductor, which is permanently in motion by thermal and hydrodynamic forces. A magnetic field exerts a Lorentz force on seawater that principally influences both the dissipation of turbulence and the flow properties by magnetic friction. Here we show by experiments on laboratory scale, that convection in
... onvection in seawater is slowed down by an external static magnetic field and leads to a reduced heat flux resulting in a increased or decreased heat content in the volume in response to influx or drain of heat, respectively. Experimentally, the application of a vertical magnetic field of 60 mT reduces the convective heat transport on the liquid-air surface within in 5 minutes by about 8% perpendicular to the field and up to 14% parallel to it. The effect is strongly correlated with the magnetic interaction parameter of the system, which relates the magnetic to the viscous volume force. In the natural environment the geomagnetic field is omnipresent. It is weaker by about three orders of magnitude compared with the magnetic field applied in the experiments. It has, however, an undisturbed and long-lasting impact on the convection, at low Reynolds numbers, in the large body of water in the deeper ocean below the mixed layer. There are no investigations regarding a possible contribution of this effect to natural saltwater flows, neither by proxy experiments nor by model calculations. The data presented raise the possibility that convective heat transport in the sea could be always slowed down by the geomagnetic field to a certain extent, besides it could be modulated by the geomagnetic secular variation on relatively short timescales like decades.