Iron transport pathways in the Ross Sea: Physical processes affecting the supply of dFe in a regional ocean model

Productivity in the Ross Sea, Antarctica is limited by the availability of dissolved iron (dFe) during the growing season. Using passive tracer dyes in a regional ocean model, we represent each main source of dFe: Circumpolar Deep Water (CDW), sea ice melt water, glacial melt water, and benthic efflux. By adding tidal forcing and varying the model horizontal resolution, we examine the effects of tides and mesoscale eddies on each source of dFe, and on the amount and spatial distribution of dFe during austral spring and early summer (DJF).

We find that the dFe sources are significantly modified with the addition of tidal forcing to simulations, while the increase in resolution from eddy-permitting to eddy-resolving (5km to 1.5km) makes little difference. With tidal forcing added, the amount of CDW advected onto the continental shelf increases, implying a similar increase in export of Antarctic Bottom Water (AABW), but the volume of on-shelf MCDW decreases, indicating enhanced on-shelf mixing. Likewise, the Ross Ice Shelf basal melt rate increases by ~13% with tidal forcing (~5% with increased horizontal resolution), also increasing the amount of glacial melt water transported across the ice shelf front. In terms of dFe supply to the surface mixed layer, tidal forcing increases the total dFe by ~17%. Benthic sources of dFe are particularly enhanced (~38%), as tides increase vertical mixing and upwelling over the shelf. The relative spatial distributions of each source do not change significantly: dFe from sea ice dominates the western and outer shelf, benthic sources dominate the inner eastern shelf and areas around Terra Nova Bay, CDW is not dominant in most locations after the sea ice melts, and glacial sources only dominate a few small areas near the front of the ice shelf. Overall, tides appear to be a significant and necessary forcing for modeling the Ross Sea and determining the role of physical processes in the supply of dissolved iron to the surface mixed layer, while increasing resolution to resolve mesoscale eddies plays much less of a role.