Antarctic Bottom Water (AABW) forms the deepest limb of the meridional overturning circulation (MOC) and is a key control on global exchanges of heat, freshwater, and carbon. Density differences that drive the MOC have their origin, in part, in coastal polynyas. Prydz Bay polynyas in East Antarctica are a key source of Dense Shelf Water (DSW) that feeds AABW to the Atlantic and Indian Oceans. However, several poorly understood mechanisms influence the pathways and change water mass properties of the DSW on its way to the abyss. To better understand these mechanisms, we release Lagrangian particles in a 10 km resolution simulation of the Whole Antarctic Ocean Model and analyze the resulting tracks using novel cluster analysis. Our results highlight the role of mixing with other water masses on the shelf in controlling the fate of DSW and its eventual contribution to AABW. When advected beneath the ice shelf, DSW can mix with fresh Ice Shelf Water (ISW), becoming less dense and making future AABW formation less likely. This study confirms that towards the shelf break along the Antarctic Slope Current, mixing with circumpolar deep water (CDW) forms modified circumpolar deep water (mCDW) and influences DSW export as AABW. Our findings indicate that the pathway from DSW to AABW is sensitive to mixing with ambient waters on the shelf. An important implication is that with future increase in ice shelf melt and CDW warming, AABW production is likely to decline, even if DSW production in coastal polynyas remains constant.

Inland waters, such as lakes, reservoirs and rivers, are important sources of greenhouse gases to the atmosphere. A key parameter that regulates the gas exchange between water and the atmosphere is the gas transfer velocity, which itself is controlled by near-surface turbulence in the water. While in lakes and reservoirs, near-surface turbulence is mainly driven by atmospheric forcing, in shallow rivers and streams it is generated by flow-induced bottom friction. Large rivers represent a transition between these two cases. Near-surface turbulence has rarely been observed in rivers and the drivers of turbulence have not been quantified. We obtained continuous measurements of flow velocity and fluctuations from which we quantified turbulence, as the rate of dissipation of turbulent kinetic energy ($\varepsilon$) over the ice-free season in a large regulated river in Northern Finland. Atmospheric forcing was observed simultaneously. Measured values of $\varepsilon$ were well predicted from bulk parameters, including mean flow velocity, wind speed, surface heat flux and a one-dimensional numerical turbulence model. Values ranged from $\sim 10^{-9}$ m$^2$ s$^{-3}$ to $10^{-5}$ m$^2$ s$^{-3}$. Atmospheric forcing and river flow contributed to near-surface turbulence a similar fraction of the time, with variability in near-surface dissipation rate occurring at diel time scales, when the flow velocity was strongly affected by downstream dam operation. By combining scaling relations for boundary-layer turbulence at the river bed and at the air-water interface, we derived a simple model for estimating the relative contributions of wind speed and bottom friction in rivers as a function of flow depth.