@article {buckley_buoyancy_2023,
	title = {Buoyancy forcing and the subpolar Atlantic meridional overturning circulation},
	journal = {Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences},
	volume = {381},
	number = {2262},
	year = {2023},
	note = {Publisher: Royal Society},
	month = {oct},
	pages = {20220181},
	abstract = {The North Atlantic meridional overturning circulation and its variability are examined in terms of the overturning in density space and diapycnal water mass transformation. The magnitude of the mean overturning is similar to the surface water mass transformation, but the density and properties of these waters are modified by diapycnal mixing. Surface waters are progressively densified while circulating cyclonically around the subpolar gyre, with the densest waters and deepest convection occurring in the Labrador Sea and Nordic Seas. The eddy-driven interaction between the convective interior and boundary currents is a key to the export of dense waters from marginal seas. Due to the multitude of pathways of dense waters within the subpolar gyre, as well as mixing with older waters, waters exiting the subpolar gyre have a wide range of ages, with a mean age on the order of a decade. As a result, interannual changes in water mass transformation are mostly balanced locally and do not result in changes in export to the subtropics. Only persistent changes in water mass transformation result in changes in export to the subtropics. The dilution of signals from upstream water mass transformation suggests that variability in export of dense waters to the subtropics may be controlled by other processes, including interaction of dense waters with the energetic upper ocean. This article is part of a discussion meeting issue {\textquoteleft}Atlantic overturning: new observations and challenges{\textquoteright}.},
	keywords = {Atlantic Meridional Overturning Circulation, buoyancy forcing, water mass transformation},
	doi = {10.1098/rsta.2022.0181},
	url = {https://royalsocietypublishing.org/doi/10.1098/rsta.2022.0181},
	author = {Buckley, Martha W. and Lozier, M. Susan and Desbruy{\`e}res, Damien and Evans, Dafydd Gwyn}
}
@article {evans_mixing_2023,
	title = {Mixing and air{\textendash}sea buoyancy fluxes set the time-mean overturning circulation in the subpolar North Atlantic and Nordic Seas},
	journal = {Ocean Science},
	volume = {19},
	number = {3},
	year = {2023},
	note = {Publisher: Copernicus GmbH},
	pages = {745{\textendash}768},
	abstract = {The overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm, salty Atlantic Water into cold, fresh North Atlantic Deep Water (NADW). The magnitude of the overturning at the OSNAP array can therefore be linked to the transformation by air{\textendash}sea buoyancy fluxes and mixing in the region north of the OSNAP array. Here, we estimate these water mass transformations using observational-based, reanalysis-based and model-based datasets. Our results highlight that air{\textendash}sea fluxes alone cannot account for the time-mean magnitude of the overturning at OSNAP, and therefore a residual mixing-driven transformation is required to explain the difference. A cooling by air{\textendash}sea heat fluxes and a mixing-driven freshening in the Nordic Seas, Iceland Basin and Irminger Sea precondition the warm, salty Atlantic Water, forming subpolar mode water classes in the subpolar North Atlantic. Mixing in the interior of the Nordic Seas, over the Greenland{\textendash}Scotland Ridge and along the boundaries of the Irminger Sea and Iceland Basin drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. Air{\textendash}sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the magnitude of the overturning within the subpolar North Atlantic and Nordic Seas. This study highlights that, for ocean and climate models to realistically simulate the overturning circulation in the North Atlantic, the small-scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model{\textquoteright}s parameterisation scheme.},
	keywords = {14 Life Below Water},
	issn = {1812-0784},
	doi = {10.5194/os-19-745-2023},
	url = {https://os.copernicus.org/articles/19/745/2023/},
	author = {Evans, Dafydd Gwyn and Holliday, N. Penny and Bacon, Sheldon and Le Bras, Isabela}
}
@article {tooth_seasonal_2023,
	title = {Seasonal overturning variability in the eastern North Atlantic subpolar gyre: a Lagrangian perspective},
	journal = {Ocean Science},
	volume = {19},
	number = {3},
	year = {2023},
	note = {Publisher: Copernicus GmbH},
	month = {jun},
	pages = {769{\textendash}791},
	abstract = {Both observations and ocean reanalyses show a pronounced seasonality in the strength of the Atlantic meridional overturning circulation (MOC) within the eastern North Atlantic subpolar gyre (eSPG). However, attributing this overturning seasonality to seasonal dense water formation remains challenging owing to the wide distribution of recirculation timescales within the Iceland and Irminger basins. Here, we investigate the nature of seasonal overturning variability using Lagrangian water parcel trajectories initialised across the Overturning in the Subpolar North Atlantic Program (OSNAP) East section within an eddy-permitting ocean sea ice hindcast. By adopting a Lagrangian perspective, we show that the seasonal minimum of the Eulerian overturning at OSNAP East in autumn results from a combination of enhanced stratification and increased southward transport within the upper East Greenland Current. This convergence of southward transport within the MOC upper limb is explained by decreasing water parcel recirculation times in the upper Irminger Sea, consistent with a gyre-scale response to seasonal wind forcing. To account for the diversity of recirculation times within the eSPG, we also quantify the Lagrangian overturning (LMOC) as the total dense water formation along water parcel trajectories. The majority of water parcels, sourced from the central and southern branches of the North Atlantic Current, fail to return to OSNAP East prior to experiencing wintertime diapycnal transformation into the lower limb, and thus they determine the mean strength of the LMOC within the eSPG (8.9 {\textpm} 2.2 Sv). The strong seasonality of the LMOC is explained by a small collection of upper-limb water parcels, circulating rapidly (<= 8.5 months) in the upper Irminger and central Iceland basins, whose along-stream transformation is determined by their month of arrival at OSNAP East.},
	issn = {1812-0784},
	doi = {10.5194/os-19-769-2023},
	url = {https://os.copernicus.org/articles/19/769/2023/},
	author = {Tooth, Oliver John and Johnson, Helen Louise and Wilson, Chris and Evans, Dafydd Gwyn}
}
