Observation‐Based Estimates of Eulerian‐Mean Boundary Downwelling in the Western Subpolar North Atlantic

Liu, Y. J.; Desbruyères, Damien; Mercier, Herlé; Spall, Michael A.

doi:10.1029/2021gl097243

Cited by 3 publications

(2 citation statements)

References 48 publications

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“…7 ), suggesting that the downwelling into the lower limb has been modulated by the thick layer of extremely dense LSW invading the Irminger Basin and its western boundary regime during the mid-1990s. The model behaviour appears in accord with recent analyses of climatological hydrographic data sets which pointed to a concentration of diapycnal downwelling connecting the upper and lower limbs of the AMOC along the continental slope of Greenland 36 . The abyssal density in the boundary current is determined by the properties of the Nordic Seas outflow and its mixing with the ambient intermediate waters, resulting in a drop in the density and an increase in the transport primarily in the downslope flow regime south of the sill 37 .…”

Section: Resultssupporting

confidence: 86%

Decadal changes in Atlantic overturning due to the excessive 1990s Labrador Sea convection

et al. 2023

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Changes in the Atlantic Meridional Overturning Circulation (AMOC) represent a crucial component of Northern Hemisphere climate variability. In modelling studies decadal overturning variability has been attributed to the intensity of deep winter convection in the Labrador Sea. This linkage is challenged by transport observations at sections across the subpolar gyre. Here we report simulations with an eddy-rich ocean model which captures the observed concentration of downwelling in the northeastern Atlantic and the negligible impact of interannual variations in Labrador Sea convection during the last decade. However, the exceptionally cold winters in the Labrador Sea during the first half of the 1990s induced a positive AMOC anomaly of more than 20%, mainly by augmenting the downwelling in the northeastern North Atlantic. The remote effect of excessive Labrador Sea buoyancy forcing is related to rapid spreading of mid-depth density anomalies into the Irminger Sea and their entrainment into the deep boundary current off Greenland.

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Section: Resultssupporting

confidence: 86%

Decadal changes in Atlantic overturning due to the excessive 1990s Labrador Sea convection

et al. 2023

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“…Downwelling along the Labrador Sea boundary currents (Spall 2004;Straneo 2006;Born and Stocker 2014;Liu et al 2022) is not an essential element in our fast connectivity mechanism. The deep penetration of density anomalies along the boundary is much weaker compared to the upper layer density anomaly and emerges late in our experiment (Figs.…”

Section: E Response Of the Lsw And Deep Labrador Sea Densitymentioning

confidence: 97%

Fast mechanisms linking the Labrador Sea with subtropical Atlantic overturning

et al. 2022

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We use an ocean general circulation model and its adjoint to analyze the causal chain linking sea surface buoyancy anomalies in the Labrador Sea to variability in the deep branch of the Atlantic meridional overturning circulation (AMOC) on inter-annual timescales. Our study highlights the importance of the North Atlantic Current (NAC) for the north-to-south connectivity in the AMOC and for the meridional transport of Lower North Atlantic Deep Water (LNADW). We identify two mechanisms that allow the Labrador Sea to impact velocities in the LNADW layer. The first mechanism involves a passive advection of surface buoyancy anomalies from the Labrador Sea towards the eastern subpolar gyre by the background NAC. The second mechanism plays a dominant role and involves a dynamical response of the NAC to surface density anomalies originating in the Labrador Sea; the NAC adjustment modifies the northward transport of salt and heat and exerts a strong positive feedback, amplifying the upper ocean buoyancy anomalies. The two mechanisms spin up/down the subpolar gyre on a timescale of years, while boundary trapped waves rapidly communicate this signal to the subtropics and trigger an adjustment of LNADW transport on a timescale of months. The NAC and the eastern subpolar gyre play an essential role in both mechanisms linking the Labrador Sea with LNADW transport variability and the subtropical AMOC. We thus reconcile two apparently contradictory paradigms about AMOC connectivity: (1) Labrador Sea buoyancy anomalies drive AMOC variability; (2) water mass transformation is largest in the eastern subpolar gyre.

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Observation-based estimates of volume, heat, and freshwater exchanges between the subpolar North Atlantic interior, its boundary currents, and the atmosphere

et al. 2023

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Abstract. The Atlantic Meridional Overturning Circulation (AMOC) transports heat and salt between the tropical Atlantic and Arctic oceans. The interior of the North Atlantic subpolar gyre (SPG) is responsible for the much of the water mass transformation in the AMOC, and the export of this water to intensified boundary currents is crucial for projecting air–sea interaction onto the strength of the AMOC. However, the magnitude and location of exchange between the SPG and the boundary remains unclear. We present a novel climatology of the SPG boundary using quality-controlled CTD (conductivity–temperature–depth) and Argo hydrography, defining the SPG interior as the oceanic region bounded by 47∘ N and the 1000 m isobath. From this hydrography we find geostrophic flow out of the SPG around much of the boundary with minimal seasonality. The horizontal density gradient is reversed around western Greenland, where the geostrophic flow is into the SPG. Surface Ekman forcing drives net flow out of the SPG in all seasons with pronounced seasonality, varying between 2.45 ± 0.73 Sv in the summer and 7.70 ± 2.90 Sv in the winter. We estimate heat advected into the SPG to be between 0.14 ± 0.05 PW in the winter and 0.23 ± 0.05 PW in the spring, and freshwater advected out of the SPG to be between 0.07 ± 0.02 Sv in the summer and 0.15 ± 0.02 Sv in the autumn. These estimates approximately balance the surface heat and freshwater fluxes over the SPG domain. Overturning in the SPG varies seasonally, with a minimum of 6.20 ± 1.40 Sv in the autumn and a maximum of 10.17 ± 1.91 Sv in the spring, with surface Ekman the most likely mediator of this variability. The density of maximum overturning is at 27.30 kg m−3, with a second, smaller maximum at 27.54 kg m−3. Upper waters (σ0<27.30 kg m−3) are transformed in the interior then exported as either intermediate water (27.30–27.54 kg m−3) in the North Atlantic Current (NAC) or as dense water (σ0>27.54 kg m−3) exiting to the south. Our results support the present consensus that the formation and pre-conditioning of Subpolar Mode Water in the north-eastern Atlantic is a key determinant of AMOC strength.

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Observation‐Based Estimates of Eulerian‐Mean Boundary Downwelling in the Western Subpolar North Atlantic

Cited by 3 publications

References 48 publications

Decadal changes in Atlantic overturning due to the excessive 1990s Labrador Sea convection

Decadal changes in Atlantic overturning due to the excessive 1990s Labrador Sea convection

Fast mechanisms linking the Labrador Sea with subtropical Atlantic overturning

Observation-based estimates of volume, heat, and freshwater exchanges between the subpolar North Atlantic interior, its boundary currents, and the atmosphere

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