TY - JOUR
T1 - Observation-based estimates of volume, heat, and freshwater exchanges between the subpolar North Atlantic interior, its boundary currents, and the atmosphere
AU - Jones, Sam C.
AU - Fraser, Neil J.
AU - Cunningham, Stuart A.
AU - Fox, Alan D.
AU - Inall, Mark E.
N1 - Funding Information:
This project was supported by the NERC research programme funding CLASS NE/R015953/1, OSNAP NE/K010700/1, and SNAP-DRAGON NE/T013494/1 (Neil J. Fraser and Stuart A. Cunningham). Additional support (Alan D. Fox) was received from the European Union Horizon 2020 Research and Innovation Programme under grant no. 727852 (Blue-Action). This output reflects only the authors' views and the European Union cannot be held responsible for any use that may be made of the information contained therein.
Publisher Copyright:
© Author(s) 2023. This work is distributed under the Creative Commons Attribution 4.0 License.
PY - 2023/2/22
Y1 - 2023/2/22
N2 - 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.
AB - 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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U2 - 10.5194/os-19-169-2023
DO - 10.5194/os-19-169-2023
M3 - Article
AN - SCOPUS:85149104243
SN - 1812-0784
VL - 19
SP - 169
EP - 192
JO - Ocean Science
JF - Ocean Science
IS - 1
ER -