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Planetary-Wave-Induced Strengthening of the AMOC Forced by Poleward Intensified Southern Hemisphere Westerly Winds

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  • 1 a Climate Change Research Centre, University of New South Wales, Sydney, New South Wales, Australia
  • | 2 b School of Geosciences, University of Sydney, Sydney, New South Wales, Australia
  • | 3 c School of Mathematics and Statistics, University of New South Wales, Sydney, New South Wales, Australia
  • | 4 d Australian Centre for Excellence in Antarctic Science, University of New South Wales, Sydney, New South Wales, Australia
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Abstract

The Atlantic meridional overturning circulation (AMOC) plays a key role in determining the distribution of heat and nutrients in the global ocean. Climate models suggest that Southern Ocean winds will strengthen and shift poleward in the future, which could have implications for future AMOC trends. Using a coupled global–ocean sea ice model at 1/4° horizontal resolution, we study the response of the North Atlantic overturning to two anomalous Southern Ocean wind-forcing scenarios; namely, a strengthening (τ+15%) and a poleward intensification (τ4°S+15%). In both scenarios, a strengthening in the North Atlantic overturning develops within a decade, with a much stronger response in the τ4°S+15% case. In τ4°S+15%, we find that the primary link between the North Atlantic response and the Southern Ocean forcing is via the propagation of baroclinic waves. In fact, due to the rapid northward propagation of these waves, changes in the AMOC in the τ4°S+15% case appear to originate in the North Atlantic and propagate southward, whereas in the τ+15% case AMOC anomalies propagate northward from the Southern Ocean. We find the difference to be predominately caused by the sign of the baroclinic waves propagating from the forcing region into the North Atlantic: downwelling in the τ+15% case versus upwelling in the τ4°S+15% case. In the τ4°S+15% case, upwelling waves propagate into the NADW formation regions along shelf-slope topography bringing dense water to the surface. This reduces vertical density gradients leading to deeper wintertime convective overturn of surface waters and an intensification of the AMOC.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: David J. Webb, d.webb@unsw.edu.au

Abstract

The Atlantic meridional overturning circulation (AMOC) plays a key role in determining the distribution of heat and nutrients in the global ocean. Climate models suggest that Southern Ocean winds will strengthen and shift poleward in the future, which could have implications for future AMOC trends. Using a coupled global–ocean sea ice model at 1/4° horizontal resolution, we study the response of the North Atlantic overturning to two anomalous Southern Ocean wind-forcing scenarios; namely, a strengthening (τ+15%) and a poleward intensification (τ4°S+15%). In both scenarios, a strengthening in the North Atlantic overturning develops within a decade, with a much stronger response in the τ4°S+15% case. In τ4°S+15%, we find that the primary link between the North Atlantic response and the Southern Ocean forcing is via the propagation of baroclinic waves. In fact, due to the rapid northward propagation of these waves, changes in the AMOC in the τ4°S+15% case appear to originate in the North Atlantic and propagate southward, whereas in the τ+15% case AMOC anomalies propagate northward from the Southern Ocean. We find the difference to be predominately caused by the sign of the baroclinic waves propagating from the forcing region into the North Atlantic: downwelling in the τ+15% case versus upwelling in the τ4°S+15% case. In the τ4°S+15% case, upwelling waves propagate into the NADW formation regions along shelf-slope topography bringing dense water to the surface. This reduces vertical density gradients leading to deeper wintertime convective overturn of surface waters and an intensification of the AMOC.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: David J. Webb, d.webb@unsw.edu.au
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