Wind sensitivity of the Atlantic meridional overturning circulation in an eddying and a non-eddying ocean
Wind sensitivity of the Atlantic meridional overturning circulation in an eddying and a non-eddying ocean
Abstract:
We compare the wind sensitivity of an eddying and a non-eddying ocean and find substantial differences for the Atlantic meridional overturning circulation (AMOC) in the northern hemisphere.
Numerous studies have already demonstrated the importance of mesoscale eddies for the response of some essential ocean features such as the southern ocean overturning, to changes in the surface wind forcing. However, the role that eddies play for the northern hemisphere wind sensitivity, notably for the response of AMOC, is less clear.
We ran an uncoupled twin experiment with a non-eddy-resolving and an eddy-resolving configuration of the same ocean model, using the standard NCEP forcing and the NCEP forcing with 2x surface wind stress. This experiment design enables us to attribute differences in the response behavior to whether mesoscale eddies are parametrized or resolved.
The response in the most common AMOC index, northward transport at 26° N, is similar in both, the eddying and the non-eddying ocean, showing an increase by about 5 Sv (30%). However, looking at the deep western boundary current (DWBC), AMOCs lower limb that closes the circulation in the southward direction, we find substantial differences in the response behavior: Without eddies, the additional mass transport in the DWBC is accomplished mainly through a fastening of the flow. In contrast, the DWBC in the eddying ocean slows down by about 0.05 m/s (25%). The excess mass transport then is realized mainly through a deepening of the flow.
We show that the DWBC slow down in the eddying model is due to mesoscale eddy fluxes of horizontal momentum that are not present in the non-eddying configuration. Though mesoscale eddies seem not to be necessarily relevant for the response magnitude of AMOC, we demonstrate that they are crucial for the 3D response structure, with potential implications for various other ocean processes such as Antarctic bottom water formation.
Numerous studies have already demonstrated the importance of mesoscale eddies for the response of some essential ocean features such as the southern ocean overturning, to changes in the surface wind forcing. However, the role that eddies play for the northern hemisphere wind sensitivity, notably for the response of AMOC, is less clear.
We ran an uncoupled twin experiment with a non-eddy-resolving and an eddy-resolving configuration of the same ocean model, using the standard NCEP forcing and the NCEP forcing with 2x surface wind stress. This experiment design enables us to attribute differences in the response behavior to whether mesoscale eddies are parametrized or resolved.
The response in the most common AMOC index, northward transport at 26° N, is similar in both, the eddying and the non-eddying ocean, showing an increase by about 5 Sv (30%). However, looking at the deep western boundary current (DWBC), AMOCs lower limb that closes the circulation in the southward direction, we find substantial differences in the response behavior: Without eddies, the additional mass transport in the DWBC is accomplished mainly through a fastening of the flow. In contrast, the DWBC in the eddying ocean slows down by about 0.05 m/s (25%). The excess mass transport then is realized mainly through a deepening of the flow.
We show that the DWBC slow down in the eddying model is due to mesoscale eddy fluxes of horizontal momentum that are not present in the non-eddying configuration. Though mesoscale eddies seem not to be necessarily relevant for the response magnitude of AMOC, we demonstrate that they are crucial for the 3D response structure, with potential implications for various other ocean processes such as Antarctic bottom water formation.