Simulated Impacts of a change in the AMOC on the global Climate; an Energy Budget Perspective.

Abstract:

This study explores the impact of anomalous northward oceanic heat transport by
the AMOC on the global climate. We first use the LMDZ5 AGCM of the
Laboratoire de Météorologie Dynamique coupled to a slab ocean, with realistic
zonal asymmetries and seasonal cycle. Two anomalous surface heatings
reproducing the impact of an idealized AMOC strenghtening are imposed: (a)
uniform heating over the North Atlantic basin, and (b) concentrated heating in
the Gulf Stream region; in both cases a compensating uniform cooling in the Southern
Ocean is applied. The magnitude of the heating, and of the implied northward
inter-hemispheric heat transport, are within the range of current natural
variability.

Both simulations show global effects, which can be interpreted as a
compensation by the atmosphere of the anomalous oceanic heat transport: the
Intertropical Convergence Zone (ITCZ) shifts north toward the heating
anomalies. This shift is accompanied by a northward shift of the jets and storm
tracks in both hemispheres, consistent with anomalous soutward heat transport
by transient eddies in the subtropics.

In the extra-tropics, the clear-sky radiative response tends to damp the
prescribed anomalies, while the cloud response acts as a large positive
feedback on the oceanic forcing, mainly due to the low-cloud induced shortwave
anomalies. In the tropics, the clear-sky response is dominated instead by
humidity changes and reinforces the ITCZ movements.

We then use a fully coupled GCM in glacial conditions, in which a freshwater
flux ("hosing") is added in the North Atlantic to weaken the AMOC. We still
observe a strong compensation between the ocean and atmosphere heat transports,
as well as a southward shift of the ITCZ, and of the Southern Hemisphere jet
and storm tracks. There is however no warming of the Southern mid and high
latitudes.

In addition to the compensation mechanisms of the slab ocean settings, new
feedbacks on the meridional energy transports by the oceanic circulation
appear. The Southern Hemisphere response also takes several decades to become
established, whereas the AMOC weakening and Northern Hemisphere cooling in
instantaneous.