Impact of anomalous northward oceanic heat transport on the global climate in a slab-ocean setting

This study explores the impact of anomalous northward oceanic heat transport on the global climate in a slab-ocean setting. To that end, the LMDZ5 AGCM of the Laboratoire de Météorologie Dynamique is coupled to a slab ocean, with realistic zonal asymmetries and seasonal cycle. Two anomalous surface heatings are imposed: (a) uniform heating over the North Atlantic basin, and (b) concentrated heating in the Gulf Stream region; with a compensating uniform cooling in the Southern Ocean in both cases. 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 that are particularly strong in the Tropics, with a northward shift of the Intertropical Convergence Zone (ITCZ), toward the heating anomalies. This shift is accompanied by a northward shift of the jets and storm tracks in both hemispheres. From the comparison between the two simulations with different anomalous surface heatings in North Atlantic, it emerges that the global climate response is nearly insensitive to the spatial distribution of the heating.

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.

The detailed meridional energy budget also shows that the extratropical Q-flux anomalies impact the tropics by the way of the transient eddy fluxes, acting at all longitudes.