| One of the outstanding problems within the climate community has been how to reconcile the geological proxy records of past warm, equable climates, characterized by both an increase in the global mean surface temperature and a decrease in the equator-to-pole temperature gradient, with numerical simulations of the same period. Recent work has shown that tectonic driven changes in the meridional ocean heat transport (OHT) may have played a significant role in the warming. Here, we study the adjustment of the climate to variations in OHT using a suite of slab-ocean aquaplanet GCM simulations spanning 24 different imposed variations in OHT and two different initial states. We find that increasing the OHT out of the tropics works to warm the global mean climate at a linear rate of 2 K/PW, insensitive to the structure of the transport. However, because of the localized nature of the warming and the compensating changes in the atmospheric heat transport (AHT), the mechanisms driving the climate change are highly dependent on the spatial pattern of the OHT. We find substantial but incomplete atmospheric compensation in which the adjustment of the Hadley circulation (HC) plays a key role, driving asymmetries in radiative feedback processes that work to offset the prescribed tropical energy sink. We decompose the HC adjustment into dynamical and thermodynamical components, analyzing causal relationships between radiation imbalance, circulation slowdown, and changes in Gross Moist Stability (GMS). In the subtropics/midlatitudes, we find that the warming is primarily driven by enhanced greenhouse trapping associated with the convective moistening of the upper troposphere in the outer flanks of the HC and in the midlatitude storm tracks. We also show that these adjustments are not dependent on the initial climate states of the model. |
