What Determines the Meridional Heat Transport? Insights from Varying Rotation Rate Experiments

The atmosphere-ocean system transports energy polewards, balancing the energy surplus in the tropics and the deficit in the extratropics. We explore the question "what determines the annual mean total meridional heat transport (MHT)?" by performing a set of rotation-rate experiments with an aquaplanet atmospheric General Circulation model (GFDL AM2.1) coupled to a slab ocean. We change the planet's rotation rate (Ω) from 1/8 to four times its present-day value (ΩE). We find that over this range of rotation rates the change of MHT with Ω falls into two regimes: a slow regime (Ω/ΩE< 0.5), in which MHT decreases with increasing Ω, and a fast regime (Ω/ΩE≥0.5), in which MHT is relatively constant. These two regimes of MHT can be understood in terms of difference between the equator-to-pole imbalance of absorbed shortwave radiation (ASR*) and the imbalance of outgoing longwave radiation (OLR*): MHT = ASR* - OLR*. In both regimes, the response is predominantly associated with the narrowing and weakening of the Hadley Cell with increasing Ω. In the slow regime, the narrowing and weakening Hadley cell reduces the heat transport by the mean meridional circulation; the resulted warming causes a local increase in OLR, which consequently increases OLR* and decreases MHT. In the fast regime the continued contraction and weakening of the Hadley Cell is also associated with a decrease in low-level tropical clouds, which increases local ASR by an amount that almost exactly compensates the local increases in OLR. Thus ASR* - OLR* and hence MHT remains approximately constant. The behavior of MHT with Ω is consistent with the change of dynamics with Ω. For the slow regime, the Hadley cell contributes significantly to MHT. The mass transport (and hence the heat transport) by the Hadley Cell decreases with increasing Ω, resulting in a decrease in MHT. In the fast regime, MHT is predominantly accomplished by atmospheric eddies. Both the eddy length scale and the velocity scale are shown to decrease with increasing Ω, rendering the eddy energy transport less efficient. However, the meridional gradient of moist entropy increases, rendering MHT relatively unchanged. We performed the same set of experiments with three different estimates of ocean heat transport, aka Q-flux. The behavior of MHT with Ω is independent of the prescribed Q-flux.