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. In the modern climate, the annual mean total meridional heat transport (MHT) is predominantly accomplished by atmospheric eddy transport, and maximizes at 35\degree~latitude in both hemispheres. In this study, we explore the question ``what determines the annual mean MHT?" by performing a series 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 ($\Omega$) from 1/8 of its present-day value ($\Omega_{E}$) to four times the present-day value. We find that over this range of rotation rates the change of MHT with $\Omega$ falls into two regimes: a slow-rotating regime ( $\Omega / \Omega_{E})< 0.5$), in which the MHT decreases with increasing $\Omega$, and a fast-rotating  regime ($\boldsymbol\Omega>= 0.5$), in which MHT is relatively constant. 

These two regimes of MHT can be understood in terms of the difference between the equator-to-pole imbalance of absorbed shortwave radiation (ASR*) and the equator-to-pole 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 rotation rate. In the slow-rotating regime, the narrowing and weakening Hadley cell reduces the heat transport by the mean meridional circulation; the resulting warming causes a local increase in OLR, which consequently increases OLR* and decreases MHT. In the fast-rotating regime the continued contraction of the Hadley Cell is also associated with a decrease in low-level tropical clouds, which increases the local ASR by an amount that almost exactly compensates the local increases in OLR. Thus ASR* - OLR* remains approximately constant, and therefore so too does MHT. 

The behavior of MHT with $\Omega$ is consistent with the change of the dynamics with increasing $\Omega$. For the slow-rotating regime, the Hadley cell contributes significantly to the MHT. In this regime, the mass transport (and hence the heat transport) by the Hadley decreases with increasing $\Omega$, resulting in a decrease in MHT with increasing $\Omega$. In the fast-rotating regime, MHT is predominantly accomplished by atmospheric eddies. Consistent with previous studies, both the eddy length scale and the velocity scale are shown to decrease with increasing $\Omega$, rendering the eddy energy transport to be less efficient with increasing $\Omega$. However, the meridional gradient of moist static energy increases with increasing $\Omega$  -- mainly due to the increase in the gradient of moisture, rendering the MHT relatively unchanged in the fast-rotating regime. 

We performed the same set of rotation rate experiments described above with three different estimates of (prescribed) ocean heat transport, aka Q-flux. The behavior of MHT with rotation rate is independent of the prescribed Q-flux.