Changes in the large-scale circulation and the clear-sky response to warming at very slow rotation rates

The clear-sky response to surface warming is generally the result of increases in tropospheric temperature and water vapour, assuming constant relative humidity (RH). While this purely thermodynamic response is fairly well understood, there has been less focus on whether the response of the large-scale circulation to surface warming can alter the clear-sky response. Therefore, in this study, we investigate how the large-scale circulation on Earth-like planets would respond to warming, whether the constant RH assumption holds for different circulation responses, and how deviations from this assumption can affect the clear-sky response. We use the ECHAM6 general circulation model in an aquaplanet configuration and modify the large-scale circulation by changing the planet's rotation rate from 1/32 to 8 times the current Earth's rotation rate. We run two sets of experiments, one with a fixed SST as a control scenario and the other with a +4K warming scenario. We analyse the radiative flux-circulation response as the difference between the warming and control scenarios.

From faster to slower rotation, the Hadley cell expands and strengthens, increasing the dryness of the atmosphere and decreasing the water vapour masking effect. Therefore, at first order, when RH is assumed to be constant, the clear-sky response increases from faster to slower rotation. However, there are second order effects at rates slower than 1/4 of the Earth's current rotation rate, which we associate with the large changes ( > 10%) in RH. At such slow rotation rates, the Hadley cell becomes global. Meanwhile, a secondary circulation develops, characterised by convergence at the equator in the lower troposphere and divergence in the mid-troposphere. We refer to this as the congestus circulation. Changes in RH correlate well with changes in the response of the congestus circulation to warming. The deep Hadley circulation weakens with surface warming like on Earth. But the congestus circulation strengthens, increasing mid-tropospheric RH, which in turn reduces the clear-sky response. We discuss to what extent this effect is due to increased upper-tropospheric radiative cooling that is not compensated by the deep circulation. Alternatively, we discuss whether this effect is due to increased convective self-aggregation with surface warming that increases the congestus outflow.