Surface Heat Capacity Controls the Existence of Summertime Radiative Convective Equilibrium in the Midlatitudes

Energy balance regimes (e.g., Radiative Convective Equilibrium or RCE) qualitatively characterize the low, mid, and high latitudes of Earth's modern climate. Currently we do not have a complete quantitative understanding of the spatio-temporal structure of energy balance regimes. Here we use the vertically-integrated moist static energy budget to define a nondimensional number that quantifies when and where RCE is approximately satisfied in Earth's modern climate. We find RCE exists yearround in the tropics and also in the Northern midlatitudes during summertime. Decomposing the seasonal RCE regime transition in the midlatitudes over land and ocean shows that RCE occurs predominantly over land. We use idealized models (energy balance and aquaplanet models) to test the hypothesis that the RCE regime occurs during midlatitude summer for land-like (small heat capacity) surface conditions. Consistent with the hypothesis, an aquaplanet model configured with a shallow mixed layer depth transitions to RCE in the midlatitudes during summertime whereas it does not for a deep mixed layer depth.