Diurnal Variability Modulates Episodic Convection in Hothouse Climates Over Ocean and Swamp-Like Surface Conditions

Hot and moist “hothouse” climates occurred in Earth's past and are expected in Earth's far future climate, driven by increasing solar luminosity. In hothouse climate regimes, precipitation transitions from a quasi-steady state, as in present-day tropical convection, to an “episodic deluge” or relaxation-oscillator (RO) regime where precipitation occurs in intense bursts separated by multi-day dry spells. Recent studies suggest that the transition to RO convection regimes is radiatively driven. However, the transition from steady state to RO convection has only been studied with radiative convective equilibrium (RCE) simulations with constant insolation, excluding the diurnal cycle. Precipitation and convection are strongly linked to the diurnal cycle in Earth's present climate over both land and ocean. We explore the impact of the diurnal cycle on the transition from steady state to RO convection using two sets of small-domain RCE simulations with ocean and swamp-like surface boundary conditions. Our RCE simulations with ocean boundary conditions show convection transitions to an episodic deluge regime at 322 K and the diurnal cycle modulates precipitation to occur during late-night or near dawn, when convective inhibition is the weakest. Our RCE simulations with swamp-like boundary conditions, which allow for mean surface temperature variations, show that as RO states emerge, the diurnal cycle modulates precipitation to primarily occur during the late-afternoon to about dusk; but as the mean SST increases, precipitation occurs during the late-night to dawn. These results show that the diurnal cycle strongly influences the timing of convection and precipitation patterns in extreme climates.