Collapse of the General Circulation in Shortwave-absorbing Atmospheres: An Idealized Model Study

The response of the general circulation in a dry atmosphere to various atmospheric shortwave absorptivities is investigated in a three-dimensional general circulation model with gray radiation. Shortwave absorption in the atmosphere reduces the incoming radiation reaching the surface but warms the upper atmosphere, significantly shifting the habitable zone toward the star. The strong stratification under high shortwave absorptivity suppresses the Hadley cell in a manner that matches previous Hadley cell scalings. General circulation changes may be observable through cloud coverage and superrotation. The equatorial superrotation in the upper atmosphere strengthens with the shortwave opacity, as predicted based on the gradient wind of the radiative–convective equilibrium profile. There is a sudden drop of equatorial superrotation at very low shortwave opacity. This is because the Hadley cell in those cases are strong enough to fill the entire troposphere with zero momentum air from the surface. A diurnal cycle (westward motion of substellar point relative to the planet) leads to acceleration of the equatorial westerlies in general, through the enhancement of the equatorward eddy momentum transport, but the response is not completely monotonic, perhaps due to the resonance of tropical waves and the diurnal forcing.