Hadley Circulation and Methane Cycle on Titan

Saturn's moon Titan has a methane cycle akin to Earth's water cycle, with methane evaporating from lakes, condensing in clouds, and precipitating as rain. Titan has been observed in detail for a decade, from its southern summer solstice through vernal equinox. In this time, key observations include methane clouds in southern middle and high latitudes, dry and dune-covered low latitudes, and lakes in polar regions, preferentially in the north. Here we show that these observations can be accounted for by a seasonally varying global Hadley circulation, which transports methane toward the summer pole. A complex of rising branches of the mean meridional circulation (including the global Hadley cells and embedded lower-level recirculation cells) lead to the observed cloud distribution. The eccentricity of Saturn's orbit leads to seasonal asymmetries in the thermal structure of Titan's atmosphere, which give rise to methane accumulation preferentially at the north pole. Methane is cold trapped preferentially at the north pole in the spring/summer, because northern spring/summer is currently colder than southern spring/summer. To understand the thermal asymmetries, analysis of the moist static energy balance is illuminating; it explains seemingly counterintuitive features such as the possibility of summer being colder than winter.