Locally enhanced precipitation organized by planetary-scale waves on Titan

Saturn’s moon Titan exhibits an active weather cycle that involves methane1–8. Equatorial and mid-latitude clouds can be organized into fascinating morphologies on scales exceeding 1,000 km (ref. 9). Observations include an arrow-shaped equatorial cloud that produced detectable surface accumulation, probably from the precipitation of liquid methane10. An analysis of an earlier cloud outburst indicated an interplay between highand low-latitude cloud activity, mediated by planetary-scale atmospheric waves11. Here we present a combined analysis of cloud observations and simulations with a three-dimensional general circulation model of Titan’s atmosphere, to obtain a physical interpretation of observed storms, their relation to atmosphere dynamics and their aggregate effect on surface erosion. We find that planetary-scale Kelvin waves arise naturally in our simulations, and robustly organize convection into chevron-shaped storms at the equator during the equinoctial season. A second and much slower wave mode organizes convection into southern-hemisphere streaks oriented in a northwest–southeast direction, similar to observations9. As a result of the phasing of these modes, precipitation rates can be as high as twenty times the local average in our simulations. We conclude that these events, which produce up to several centimetres of precipitation over length scales exceeding 1,000 km, play a crucial role in fluvial erosion of Titan’s surface. Titan’s slow rotation (16 terrestrial days) and small radius (40% that of Earth) conspire to allow a global Hadley circulation, the tropical meridional overturning circulation of the atmosphere. As a result, Titan’s strongest zonal winds are shifted polewards relative to Earth’s, meridional temperature gradients are weak12, and baroclinic cyclogenesis associated with storm-track weather is suppressed13. This dynamical configuration gives Titan an ‘all tropics’ climate14. Titan’s intertropical convergence zone (ITCZ) migrates fromone summer hemisphere to the other.Models predict a transient phase just following equinoxes when the ITCZ passes over the equator14–16 and, during this time, Earth-like tropical disturbances would be expected at Titan’s equator9,17. Classical shallow-water theory18 predicts the existence of a broad spectrum of linear equatorial wave modes, including equatoriallytrapped Rossby, Kelvin and mixed Rossby-gravity waves. All of these modes can be detected in Earth’s atmosphere through spectral analysis of observations, which show power concentrated at planetary scales (>104 km; ref. 19). Through their surface convergence and induced vertical motion, the modes collectively organize the location and timing of clouds and precipitation in Earth’s tropics on intraseasonal timescales. As a result of low insolation and a stabilizing antigreenhouse effect20, moist