Supplementary InformationOur GCM is based on the Flexible Modeling System of the Geophysical Fluid Dynamics Laboratory (http://fms.gfdl.noaa.gov). It is similar to standard models for Earth's atmosphere, but with Titan's radius, planetary rotation rate, material properties, and seasonally varying insolation, and with a methane cycle instead of a water cycle. The GCM has similarities to the models in refs. 16, 17, but unlike those, it is 3D and eddy-resolving and has a different representation of radiative transfer and surface processes.Resolution. The GCM solves the hydrostatic primitive equations in vorticity-divergence form, using the spectral transform method in the horizontal and finite differences in the vertical 31 . The horizontal spectral resolution is T21 (corresponding to about 5 • × 5 • resolution of the transform grid). The vertical coordinate is σ = p/p s (pressure p normalized by surface pressure p s ); it is discretized with 18 unequally spaced levels 32 . The top of the model is at σ = 0; the uppermost full level has a mean pressure of 15 mbar (altitude ∼90 km). The time-stepping scheme is a semiimplicit leapfrog scheme (timestep 1600 s).

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