Topographically Forced Stationary Waves Can Explain the Near-Surface Zonal Winds Observed by Huygens on Titan

Abstract

Titan's equatorial superrotation is associated with climatological retrograde (westward) near-surface winds at low latitudes, as consistently simulated by Titan general circulation models. However, the Huygens probe measured prograde (eastward) near-surface winds, contrary to the expected circulation. We investigate whether these observations reflect localized forcing by large-scale topography, using paired three-dimensional dry simulations with and without topography in the Titan Atmospheric Model. We find that topography generates longitude-locked, vertically structured zonal wind anomalies that reproduce key features of the Huygens wind profile in the lowest few kilometers. Helmholtz and spectral analyses show that this structure arises from the superposition of several vertically varying low wavenumber modes. A predominantly rotational, topographically forced stationary zonal wavenumber s=1 response provides a major eastward anomaly throughout the lowest 2 km, whereas a locally divergent s=4 response consistent with a gravity wave, opposes it near the surface. Weaker s=2 and s=3 responses also vary vertically and contribute eastward anomalies near the surface. Combined with the retrograde zonal-mean circulation, these modes produce a Huygens-like profile characterized by prograde flow within the lowest 1 km and shear toward retrograde flow aloft. The paired simulations establish their topographic origin, while coherence with the corresponding terrain-forcing components demonstrates that the responses remain phase locked to the forcing. Similar but weaker stationary responses are simulated near the planned Dragonfly landing site, providing baseline expectations for interpreting its future atmospheric measurements. This study demonstrates how large-scale topography can generate localized departures from Titan's zonal-mean circulation and highlights the importance of stationary waves.