The Heat and Momentum Budgets of Titan's Middle Atmosphere.

The thermal and dynamical structure of Titan's middle atmosphere (the stratosphere and mesosphere) has been observed to evolve over seasonal timescales. Measurements from the Composite Infrared Spectrometer on the Cassini spacecraft indicated the presence of a westerly jet with the strongest winds exceeding 200 m s−1 near the autumn and spring poles. The strength of the winds varied substantially with latitude and altitude, and weakened throughout most of winter. The strong winds also served to trap short‐lived trace molecules near the winter pole, leading to a chemically enriched environment. Here, to better understand the evolution of the middle atmosphere jet, we quantify the heat and zonal momentum budgets in Titan's stratosphere and mesosphere over the course of a Titan year using a three‐dimensional general circulation model. We confirm that the dominant heating balance is between the net radiative and adiabatic heating rates, and also show that the convergence of sensible heat by the atmospheric flow is important at low stratospheric altitudes above the winter pole. We show that the polar jet is maintained by the convergence of zonal momentum by the mean meridional flow, while the low‐latitude winds are maintained by an up‐gradient transport of momentum by eddies that occur on time scales of less than one Titan day. The heat and momentum budgets we determine here will be useful in constraining the factors controlling the evolution of Titan's middle atmosphere over the coming decades. Plain Language Summary: Titan is the largest moon of Saturn, and possesses a thick, nitrogen dominated atmosphere with surface pressure that is greater than Earth's. The middle atmosphere of Titan includes the stratosphere and mesosphere and extends from about 60 km through about 600 km above its surface. Winds in Titan's middle atmosphere can be up to 200 m s−1 (∼450 miles per hour) and play an important role in moving trace molecules throughout the atmosphere. In this paper, we perform the first numerical study to determine what causes the temperature and winds in Titan's middle atmosphere to change over the course of Titan's 29.5 Earth‐year trip around the sun. We find that the temperatures in Titan's middle atmosphere are driven primarily by solar heating and infrared cooling, and by compressional heating of descending air; however, the transport of heat by the winds is also important. The winds are primarily accelerated by the large‐scale transport of momentum toward the high winter latitudes, but are balanced by the transport of momentum toward the equator by atmospheric waves. Our results will be important to predicting the changes that will be seen in Titan's atmosphere as it progresses through northern autumn in 2025. Key Points: Heating in the middle atmosphere is balanced between radiative and adiabatic effects, but convergence of heat is important near the polesThe acceleration of the polar jet is balanced between the transport of momentum by the mean flow and deceleration by transient eddiesThe transition of the polar jet from one hemisphere to the other is a complex process that involves changing structure over all latitudes [ABSTRACT FROM AUTHOR]