Titan’s annular modes of climate variability
Titan also experiences annular modes that explain larger percentages of variance than Earth’s, up to 65%. Battalio and Lora (2021) showed that annular modes are even more important on Mars and potentially Titan than they are for Earth. We found that the “barotropic” mode represents vertical shifts of the jet stream, instead of the north-south shifts found on Mars and Earth. While the “baroclinic” mode does coincide with increased amplitudes of eddies, the main center of action of the mode does not reside in the location of baroclinic eddies very close to the surface (Fig. 1).
Global wave activity caused by convection
The occurrence of convection on Titan can have global implications for the circulation (Battalio and Lora, 2021b). The movie below shows that convection happens in the moist sectors of traveling baroclinic Rossby waves, but once convection initiates, the thermodynamics take over. The wave slows and then becomes stationary for about 2 Titan days (Tsols) until the wave scours the entire atmosphere of convective instability (as measured by the Convective Available Potential Energy, CAPE). The enormous energy produced by the release of latent heat (of methane) forces first an equatorial and then global wave mode that then slowly decays.
Once the convective event takes place, complex behavior emerges as the wave slows, grows, and interacts with both equatorial and global wave modes. Phasing of the northern and southern waves, plus interactions with the mean flow produces reversals in wave motion (blue line, Fig. 2). Initially the waves travel east (times Tsol=-20 – -2). At Tsol=0, the wave stops while convection occurs. The wave then briefly moves east but then swaps direction to the west (Tsols=4–12) at which point eastward motion returns as the atmosphere relaxes back to the pre-event state. Amazingly all of this swapping of motion can be explain by Rossby wave theory (orange line, Fig. 2).
This means is that convective events have global implications. The outgoing long wave radiation changes by up to 1% regardless of where the convective event takes place. And these events are large enough to perhaps be detected by the Dragonfly quadcopter at the equator after it lands in the 2030s.
Impact of convection on Titan’s circulation
Current global circulation models from Titan must use parametrization schemes to simulate the process of methane convection. TAM uses (as of 2021) a simplified Betts-Miller scheme. I tuned this scheme for Titan (Battalio et al., 2021) and produced a simulation that successfully replicates many features of the zonal-mean circulation. This simulation allows for an in-depth investigation of the role of convection on the circulation. I found that convection warms the lower levels between 1400–600 hPa but cools the surface. This results in strengthening of the ascending branch of the solstitial Hadley cell.