Repeated Cyclogenesis on Hot-Exoplanet Atmospheres with Deep Heating

Most current models of hot-exoplanet atmospheres assume shallow heating, a strong day-night differential heating near the top of the atmosphere. Here we investigate the effects of energy deposition at differing depths in a model tidally locked gas-giant exoplanet. We perform high-resolution atmospheric flow simulations of hot-exoplanet atmospheres forced with idealized thermal heating representative of shallow and deep heating (i.e., stellar irradiation strongly deposited at $\sim {10}^3\mathrm{Pa}$ and $\sim {10}^5\mathrm{Pa}$ pressure levels, respectively). Unlike with shallow heating, the flow with deep heating exhibits a new dynamic equilibrium state, characterized by repeated generation of giant cyclonic storms that move away westward once formed. The formation is accompanied by a burst of heightened turbulence, leading to the production of small-scale flow structures and large-scale mixing of temperature on a timescale of $\sim 3$ planetary rotations. Significantly, while effects that could be important (e.g., coupled radiative flux and convectively excited gravity waves) are not included, over a timescale of several hundred days the simulations robustly show that the emergent thermal flux depends strongly on the heating type and is distinguishable by current observations.

Figure 1

Vorticity $\zeta$ and temperature $T$ fields [(a) and (b), respectively] with deep heating at time $t=155$ and at the ${10}^5\mathrm{Pa}$ pressure level. The generic response to deep heating is large-scale vortices rolling up, breaking off, and then moving westward away from the point of formation at midlatitudes. The point may be slightly shifted in longitude at other times or in simulations with slightly different numerical parameters; a movie is available [34].

Figure 2

Temperature profiles $T(p)$ of shallow heating (dashed lines) and deep heating (solid lines). Colors refer to different points in longitude. The dayside-nightside difference is larger and the thermal relaxation time is smaller in the deep region for deep heating.

Figure 3

$\zeta$ fields at the ${10}^5\mathrm{Pa}$ level with shallow and deep heating [(a) and (b), respectively], showing the flows dominated by different organized structures (after $t\approx 10$). In (a) the flow is dominated by a modon and a much weaker antimodon, whereas in (b) the flow is dominated by a quartet of cyclones in a von Kármán vortex streetlike formation. In the latter, the cyclone quartet exhibits a quasiperiodic life cycle (of period $\sim 10$) that begins with the flow similar to that seen in Fig. 1.

Figure 4

Time series of disk-averaged thermal flux ($\propto \sigma T^4$, where $\sigma$ is the Stefan-Boltzmann constant) at the $p={10}^5\mathrm{Pa}$ level, with disks centered on the dayside (top) and nightside (bottom), for atmospheres with shallow heating (blue) and deep heating (red). Each series is normalized by the initial terminator value at the $p={10}^5\mathrm{Pa}$ level.

Figure 5

High-resolution version of Fig. 1 in the main text. Vorticity $\zeta$ and temperature $T$ fields [(a) and (b), respectively] of a hot, $1:1$ spin–orbit synchronized exoplanet atmosphere with deep heating at time $t=155$, in units of planetary rotation period $\tau$. The fields are in Mollweide projection, centered on the substellar point at the $p={10}^5\mathrm{Pa}$ level; the western and eastern terminators are on the left and right of the dayside, respectively (dashed circle). The generic response to deep heating is shedding of a large vortex, spinning away westward from the substellar point to the north or south, depending on the hemisphere in which the shedding occurs. The temperature tracks the vorticity very closely. The shedding is also accompanied by bursts of small-scale storms and very sharp vorticity and thermal fronts, which are layered and sweeping. The overall result is a much more complex and widespread mixing of the temperature outside the vortices (as well as the usual transport in their interiors)—seen only sporadically in the atmosphere without deep heating [13, 32]. Here it is the principle state.

Figure 6

High-resolution version of Fig. 3 in the main text. $\zeta$ fields at the ${10}^5\mathrm{Pa}$ level with shallow and deep heating [(a) and (b), respectively], showing the flows dominated by different organized structures (after $t\approx 10$). In (a) the flow is dominated by a modon and a much weaker antimodon, whereas in (b) the flow is dominated by a quartet of cyclones in a von Kármán vortex streetlike formation. In the latter, the cyclone quartet exhibits a quasiperiodic life cycle (of period $\sim 10$) that begins with the flow similar to that seen in Fig. 1.