Hammond Mark, Lewis Neil T
Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637.
Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, OX1 3PU Oxford, United Kingdom
Proc Natl Acad Sci U S A. 2021 Mar 30;118(13). doi: 10.1073/pnas.2022705118.
Tidally locked exoplanets likely host global atmospheric circulations with a superrotating equatorial jet, planetary-scale stationary waves, and thermally driven overturning circulation. In this work, we show that each of these features can be separated from the total circulation by using a Helmholtz decomposition, which splits the circulation into rotational (divergence-free) and divergent (vorticity-free) components. This technique is applied to the simulated circulation of a terrestrial planet and a gaseous hot Jupiter. For both planets, the rotational component comprises the equatorial jet and stationary waves, and the divergent component contains the overturning circulation. Separating out each component allows us to evaluate their spatial structure and relative contribution to the total flow. In contrast with previous work, we show that divergent velocities are not negligible when compared with rotational velocities and that divergent, overturning circulation takes the form of a single, roughly isotropic cell that ascends on the day side and descends on the night side. These conclusions are drawn for both the terrestrial case and the hot Jupiter. To illustrate the utility of the Helmholtz decomposition for studying atmospheric processes, we compute the contribution of each of the circulation components to heat transport from day side to night side. Surprisingly, we find that the divergent circulation dominates day-night heat transport in the terrestrial case and accounts for around half of the heat transport for the hot Jupiter. The relative contributions of the rotational and divergent components to day-night heat transport are likely sensitive to multiple planetary parameters and atmospheric processes and merit further study.
潮汐锁定的系外行星可能存在全球大气环流，包括超旋转赤道急流、行星尺度的驻波和热驱动的翻转环流。在这项工作中，我们表明，通过亥姆霍兹分解可以将这些特征中的每一个与总环流分离，亥姆霍兹分解将环流分为旋转（无散度）和发散（无涡度）分量。该技术应用于类地行星和气态热木星的模拟环流。对于这两种行星，旋转分量包括赤道急流和驻波，发散分量包含翻转环流。分离出每个分量使我们能够评估它们的空间结构以及对总气流的相对贡献。与之前的工作相比，我们表明，与旋转速度相比，发散速度不可忽略，并且发散的翻转环流采取单个大致各向同性的气团形式，在昼侧上升，在夜侧下降。这些结论适用于类地情况和热木星。为了说明亥姆霍兹分解在研究大气过程中的实用性，我们计算了每个环流分量对从昼侧到夜侧的热量传输的贡献。令人惊讶的是，我们发现，在类地情况下，发散环流主导了昼夜热量传输，并且在热木星的热量传输中占大约一半。旋转和发散分量对昼夜热量传输的相对贡献可能对多个行星参数和大气过程敏感，值得进一步研究。
