潮汐锁相行星的大气环流与气候

Atmospheric circulations and climate of tidal-locking exoplanets

红矮星是宇宙中数量最多的恒星,其表面辐射温度比太阳的辐射温度低得多,因此,红矮星的宜居带与恒星的距离非常近.因为距离其恒星较近,在强大引力的作用下,位于红矮星宜居带内的宜居行星很可能是潮汐锁相行星.锁相行星的特征是其一侧永远面对恒星,另一侧则永远背对恒星,从而形成极端不均匀的加热.我们在本文中使用一个真实的大气环流模式与平板海洋相耦合来研究锁相行星表面的气候要素和大气环流特征.我们特别针对两种自转速度进行了模拟试验.结果发现,在慢速自转条件下(自转和公转周期均为243天),大气环流基本是热力驱动的下层辐合和上层辐散的运动,行星的表面仅有恒星直射点附近比较湿润,其他区域均比较干燥,背阳面尤其寒冷.在快速自转条件下(自转和公转周期均为1天),科里奥利力的作用比较显著,大气环流以纬向西风为主,并呈现波动特征,赤道上空出现超级旋转的西风急流,湿润区域显著扩大.EP通量诊断分析表明,赤道超级旋转急流是由于热带Rossby波随高度向极地方向传播所产生的EP通量在赤道附近的辐散造成的.

M-dwarfs have the highest population in the Universe. Because M-dwarfs have much lower luminosity than Sun＇s, the habitable zone around M-dwarfs is very close to stars. Thus, habitable exoplanets orbiting M-dwarfs are very likely to be tidally locked to their primaries due to strong gravity force, causing one side of these habitable exoplanets facing stars permanently and the other side remaining dark. The very uneven heating causes strong temperature contrast between day and night sides. In the present paper, we use a modified Earth atmospheric general circulation model, which is coupled with a slab ocean, to study surface climate and atmospheric circulations of tidal-locking exoplanets around M-dwarfs. Two types of simulations are carried out. One is slow rotation with a rotating and orbiting period of 243 Earth days, and the other one is fast rotation with rotating period of 1 Earth day. Results show that in the case of slow rotation, atmospheric circulations are mainly driven by thermal contrast with convergent flows at lower layers and divergent flows at higher layers, and that the region with liquid water is a small area around the substellar point. By contrast, in the case of fast rotation, the Coriolis force becomes effective, atmospheric circulations are dominated with strong zonal flows and tropical wave modes, and regions with liquid water are much broader. Especially, zonal flows demonstrate equatorial super-rotation jet stream. EP flux diagnostics show that the equatorial super-rotation flow is due to poleward propagation of tropical Rossby waves, which generates EP flux divergence over the equator. It is the EP flux divergence that generates and maintains the equatorial super-rotation.