Two Atmospheric General Circulation Problems on Earth and High Obliquity Exoplanets
CitationKang, Wanying. 2020. Two Atmospheric General Circulation Problems on Earth and High Obliquity Exoplanets. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractIn this thesis, two large-scale general circulation problems are addressed. The first is understanding how dominant tropical intraseasonal variability, the Madden-Julian Oscillation (MJO), affects the Arctic polar stratospheric vortex, particularly the Sudden Stratospheric Warming events (SSW) during winter. In observation, SSW events have been found to increase the probability of extreme weathers events, but the trend of its frequency in a future warmer climate is still highly uncertain.
MJO has been projected to strengthen and to occupy a wide longitudinal range in a future warmer climate. I find that a stronger MJO constrained to the west-to-mid Pacific, where the MJO is presently active, can amplify the mid-latitude stationary wave pattern, and thereby trigger SSW more frequently. The results are consistent in both an idealized dry model and a full general circulation model (GCM). I find the response of the mid-latitude stationary pattern play a decisive role in the response of polar vortex and SSWs: a stronger MJO in the current active region can enhance and the mid-latitude stationary pattern and hence SSWs, while a strong circumglobal MJO, expected in a warmer climate, may do the opposite. This interaction between the MJO and the mid-latitude stationary pattern turns out to be accomplished by the meridional eddy momentum transport, suggesting that a shallow water model may be helpful for further investigations.
The second problem I examine is climate on exoplanets with high obliquity, which may be a common configuration in the absence of a large moon. I first find high obliquity planets to be warmer than their low obliquity equivalents for a wide range of insolation. Through a series of mechanism-denying experiments, I conclude that, besides the ice-albedo feedback, a significant part of the relative warmness stems from the low cloud albedo under high obliquity, which in turn is caused by the high surface heat capacity, making cloud formation lag behind the substellar point migration. I then investigate whether the ocean on warmer high obliquity planets are more vulnerable to strong insolation. In addition to the overall warmer climate, the extremely warm period during permanent days, and the misalignment between the cold trap at low latitudes and the water vapor ventilation at high latitudes, are each responsible for another order of magnitude increase of the stratospheric humidity relative to the low obliquity equivalents.
The general circulation on high obliquity planets may have observable consequences. I use a model hierarchy, from a 2D generalized Eady model, to a 3D dry dynamic model with normal and reversed meridional temperature gradients, and finally to a fully coupled GCM, to explore and understand the exotic general circulation on such planets. With the equator being the coldest place on high obliquity planets, rising motion still prevails there when the planets are relatively cool and fast-rotating. This thermally-indirect circulation is found to be momentum driven. In the opposite limit (warm and slow rotating), the circulation turns thermally-direct, as the amplified latent heating gets the meridional heat contrast through into the free atmosphere. Changes of cloud fraction and upper air zonal wind may be observable with the broadband thermal phase curve in the future.
Citable link to this pagehttps://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37365122
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