Thursday, October 23, 2014

Exoplanet Climate at High-obliquity

Climate at high-obliquity


Ferreira et al


The question of climate at high obliquity is raised in the context of both exoplanet studies (e.g. habitability) and paleoclimates studies (evidence for low-latitude glaciation during the Neoproterozoic and the “Snowball Earth” hypothesis). States of high obliquity, ϕ , are distinctive in that, for ϕ⩾54°ϕ⩾54°, the poles receive more solar radiation in the annual mean than the equator, opposite to the present day situation. In addition, the seasonal cycle of insolation is extreme, with the poles alternatively “facing” the Sun and sheltering in the dark for months.

The novelty of our approach is to consider the role of a dynamical ocean in controlling the surface climate at high obliquity, which in turn requires understanding of the surface winds patterns when temperature gradients are reversed. To address these questions, a coupled ocean–atmosphere–sea ice GCM configured on an Aquaplanet is employed. Except for the absence of topography and modified obliquity, the set-up is Earth-like. Two large obliquities ϕ, 54° and 90°, are compared to today’s Earth value, ϕ = 23.5°.

Three key results emerge at high obliquity: (1) despite reversed temperature gradients, mid-latitudes surface winds are westerly and trade winds exist at the equator (as for ϕ = 23.5°) although the westerlies are confined to the summer hemisphere, (2) a habitable planet is possible with mid-latitude temperatures in the range 300–280 K and (3) a stable climate state with an ice cap limited to the equatorial region is unlikely.

We clarify the dynamics behind these features (notably by an analysis of the potential vorticity structure and conditions for baroclinic instability of the atmosphere). Interestingly, we find that the absence of a stable partially glaciated state is critically linked to the absence of ocean heat transport during winter, a feature ultimately traced back to the high seasonality of baroclinic instability conditions in the atmosphere.

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