Tropical and Extratropical General Circulation with a Meridional Reversed Temperature Gradient as Expected in a High Obliquity Planet. (arXiv:1902.06832v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kang_W/0/1/0/all/0/1">Wanying Kang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cai_M/0/1/0/all/0/1">Ming Cai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tziperman_E/0/1/0/all/0/1">Eli Tziperman</a>
Planets with high obliquity receive more radiation in the polar regions than
at low latitudes, and thus, assuming an ocean-covered surface with sufficiently
high heat capacity, their meridional temperature gradient was shown to be
reversed for the entire year. The objective of this work is to investigate the
drastically different general circulation of such planets, with an emphasis on
the tropical Hadley circulation and the mid-latitude baroclinic eddy structure.
We use a 3D dry dynamic core model, accompanied by an eddy-free configuration
and a generalized 2D Eady model. When the meridional temperature gradient $T_y$
is reversed, the Hadley cell remains in the same direction, because the surface
wind pattern and hence the associated meridional Ekman transport are not
changed, as required by the baroclinic eddy momentum transport. The Hadley cell
under reversed $T_y$ also becomes much shallower and weaker, even when the
magnitude of the gradient is the same as in the normal case. The shallowness is
due to the bottom-heavy structure of the baroclinic eddies in the reverse case,
and the weakness is due to the weak wave activity. We propose a new mechanism
to explain the mid-latitude eddy structure for both cases, and verify it using
the generalized Eady model. With seasonal variations included, the annual mean
circulation resembles that under perpetual annual mean setup. Approaching the
solstices, a strong cross-equator Hadley cell forms in both cases, and about
2/3 of the Hadley circulation is driven by eddies, as shown by eddy-free
simulations and using a decomposition of the Hadley cell.
Planets with high obliquity receive more radiation in the polar regions than
at low latitudes, and thus, assuming an ocean-covered surface with sufficiently
high heat capacity, their meridional temperature gradient was shown to be
reversed for the entire year. The objective of this work is to investigate the
drastically different general circulation of such planets, with an emphasis on
the tropical Hadley circulation and the mid-latitude baroclinic eddy structure.
We use a 3D dry dynamic core model, accompanied by an eddy-free configuration
and a generalized 2D Eady model. When the meridional temperature gradient $T_y$
is reversed, the Hadley cell remains in the same direction, because the surface
wind pattern and hence the associated meridional Ekman transport are not
changed, as required by the baroclinic eddy momentum transport. The Hadley cell
under reversed $T_y$ also becomes much shallower and weaker, even when the
magnitude of the gradient is the same as in the normal case. The shallowness is
due to the bottom-heavy structure of the baroclinic eddies in the reverse case,
and the weakness is due to the weak wave activity. We propose a new mechanism
to explain the mid-latitude eddy structure for both cases, and verify it using
the generalized Eady model. With seasonal variations included, the annual mean
circulation resembles that under perpetual annual mean setup. Approaching the
solstices, a strong cross-equator Hadley cell forms in both cases, and about
2/3 of the Hadley circulation is driven by eddies, as shown by eddy-free
simulations and using a decomposition of the Hadley cell.
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