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Tropical and Extratropical General Circulation with a Meridional Reversed Temperature Gradient as Expected in a High Obliquity Planet

Will the general circulation be just reversed when reversing the meridional temperature gradient?

The simple answer is NO. With a reversed meridional temperature gradient, circulation becomes thermally indirect, shallow, and weak. Explanation is as follows:

  • Mid-latitude baroclinic eddies become bottom amplified, and this can be explained by the changes of the most unstable mode in the generalized Eady model.

 

baroclinic_eddy_reverse.png

 

  • Driven by surface friction (which is required to balance u’v’), the Hadley cell is thermally indirect while the Ferrel cell is thermally direct, in the reverse temperature gradient case, as opposed to the normal gradient case.

 

meridional_circulation_normal-reverse.png
Zonal mean meridional circulation streamfunction for the normal case (a,c) and reverse case (b,d). (a,b) are for full eddy simulations and (c,d) are for eddy-free simulations. The streamfunction for the reverse cases (b,d) are in unit 0.2 svp rather than 1 svp as in the normal case. The maxima values are 89 svp, 20 svp, 19 svp and 19 svp respectively for the panel (a,b,c,d).

 

  • The eddy-driven Hadley circulation aids to the surface friction driven circulation in the normal case, but cancels it in the reverse case. This partially explained why the reverse case has a weak and shallower circulation compared to the normal case.
  • The meridional circulation in the reverse case is shallow also because the eddies are bottom amplified in the reverse case and thus there is no strong eddy drag in the upper atmosphere to drive circulation.
  • The mid-lat Ferrel cell is thermally indirect (direct) in the normal (reverse) case, acting to restore (reduce) the meridional temperature gradient there. This may partially explain why the eddy activity gets much weaker in the reverse case, which then leads to a much weaker meridional circulation.

With seasonal variability (Held Suarez experiment with equilibrium temperature varying with season)…

  • Annual mean meridional overturning circulation is almost identical to the circulation in the perpetual annual mean simulation.
  • Solstice Hadley cell is much stronger than the annual mean, and it is driven by eddy heat transport rather than the eddy momentum transport as in the perpetual annual mean simulation.

Reference

Kang, W., M. Cai and E. Tziperman, 2018, Tropical and Extratropical General Circulation with a Meridional Reversed Temperature Gradient as Expected in a High Obliquity Planet, Icarus (und rev.)

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