Sack Lunch Seminar (SLS)

Climate at high obliquity


One method of studying earth-like exoplanets is to view earth as an
exoplanet and consider how its climate might change if, for example,
its obliquity were ranged from 0 to 90 degrees. High values of
obliquity particularly challenge our understanding of climate dynamics
because if obliquity exceeds 54 degrees, then polar latitudes receive
more energy per unit area than do equatorial latitudes. Thus the pole
will become warmer than the equator and we are led to consider a world
in which the meridional temperature gradients, and associated
prevailing zonal wind, have the opposite sign to the present earth,
and the equatorial Hadley circulation exists where it is cold rather
than where it is warm. And all this is going on in the context of a
very pronounced seasonal cycle.


The problem becomes even richer when one considers the dynamics of an
ocean, should one exist below. A central question for the ocean
circulation is: what is the pattern of surface winds at high
obliquities?, for it is the winds that drive the ocean currents and
thermohaline circulation. How do atmospheric weather systems growing
in the easterly sheared middle latitude jets and subject to a global
angular momentum constraint, combine to determine the surface wind
pattern? Should one expect middle latitude easterly winds? If not, why
not?


Finally, a key aspect with regard to habitability is to understand how
the atmosphere and ocean of this high obliquity planet work
cooperatively together to transport energy meridionally, mediating the
warmth of the poles and the coldness of the equator. How extreme are
seasonal temperature fluctuations? Should one expect to find ice
around the equator?


Possible answers to some of these questions have been sought by
experimentation with a coupled atmosphere, ocean and sea-ice General
Circulation Model of an earth-like aquaplanet: i.e. a planet like our
own but on which there is only an ocean but no land. The coupled
climate is studied across a range of obliquities (23.5, 54 and 90). We
present some of the descriptive climatology of our solutions and how
they shed light on the deeper questions of coupled climate dynamics
that motivate them. We also review what they tell us about
habitability on such planets.