Summary of A Gray-Radiation Aquaplanet Moist GCM. Part II: Energy Transports in Altered Climates
by Frierson, Held, and Zurita-Gotor, which appears
in Journal of the Atmospheric Sciences.
Primary arguments:
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Increases in moisture content in the simplified moist GCM are associated with a
large decrease in dry static energy flux, to almost perfectly compensate
the increase in moisture fluxes.
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Increases in atmospheric moisture is also associated with a poleward shift in
the midlatitude jet stream and eddy activity.
Discussion:
We further study the same simulations used in Part I,
which utilize the simplified moist general circulation model (GCM) that I
constructed for my thesis. We originally constructed these simulations to
study questions such as "what determines the poleward heat transport in the
climate system," or equivalently, "what determines the north-south
temperature gradients on Earth." A closely related question is what
determines the partition of the poleward heat flux into different types of
energy, including sensible and latent heat. Moisture
fluxes contribute to weakening the pole-to-equator temperature gradient
because the moisture that is transported polewards eventually condenses out
and warms higher latitudes.
While moisture is often ignored in simple models of the atmosphere
(especially in studies of midlatitude dynamics), the transport of moisture
actually plays a dominant role in transporting heat in
the midlatitudes in the atmosphere-ocean system. Oceanic heat fluxes
are large within the tropics, but the ocean transports a quite small
amount of energy in midlatitudes
as compared to atmospheric fluxes. The atmospheric flux is then roughly
divided in half between poleward transports of latent heat and dry static
energy (DSE). This ratio can only be expected to increase with increased
temperatures and moisture contents with global warming. The goal of this
work was to study the heat transports and their partitions into dry and
moist fluxes over a very wide range of atmospheric moisture contents, from a
completely dry atmosphere up to arbitrarily large moisture contents, to
develop a better appreciation of what controls the partition of energy
fluxes in the Earth's atmosphere over a wide parameter range.
We constructed this model and these simulations, and found that while it
is simple to change the energy transports when varying certain parameters,
that when we varied moisture content, the atmospheric energy transport
didn't change at all. When the atmospheric moisture content increases,
the moisture tranport increases (not surprisingly; see the lowest plot to
the right). However, there is a large compensating decrease of the dry
static energy flux (center panel to the right), which leaves the total
moist static energy flux nearly identical at all latitudes in all simulations
(top panel to the right). We examined some simple models of energy fluxes
in this paper, including an energy balance model (EBM) with constant
diffusivity that predicts exact compensation, and a more sophisticated
"theory of everything" energy balance model which predicts energy fluxes,
diffusivities, length scales, the location of the jet stream, and eddy
energy levels.
A common misconception that we have experienced with this work is that
the paper is claiming compensation will occur over all other climate
regimes and other parameter studies. This is not the case: in fact,
both the simplified GCM and the EBMs predict
changes in both the energy fluxes and the outgoing longwave radiation
when either the short wave radiation distribution changes (for instance,
by growing an ice sheet in the NH), the optical depths change (for instance,
with global warming; this changes the OLR-T relation in our EBM), or when
other changes occur in the model. While perfect compensation will not
occur over all regimes, some degree of compensation is always expected.
This study focuses on the midlatitude energy transports, but we have
studied the determination of tropical energy fluxes and the strength of the
Hadley circulation in
Frierson 2007a.
We've also continued the work on what determines the latitude of the storm
tracks, Hadley cell edge, and jet stream within the model in a series of
papers including Frierson,
Lu, and Chen 2007.
Full citation:
Frierson, D. M. W., Held, I. M., and P. Zurita-Gotor.
A Gray-Radiation Aquaplanet Moist GCM. Part II: Energy Transports in Altered Climates.
Journal of the Atmospheric Sciences, 64, 1680-1693,
2007.
The official journal link can be found
here.
A PDF download of the full paper can be found here.
This download is courtesy of the American Meteorological Society, who owns sole
rights to it.
The download is subject to copyright laws and statutes. For more
information, please visit the Allen Press/AMS Journals Online website.
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