Geoscience Reference
In-Depth Information
future sea ice distributions in association with the polar amplification of the
global warming signal.
The formation of sea ice is very different from the formation of ice on land.
The most complete sea ice models are based on a set of differential equations.
Similar to the equations that govern AGCMs and OGCMs (Box 12.1), two of
the governing equations for a sea ice model are the thermodynamic equation
based on the first law of thermodynamics and a continuity equation express-
ing conservation of mass. However, the momentum equations that govern the
movement of sea ice
are highly nonlinear and non-Newtonian. They are taken
from the equations that govern plastic flow, which describes the complicated
ice interactions that occur in nature.
The AGCM and the sea ice model interact not only through the surface tem-
perature field but also through the wind field, which modifies the ice dynamics.
The sea ice model must also be coupled both dynamically and thermodynami-
cally to the OGCM. Winds and currents partly determine how ice breaks up
and the degree to which collisions occur in the sea ice field. In addition, brine
exclusion processes, which are temperature dependent, modify the salinity of
the ocean waters and influence the thermohaline circulation so the microphys-
ics of ice formation must be included.
12.4 REGIONAL CLIMATE MODELS
Climate modeling at higher resolution than is typical for global models is use-
ful for more accurately representing climate processes and for producing infor-
mation about climate change on space scales that are more relevant for impacts
analysis and policy decisions. For example, a climate model with grid spacing
of 4 km or less can resolve convective processes within the governing equations
(Box 12.1) and can become independent of the convective parameterizations
known to be responsible for significant model error. Computational resources
currently limit this level of resolution at the global scale; it is used in only a few
high-resolution global simulations. But the governing equations can be applied
over a regional domain and achieve this resolution. The trade-offs are loss of
global connectivity and the need to make assumptions about conditions on the
lateral boundaries of the domain.
12.5 EARTH SYSTEM MODELS
GCMs represent the physical climate system, but they do not track the flow of
carbon and other elements among the climate system components. The atmo-
spheric concentrations of carbon dioxide, methane, and other greenhouse gases
are specified, often as a function of time, in GCM simulations.
To portray the climate system more completely, including the flow of
chemical elements,
earth system models
(ESMs) combine fully coupled GCMs
(including AGCMs, OGCMs, LSMs, and ice models) with
biogeochemistry
models that track the cycling of chemical elements in the climate system.
Fig-
ure 1.2
includes many of the processes determined by biogeochemical cycling,
