ESSAY: Model interpretation of climate signals: an application to the Asian monsoon climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 293

state variables and physical processes. The grid spacing and vertical interval
in the atmosphere, ocean, and the land are often different because of the
different fundamental spatial and temporal scales of dynamical and physical
processes in each component, as shown schematically in Figure 9.2 . The
dynamical equations are driven by physical processes that constitute the
forcing functions of the climate system. These processes, which are repre-
sented as physics modules or ''parameterization'', include absorption and
reflection of solar energy, emission of terrestrial radiation, aerosols, atmo-
spheric composition and chemistry, latent heat release, moisture transport,
processes underlying the formation of clouds, rain and water vapor, boundary
layer processes, surface fluxes of heat and water, ocean salinity, temperature
and currents, and sea ice as well as land surface processes including soil
moisture, river run-off, land vegetation and biomass photosynthetic processes,
and many others. Given the proper initial and boundary conditions, and
external forcing functions, e.g. solar radiation, and time history of its atmo-
spheric composition, a climate model can be integrated forward in time,
starting at some time in the past up to the present to simulate past history of
the Earth's climate. These climate history simulations are important to ensure
that the climate models have the capability to predict future climates.
Currently, climate models are routinely being run in major research institutions
to provide guidance for seasonal-to-interannual, e.g. El Ni˜o and related
regional climate, predictions. Under the Intergovernmental Panel on Climate
Change (IPCC) initiative, models have also been run for hundreds of simulated
years into the future, subject to different scenarios of climate change regarding
the different rate of increase of carbon dioxide in the atmosphere, to provide
projections for future climates associated with global warming.
9.2.3 Experimental design
In a fully coupled model, all component models are interactive. In principle,
once the initial and boundary conditions are specified, a climate model can be
integrated indefinitely into the future, to produce the so-called ''nature'' or
control run. Very often to test out a given hypothesis, a climate model has to
be re-run with one or more components held fixed, and the results compared
with the control run. Table 9.1 shows possible configurations in which a
climate model with three major components, i.e. atmosphere, ocean and land,
can be run to test climate sensitivity to anomalous SST forcings due to
El Ni˜o, and land surface processes.
In all model sensitivity or climate simulation studies, a control experiment
has to be defined first. In Table 9.1 , the control experiment (Exp-I) is one in
which the SST field is prescribed as the climatology, i.e. the mean over many
years. The climatological SST describes the part of the variation that is due to