Sea ice effects on climate sensitivity and low frequency variability
Gerald A. Meehl, Julie M. Arblaster and Warren G. Strand Jr.
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
A change in a sea ice parameter in a global coupled climate model results in a
reduction in amplitude (of about 60%) and a shortening of the predominant period
of decadal low frequency variability in the time series of globally averaged
surface air temperature. These changes are global in extent and also are
reflected in time series of area-averaged SSTs in the equatorial eastern Pacific
Ocean, the principal components of the first EOFs of global surface air
temperature and sea level pressure, and other quantities. Coupled
ocean-atmosphere-sea ice processes acting on a global scale are modified to
produce the change of climate sensitivity and low frequency decadal timescale
oscillations in the model. Global climate sensitivity is reduced when ice albedo
feedback is weakened due to the change in sea ice that makes it more difficult
to melt. The changes in the amplitude and timescale of the low frequency
variabilty in the model are traced to changes in the base state of the climate
simulations as affected by modifications associated with the changes in sea ice.
Making sea ice more difficult to melt results in increased sea ice area, colder
high latitudes, increased meridional surface temperature gradients, and, to a
first order, stronger surface winds in most regions which strengthens
near-surface currents, particularly in the Northern Hemisphere, and decreases
the advection timescale in the upper ocean gyres. Additionally, in the North
Atlantic there is enhanced meridional overturning due to increased density
mainly in the Greenland Sea region. This also contributes to an intensified
North Atlantic gyre. The changes in base state due to the sea ice change result
in a more predominant decadal timescale of near 14 years and reduced
contributions from the lower frequencies.
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Hongjun Zhang:
zhangho@ucar.edu