Sea-ice effects on climate model sensitivity and low frequency variability

A change in a sea-ice parameter in a global coupled climate model results i n a reduction in amplitude (of about 60%) and a shortening of the predomina nt period of decadal low frequency variability in the time series of global ly 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, Asian monsoon precipitatio ns and other quantities. Coupled ocean-atmosphere-sea ice processes acting on a global scale are modified to produce these changes. Global climate sen sitivity 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 amplitu de and time scale of the low frequency variability in the model are traced to changes in the base state of the climate simulations as affected by modi fications associated with the changes ill sea ice. Making sea ice more diff icult to melt results in increased sea-ice area, colder high latitudes, inc reased meridional surface temperature gradients, and, to a first order, str onger surface winds in most regions which strengthen near-surface currents, particularly in the Northern Hemisphere, and decreases the advection time scale in the upper ocean gyres. Additionally, in the North Atlantic there i s enhanced meridional overturning due to increased density mainly in the Gr eenland 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 time scale of near 14 years and significantly reduced c ontributions from lower frequencies in the range of 15-40 year periods.