Geoscience Reference
In-Depth Information
the amount of ice that is melted on top in summer stays about the same, while,
because of the increasing thickness, less and less ice forms on the bottom each win-
ter. Assuming a steady climate, a rough equilibrium is ultimately achieved, with the
winter addition of thickness (primarily at the bottom) equaling the thinning during
summer (primarily from the top). How thick the ice gets basically depends on the
air temperature and the vertical oceanic heat flux. A further critical factor is the
amount of ice deformation induced by winds and ocean currents. This is discussed
in
Section 7.3.3
. Whereas the typical thickness is in the range of 3.5-4.5 meters; in
sheltered fiord sites in the Arctic where the oceanic heat flux is very small, it can
be much thicker. However, as is clear from previous discussion and from
Figure 7.4
that the idea of an equilibrium thickness no longer appears to be valid; the Arctic
sea ice cover is losing its older MYI, and the MYI that is present seems to be thin-
ning. Some of the first observational evidence for thinning came from analysis of
submarine sonar records. Comparisons between sea ice draft data acquired during
submarine cruises between 1993 and 1997 with earlier records (1958-1976), indi-
cate that the mean ice draft at the end of the summer melt period between the two
periods had decreased by 1.3 m in most of the deep water regions of the Arctic
Ocean (Rothrock, Yu, and Maykut
1999
). Ice draft is the part of the ice below the
water surface.
Maykut and Untersteiner (
1971
) developed the first general thermodynamic
ice model. Their model accounts for nonlinear temperature gradients attributed to
lagged responses to seasonal forcing, penetration of shortwave radiation into the
ice, and the presence of snow cover. It also incorporates a treatment of brine pockets
trapped in the ice. Cooling of the ice causes brine to freeze, releasing latent heat and
slowing the cooling rate. Increasing temperatures cause melting of ice surrounding
brine pockets, slowing the rate of warming. Put differently, brine pockets act as ther-
mal buffers to retard temperature changes in either direction (Maykut,
1986
). Today,
nearly all coupled global climate models include a dynamic-thermodynamic sea ice
model, meaning that along with thermodynamic growth and melt, the ice moves and
deforms in response to winds and ocean currents. The following sections address
ice motion and deformation (convergence, ridging, shearing, and divergence); mod-
eling the sea ice cover will be explored in
Chapter 9
.
7.2
Mean Circulation, Ice Zones, Concentration
and Thickness
7.2.1
Mean Annual Circulation
Since 1979, the IABP has maintained a network of drifting buoys in the Arctic
Ocean. Ice drift records are also available from the NP program. Additional data
have been collected by manned U.S. drifting camps (e.g., T-3).
Figure 7.5
shows the
mean annual pattern based on these data. Recall that ice drift (and age) can also be
tracked by applying algorithms to satellite imagery.
