Wobbly winds in an ice age : the mutual interaction between the great continental ice sheets and atmospheric stationary waves
Author(s)Roe, Gerard Hugh, 1971-
Massachusetts Institute of Technology. Dept. of Earth, Atmospheric, and Planetary Sciences.
Richard S. Lindzen.
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The ice sheets of the last glacial maximum (about 21 thousand years ago) covered a significant fraction of the high latitude land mass, reached up to 3km in height, and had length scales of thousands of kilometers. They represented significant obstacles to the westerly flow of the atmosphere. As the atmospheric flow is forced to deviate around such topographic features, stationary waves-large scale standing patterns in the winds and temperatures-are established within the atmosphere. The largest of the ice sheets, the Laurentide (over North America), was approximately equal in both horizontal extent and height to the Tibetan Plateau, which is known to be a significant contributor to the stationary wave pattern in today's climate. As the ice sheet evolves, the patterns of temperature and winds due to the stationary wave change, and therefore the distribution of precipitation and ablation (melting) is altered over the ice sheet. These altered distributions will, in turn, change the shape of the ice sheet itself, given sufficient time over which to act. It is not possible to integrate full dynamical climate models for the long time scales appropriate to ice sheet dynamics (> 103yrs). Previous studies have typically either used general circulation models (GCMs) for 'snapshot' climate simulations with prescribed ice age insolation and boundary conditions, or used long integrations of energy balance models (EBMs), which do not account for atmospheric dynamics. We aim for an intermediate approach-including some of the important dynamical features of the climate within a framework which is nonetheless simple enough to do long term calculations with. In the most reduced approach, an ice sheet is treated as a perfectly plastic material, lying in the north-south direction. Simple representations of ablation and accumulation show that in equilibrium the southern margin of the ice sheet is tied quite strongly to a particular annually averaged isotherm. For a topographically forced stationary wave of reasonable amplitude, this implies that the potential effect of the stationary wave is to double the extent of the ice sheet over and above that which would exist without the stationary wave. The effects explored above in the rather restrictive two-dimensional approach are further studied using a fully three-dimensional ice sheet model coupled to a #-plane channel stationary wave model, which is quasi-geostrophic, steady state, and linear. The two components of the model interact via the accumulation and ablation parameterizations which are, of necessity, very simplified representations. The ablation parameterization is the positive degree day model which has been used to model the modern ice sheets. The accumulation parameterization places particular emphasis on the topographic influence on precipitation. This more sophisticated approach shows that, in an idealized rectangular geometry, the height, shape, and orientation of the ice sheet are all dependent on the stationary wave that it creates. The fundamental competing balance is between the enhanced precipitation on the windward slopes, and the cold temperatures due to the atmospheric flow in the lee of the ice sheet, which allows the ice there to flow to lower latitudes than it otherwise could. When the stationary wave model is applied to a reconstruction of the topography at the last glacial maximum, the results suggest that the stationary wave patterns due to Tibet and the Rockies may have played a role in preconditioning different regions for ice sheet initiation. Once established, the Laurentide ice sheet exerted a strong influence on the climate over the Fennoscandian ice sheet (over northern Europe) due to the downstream propagation of the stationary wave it created. Simulations with the ice sheet model over North America show that the atmospheric stationary wave creates a tendency for the ice sheet to have the shape of the Laurentude at the last glacial maximum. However, the simplicity of the model, together with the lack of knowledge about the glacial atmosphere, means it is not possible to conclude that the interaction was sufficient to create the observed configuration.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1999.Includes bibliographical references (p. 231-236).
DepartmentMassachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
Massachusetts Institute of Technology
Earth, Atmospheric, and Planetary Sciences.