Response of Western North America Surface Processes to a Canonical Heinrich Event

S.W. Hostetler (USGS, 200 SW 35th St., Corvallis, OR 97333; ph. 541-754-4370; Internet: steve@sage.cgd.ucar.edu); P.U. Clark (Geosciences, Oregon State University, Corvallis, OR); P.J. Bartlein (Department of Geography, University of Oregon, Eugene, OR); A.C. Mix and N.J. Pisias (College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR)

We are using a suite of numerical models in an attempt to quantify the response of lakes, vegetation, glaciers, and coastal upwelling in and along western North America to a canonical representation of Heinrich event 2 (H2). The hierarchy of our model suite comprises an atmospheric general circulation model [AGCM, GENESIS V2.0, resolution of T31 (3.75oX 3.75o latitude by longitude) atmosphere, and 2o X 2o surface], a regional climate model (RegCM, resolution of 60 km X 60 km) that is driven by boundary conditions derived from GENESIS, and a series of process models (lake thermal/water balance, BIOME2, empirical glacier mass-balance, wind-stress curl) that are driven directly by output from the RegCM.

Our modeled H2, which takes place under nominal 21 k boundary conditions, is specified in the AGCM as the discrete progression of lowering of the Laurentide ice sheet (LIS) over Hudson Bay, followed by a warming of the North Atlantic (SSTs specified as 3/4 of the difference between CLIMAP and modern values) that represents termination of H2 and a marked increase of NADW formation. Relative to our nominal 21 ka "control" experiment in which we specified a full-height LIS and CLIMAP SSTs, both the simulation with the lower ice sheet and the simulation with the lower ice sheet and warmer North Atlantic yield significant changes in atmospheric circulation patterns, temperature, and the surface water balance. These changes are evident within the North Atlantic region and elsewhere in the Northern Hemisphere and are transmitted across the equator. In some regions, the climatic responses from lower ice are amplified by the warmer SSTs whereas in other regions the responses are cancelled or reversed.

The transmission of events from the North Atlantic to the western U.S. yields changes in temperature, precipitation and wind patterns that may generate regional or mesoscale climatic patterns of sufficient magnitude to induce heterogeneous responses in surface processes and systems. Preliminary results from multi-year RegCM simulations suggest that this is, in fact, the case. Although the amplitude of the anomalies in the H2 simulations are smaller than those between 21 ka and modern, they are sufficient to force (modeled) changes in lake level, glacier mass balance, vegetation distribution, and the strength of coastal upwelling. Moreover, it appears that regionally opposing responses of some of these systems may have originated from a common event in the North Atlantic.