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The North American Cordillera—An Impediment to Growing the Continent-Wide Laurentide Ice Sheet

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The North American Cordillera—An Impediment to Growing the Continent-Wide Laurentide Ice Sheet 1DECEMBER 2015 L Ö FVERSTRÖ METAL. 9433 The North American Cordillera—An Impediment to Growing the Continent-Wide Laurentide Ice Sheet 1 # MARCUS LÖFVERSTRÖM,* JOHAN LIAKKA, AND JOHAN KLEMAN Department of Meteorology, and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden (Manuscript received 14 January 2015, in final form 1 September 2015) ABSTRACT This study examines the evolution of a continental-scale ice sheet on a triangular representation of North America, with and without the influence of the Cordilleran region. Simulations are conducted using a com- prehensive atmospheric general circulation model asynchronously coupled to a three-dimensional thermo- mechanical ice-sheet model. The atmospheric state is updated for every 2 3 106 km3 increase in ice volume, and the coupled model is integrated to steady state. In the first experiment a flat continent with no background topography is used. The ice sheet evolves fairly zonally symmetric, and the equilibrium state is continent-wide and has the highest point in the center of the continent. This equilibrium ice sheet forces an anticyclonic circulation that results in relatively warmer (cooler) summer surface temperatures in the northwest (south- east), owing to warm (cold) air advection and radiative heating due to reduced cloudiness. The second ex- periment includes a simplified representation of the Cordilleran region. The ice sheet’s equilibrium state is here structurally different from the flat continent case; the center of mass is strongly shifted to the east and the interior of the continent remains ice free—an outline broadly resembling the geologically determined ice margin in Marine Isotope Stage 4. The limited glaciation in the continental interior is the result of warm summer surface temperatures primarily due to stationary waves and radiative feedbacks. 1. Introduction suggest that the ice volume increased in a stepwise fashion over the subsequent 90 kyr, with rapid growth The Quaternary period is characterized by the cyclic bursts in stadials—cold periods—centered around expansion and retreat of massive ice sheets in the mid- ;110, ;70, and ;25 kyr BP that were followed by and high-latitude continents (Gibbard and Kolfschoten interstadials—warmer periods—when the ice volume 2004). The last glacial inception occurred about 115 000 remained constant or even decreased slightly (Peltier years before present (115 kyr BP), when ice sheets and Fairbanks 2006; Stokes et al. 2012; Kleman et al. started to form in the central Canadian Arctic; Quebec, 2013). The stadials are commonly referred to as the Canada; Scandinavia; and in the coastal regions of the Marine Isotope Stages 5b, 4, and 2 (MIS5b, MIS4, and Barents and Kara Seas (Svendsen et al. 2004; Kleman MIS2, respectively), where the latter is the culmination et al. 2002, 2013; Stokes et al. 2012). The data records of the glacial cycle at the Last Glacial Maximum (LGM, about 23–19 kyr BP). The incipient Eurasian ice sheet (EIS) is believed to * Current affiliation: Climate Change Research Section, Climate and Global Dynamics Division, NCAR, Boulder, Colorado. have been longitudinally extended along the Arctic 1 Current affiliation: Biodiversity and Climate Research Centre coast, from Scandinavia in the west to central Siberia in (BiK-F), and Senckenberg Gesellschaft für Naturforschung, the east, and the whole footprint of the ice sheet appears Frankfurt, Germany. to have migrated southwestward in time (Fig. 1); see # Current affiliation: Department of Physical Geography and discussion in Svendsen et al. (2004) and Kleman et al. Quaternary Geology, and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden. (2013). Reasons for this migration remain unresolved but the Atlantic storm track was likely the primary moisture source for the ice sheet, hence naturally shift- Corresponding author address: Marcus Löfverström, Climate Change Research Section, Climate and Global Dynamics Division, ing the center of mass toward the European side where NCAR, 1850 Table Mesa Drive, Boulder, CO 80305. the cyclones make landfall. Also, Löfverström et al. E-mail: [email protected] (2014) showed modeling results, suggesting that the DOI: 10.1175/JCLI-D-15-0044.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 09/28/21 11:21 PM UTC 9434 JOURNAL OF CLIMATE VOLUME 28 FIG. 1. The Northern Hemisphere topography in the (a) MIS4 and (b) LGM glacial states; the topography is derived from the geologically constrained ice-sheet reconstructions by Kleman et al. (2013). Ice sheets are indicated by shaded areas enclosed by heavy contours. Light contours show the surface elevation in 500-m intervals, starting at 500 m. ice-sheet topography induced warm summer tempera- d Why was the ice invasion in the western Laurentide tures in Siberia that possibly could help explain this area (prairies and interior plains) slow and late development. The North American ice sheet is be- compared to the rapid and repeated expansion of lieved to have had a rather different growth trajectory. the Quebec Dome in the east (Kleman et al. 2002; It initially formed in the northeastern corner of the Stokes et al. 2012)? continent and the center of mass appears to have re- d What caused Alaska and northeastern Siberia to mained in this region from the inception through MIS4 remain largely ice free over the last glacial cycle (Fig. 1a)—roughly the first 75–85 kyr of the glacial (Clague 1989; Svendsen et al. 2004), but at the same 8 cycle, though the exact timing is debated (Kleman time allowed ice-sheet expansion to 40 N over the et al. 2002, 2010; Stokes et al. 2012). The geological eastern North American continent? data further suggests that the continental interior and d To what extent did the evolution of the ice sheets the highland regions in the Cordillera were largely ice influence the atmospheric circulation and induce free over this time period (Kleman et al. 2002; Clague changing mass-balance patterns? Simpler put, did and James 2002; Clague et al. 2005), and the massive the ice sheets create their own growing conditions LGM Laurentide ice sheet (Fig. 1b), covering the (Sanberg and Oerlemans 1983; Lindeman and NorthAmericancontinentfromcoasttocoast,isbe- Oerlemans 1987; Roe and Lindzen 2001b; Liakka lieved to have been present for a mere 5–15 kyr before et al. 2012; Liakka 2012)? the subsequent glacial collapse (Kleman et al. 2002, The zonally asymmetric ice-sheet development in North 2010, 2013). America, in particular the southeastward location of the The strong asymmetry of the ice sheets toward the major mass center (Quebec Dome) and the late ice in- North Atlantic sector is enigmatic (Svendsen et al. vasion of the prairies, points to an atmospheric station- 2004; Kleman et al. 2010) and this configuration ary wave influence on the buildup of the Laurentide ice has also been shown difficult to capture in conven- sheet, that is, large-scale zonal asymmetries in the time- tional numerical ice-sheet modeling experiments mean circulation that are forced by heterogeneities in (Huybrechts and T’siobbel 1995; Marshall et al. 2000; the planetary boundary conditions (Held 1983; Held Zweck and Huybrechts 2005), and in experiments with et al. 2002; Kaspi and Schneider 2011; Ting 1994; White coupled atmosphere–ice-sheet models (Bonelli et al. 1982). The present-day winter circulation is character- 2009; Beghin et al. 2014). Hence, important questions ized by a northwesterly flow of Arctic air over North regarding the ice build-up patterns are prompted by America that gives rise to temperature differences of observations and experiments within both the atmo- several tens of degrees Celsius between the ‘‘cold’’ spheric circulation modeling and the glacial geology eastern and ‘‘warm’’ western sides of the continent research communities: (Löfverström 2014). This circulation pattern is primarily Unauthenticated | Downloaded 09/28/21 11:21 PM UTC 1DECEMBER 2015 L Ö FVERSTRÖ METAL. 9435 induced by the flow interaction with the Cordillera, in et al. 2011; Claussen et al. 2006; Colleoni et al. 2009). conjunction with the thermal forcing over the north- However, in a recent paper, Löfverström et al. (2014) western Atlantic Ocean (Held 1983; Held et al. 2002; studied the interactions between first-order paleo- Kaspi and Schneider 2011). It is tempting to suggest that topography (only considering the general outline and these temperature asymmetries can explain the north- elevation of the paleotopography) and the evolution of eastern location of the glacial inception and the asym- the planetary-scale atmospheric circulation over a se- metric ice-sheet development. However, it is really the quence of snapshots of the buildup of the last glacial. summer temperatures that control where an ice sheet They argued that the extensively studied LGM climate can form, as it is in the warm season that the ablation is only reflects a rather short-lived glacial extreme, not most significant. The recipe for a glacial inception is necessarily typical for most of the glacial cycle (Porter simple: the surface mass balance has to be positive over a 1989). Hence, because ice buildup is a slow precipitation- long period of time (order of a thousand years), which limited cumulative process, the LGM ice configuration requires cool summers and abundant precipitation; with a continent-wide LIS may potentially have been the though straightforward in theory, the triggering mech- result of long periods of ‘‘non-LGM-like’’ circulation anism for the last glacial inception is debated and several conditions (Löfverströmetal.2014). theories have been put forth. For a more complete understanding of the interaction One of the most widely accepted theories is that between topography, stationary waves, and the ice-sheet proposed by Milankovitch, suggesting that glacial cycles evolution, the coupled atmosphere–ice-sheet system are controlled by variability in Earth’s orbital parameters— must be studied over a sequence of conditions that re- eccentricity, obliquity, and axial precession—and the last alistically mimics the whole build-up phase.
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