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

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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 comprehensive atmospheric general circulation model asynchronously coupled to a three-dimensional thermomechanical 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 (southeast), owing to warm (cold) air advection and radiative heating due to reduced cloudiness. The second experiment 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

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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. To date, glacial inception occurred in a period with relatively relatively few studies exist that investigate glacial states low summer insolation in the Northern Hemisphere prior to the LGM: Jochum et al. (2012) studied the cli- (Loutre 2003; Berger and Loutre 2004). It has also been mate conditions of the last glacial inception in a fully shown that the ‘‘Milankovitch signal’’ is amplified by coupled atmosphere–ocean model and Gregory et al. changes in the regional surface albedo when tundra- (2012) simulated the inception and early developments and cold-resistant vegetation types migrate equator- using a circulation model coupled to an ice-sheet model. ward in colder climates (e.g., Desprat et al. 2005; However, because of the length of a glacial cycle (typical Kageyama et al. 2004; Colleoni et al. 2014; Vettoretti time scales involved are on the order 40–100 kyr; and Peltier 2003). Other theories involve atmospheric Lisiecki and Raymo 2005), transient simulations over circulation anomalies induced by local and remote to- the whole build-up phase from inception to glacial pographic and thermal sources (Roe 1999; Huybers and maximum are rare and the modeling strategy often Molnar 2007). Model experiments have shown that involves a three-dimensional ice-sheet model forced by they too can induce cold summer temperatures in the climate data from a simplified atmospheric model (Roe inception region and therefore could be part of the and Lindzen 2001a), an energy balance model (Tarasov explanation. and Peltier 1999), or time-interpolated data between Numerous modeling experiments, notably within the snapshot simulations of two glacial extremes (typically Paleoclimate Modelling Intercomparison Projects the interglacial and the glacial maximum; Charbit et al. (PMIP 1–3; Braconnot et al. 2007, 2012), suggest that 2007). Some studies have also utilized an ice-sheet LGM climate was quite different from the present, with model coupled to a simplified but interactive atmo- larger stationary waves and more zonally oriented jet spheric circulation model (Roe and Lindzen 2001b; streams and storm tracks (e.g., Cook and Held 1988; Liakka et al. 2012; Liakka 2012) or a low-resolution in- Kageyama and Valdes 2000; Otto-Bliesner et al. 2006; Li termediate complexity model in which almost all at- and Battisti 2008; Rivière et al. 2010; Kageyama et al. mospheric processes are parameterized (Bonelli et al. 2013; Brady et al. 2013; Löfverström et al. 2014). These 2009; Beghin et al. 2014). changes are commonly attributed to the massive Lau- The studies by Roe and Lindzen (2001a,b), Liakka rentide ice sheet (LIS) in North America (Manabe and et al. (2012), and Liakka (2012) explored the mutual Broccoli 1985; Pausata et al. 2011; Ullman et al. 2014; interactions between atmospheric stationary waves and Merz et al. 2015), although both the local and global the buildup of the Laurentide ice sheet but did not ex- climate conditions are also influenced by the sea surface plicitly consider the topographic influence of the Cor- temperatures (SSTs), the distribution of sea ice, the dillera. However, in Roe and Lindzen’s (2001b) surface albedo (snow, ice, and vegetation), and the in- simulations the (linear mechanical) stationary wave solation and greenhouse gas concentrations (e.g., Yin feedback alone was capable of generating a southeast- and Battisti 2001; Huybers and Molnar 2007; Pausata heavy ice sheet, to first order similar to the pre-LGM

Unauthenticated | Downloaded 09/28/21 11:21 PM UTC 9436 JOURNAL OF CLIMATE VOLUME 28 configuration that is documented by geological and This is also the typical naming convention used in me- geomorphological data (Kleman et al. 2002, 2010). Here teorological literature. we address the same problem using a comprehensive The paper is organized as follows: In section 2 we atmospheric general circulation model coupled to an discuss the atmosphere and ice-sheet models, and we ice-sheet model. We resort to an idealized zonally also give a description of the modeling strategy. The symmetric aquaplanet setup with a simple triangular results are presented in section 3, and a more thorough continent representing North America. Although this discussion is provided in section 4. We summarize our setup takes a step back from reality, it is deemed a conclusions in section 5. necessary compromise in order to isolate and un- derstand how asymmetries in the ice-sheet development may have been induced by local interactions over the 2. Models and experiment design continent. We run two transient simulations with the a. Atmospheric model coupled atmosphere–ice-sheet model. In the first experiment we let the ice sheet evolve on an initially flat We employ the National Center for Atmospheric Re- continent where all structural asymmetries are self- search (NCAR) Community Atmosphere Model, version 3 induced by the ice-sheet topography—this setup is (CAM3.0; Collins et al. 2004, 2006), using a spectral dy- similar in spirit to Roe and Lindzen (2001b), Liakka namical core with T42 (;2.8832.88) horizontal resolution et al. (2012), and Liakka (2012). In the second experi- and 26 hybrid sigma-pressure levels in the vertical. Land ment we make the setup slightly more representative for surface processes are represented by NCAR’s Community North America and also include the Cordilleran range Land Model, version 3 (CLM3; Oleson et al. 2004). along the continent’s west coast. Both experiments are The lower boundary is a simplified aquaplanet with a carried out with an asynchronous coupling between the single triangular-shaped continent representing North atmosphere and ice-sheet models, and extend from the America. This idealized model geometry is used to iso- glacial inception to the ice sheet’s equilibrium state. late the local interactions between the atmospheric cir- The North American Cordillera consists of several culation, the background topography, and the evolving individual mountain ranges partitioned into three main ice sheets. The land surface is prescribed with bare belts: the Pacific Coast Ranges in the west, the central ground conditions, where the soil temperature and Nevadan belt, and the Laramide belt in the east, where moisture are allowed to evolve freely. The ocean is the Rocky Mountains are part of the latter. However, represented by a fixed zonally symmetric SST distribu- for a simpler nomenclature we refer to the entire high- tion, using Eq. (1) in Neale and Hoskins (2000), but land region as the ‘‘Rockies’’ in the rest of this paper. modified to include a seasonal cycle:

p SST(f, l, t) 5 SST 1 2 sin2 [f 2 df(t)] : 2 f , f 2 df(t) , f , (1) max f 0 0 2 0 where f and l denote the latitude and longitude, re- annual-mean conditions. Ocean grid cells poleward of spectively, and the sea ice edge are here assumed to be covered by a 2-m uniformly thick ice cover. Note that the perpetual df 5 df p 2 5 ... (t) 0 sin[2 (t 5)/12]: t 1, 2, , 12 (2) annual-mean conditions in Neale and Hoskins (2000) is df 5 8 f 5 8 given by 0 and 0 60 . The more equatorward describes seasonal changes in the SST field. Here the sea ice extension used here is motivated by the glacial warmest (coldest) Northern (Southern) Hemisphere context of the study. Unlike the prescribed SSTs, the SSTs occur in August (t 5 8). A representation of a surface temperature over land and sea ice is dynamic seasonal cycle is necessary to yield a more realistic cli- and governed by the surface energy balance. mate for the ice-sheet development, as the annual sur- The sea surface climate is somewhat similar to the face mass balance is strongly dependent on the summer CLIMAP (1976) LGM reconstruction (used in the initial ablation. The seasonal shift of the latitude of maximum phase of PMIP1), which has a largely zonally symmetric SST (SSTmax 5 278 C) is represented by the parameter SST field and a sea ice cover extending well into the df 5 8 0( 6 ), which yields a maximum summer-to-winter midlatitudes in both the Pacific and Atlantic basins; the difference of approximately 108C. The parameter CLIMAP Atlantic sea ice edge is at approximately 408N f 5 8 0( 50 ) represents the latitude of the sea ice edge for in the winter season [December–February (DJF)].

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The orbital parameters and greenhouse gas concen- (integrated sum of all temperatures above freezing) is trations are speciﬁed for preindustrial conditions: calculated semianalytically using the monthly mean

CO2 5 280 ppmv (parts per million by volume), CH4 5 temperatures and a standard deviation of temperature 760 ppbv (parts per billion by volume), and N2O 5 270 representing day-to-day variability. The standard de- ppbv throughout our simulations. Note that the pre- viation of temperature is set to 4.58C, and the degree- industrial (recent past) summer insolation is slightly day constants that relate the PDDs to actual melt are set lower than the glacial mean (see Fig. 1 in Löfverström to 3 mm day–1 K–1 for snow and 12 mm day–1 K–1 for ice. et al. 2014), thus making it a decent representative for Following Reeh (1991), the available PDDs are used to the ‘‘average’’ glacial insolation. However, the in- melt snow and ablate ice in the following order: as a first solation (orbital forcing) is known to vary strongly over step, the PDDs are used to melt the snow layer on top of time scales of a glacial cycle and the last glacial inception the ice sheet. The meltwater (also liquid precipitation) occurred in a period with relatively low summer in- percolates down into the snowpack and refreezes as solation at high latitudes in the Northern Hemisphere superimposed ice. However, the saturation factor for the (Loutre 2003; Berger and Loutre 2004). snowpack is set to 60%, and surplus liquid water is assumed to run off the ice sheet immediately. The re- b. Ice-sheet model maining PDDs (if any) are then used to ablate the We use the three-dimensional land-based ice-sheet superimposed ice and, as a last step, the glacier ice. model Simulation Code for Polythermal Ice Sheets The ice-sheet model uses a similar but smaller conti- (SICOPOLIS), version 2.9, which treats ice as an nent as the atmospheric model that is truncated to not incompressible, viscous, and heat-conducting fluid include the westernmost part where the Rockies are (Greve 1997). The model uses a 1/38 horizontal resolution located. Hence, the Pacific west coast in the ice-sheet and 81 levels in the ice and 11 levels in the bedrock. model coincides with the easternmost points of the SICOPOLIS solves the time-dependent equations gov- Rockies in the atmospheric model. The ice-sheet model erning the thickness, flow velocity, and temperature for thus uses exactly the same continental domain in both grounded ice in response to external forcing, which is experiments regardless of whether or not we include the given by the surface mass balance, the eustatic sea level, Rockies (see description of the experiment design in and the geothermal heat flux. The model equations are section 2c). This is important because we want to isolate subject to the shallow-ice approximation, which means the effect of the mountain range on the atmospheric flow that only the lowest-order terms are accounted for without altering other feedbacks affecting the ice (Hutter 1983). It uses Glen’s flow law relating the strain growth. Allowing the ice sheet to freely evolve on the rates to the third power of the applied stresses with the Rockies, it would initially grow much faster on the same flow parameters as in Greve et al. (1998).A mountain range than elsewhere owing to the high alti- Weertman-type sliding scheme is used to calculate the tude (Bonelli et al. 2009). Also, since the Rockies re- basal flow velocities (Weertman 1957, 1964). It is also mained largely ice free during most of the build-up assumed that the bedrock and ice sheet relax toward phase of the last glacial (Clague 1989; Clague and James isostatic equilibrium with a time scale of 3 kyr and in- 2002; Clague et al. 2005), ice growth on the mountain stant calving (zero ice thickness) is assumed at the range would potentially yield an unrealistic forcing of continental margins. the atmospheric circulation. To localize the glacial in- We use the default parameter settings (see Greve ception, we prescribe a 500-m-high Gaussian hill in the 1997) with the following exceptions: (i) ice is simulated northeastern corner of the continent. in the ‘‘cold ice mode,’’ which means that if the basal c. Coupling procedure and experimental design melting temperature exceeds the pressure melting point, it is artificially reset to the pressure melting point; (ii) for To examine the influence of the Rockies on the simplicity—similar to Liakka et al. (2012) and Liakka downstream ice-sheet evolution, we conduct two ex- (2012)—we use a spatially uniform geothermal heat flux periments with the coupled atmosphere–ice-sheet of 55 mW m–2; and (iii) the sea level is assumed to be model. The first experiment uses a flat triangular rep- constant. resentation of North America, an experimental setup The ice sheet’s surface mass balance is defined as the similar in spirit to Roe and Lindzen (2001b), Liakka difference between accumulation (precipitation) and et al. (2012), and Liakka (2012), though they all used a ablation (melting). The ablation is parameterized using flat rectangular continent. The second experiment setup the positive degree-day (PDD) approach developed by is identical except for a prescribed 2500-m-high and 158- Braithwaite (1985), Reeh (1991), and Calov and Greve wide Gaussian mountain range along the continent’s (2005). In their approach, the sum of all PDDs in a year west coast in the atmospheric model, which represents

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FIG. 2. Evolution of the ice volume in the two experiments. Small circles indicate where the atmospheric climatology is updated (steps of 2 3 106 km3 increase in ice volume), and larger circles mark the cases presented in Fig. 3 where the ice volume is approximately 0%, 25%, 50%, 75%, and 100% of the equilibrium state. the Rockies (more precisely the North American considerably faster on the flat continent and the equi- Cordillera). librium ice volume is greater by almost a factor of 2, Since the growth rate is faster when the ice sheet is far approximately 84 3 106 km3 compared to 44 3 106 km3 from equilibrium, we update the atmospheric state in the simulation with the Rockies. based on the ice volume instead of a fixed time interval. Figure 3 shows a selection of snapshots of the ice sheet’s We motivate the use of ice volume as a proxy for the ice spatial evolution in the two experiments (the specific time sheet’s size because the climatological atmospheric slices are indicated in Fig. 2). The initial phase is similar in temperature anomalies forced by the ice sheet are es- both experiments: the glacial inception is localized by the sentially proportional to the total ice volume, at least for Gaussian hill in the northeast and the embryonic ice sheet small- to intermediate-sized ice sheets (Liakka and rapidly expands westward and becomes continent-wide Nilsson 2010). The atmospheric fields are updated for after only a few centuries. The almost instantaneous gla- every 2 3 106 km3 change in ice volume, implying that ciation of the high latitudes is the result of low ablation the coupling frequency varies from approximately 500– rates (Figs. 4a,b) due to subfreezing (,08C) surface tem- 1000 yr–1 for the smaller ice sheets to about 5000 yr–1 peratures in the summer season [June–August (JJA); see when the ice sheet is close to equilibrium (see Fig. 2). Figs. 5a, 6a], which allow a perennial snow cover to form At each coupling event we compute an atmospheric along the Arctic coast (not shown). monthly climatology (based on 10 years of data) using However, subsequent to the glacial inception, the ice the ice-sheet topography and glacial mask from the last sheets’ growth trajectories begin to diverge significantly. time step in the ice-sheet model integration. The fields On the flat continent (left column in Fig. 3), the incipient/ are bilinearly interpolated between the model grids. early ice sheet evolves fairly zonally symmetric (note the largely zonal ice ablation in Fig. 4a) but with a slightly east-heavy disposition. The early and intermediate stages 3. Results shown in Figs. 3c,e bear some structural resemblance to the equilibrium ice sheet in Roe and Lindzen (2001b;cf. a. Ice-sheet evolution their Fig. 13). As time progresses, a discernible double- Figure 2 shows the temporal evolution of the ice dome structure emerges with focal points on either side of volume in the two experiments. The ice sheet grows the continent, and the southern ice margin gradually

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FIG. 3. The spatial evolution of the ice sheet in the two experiments (cases indicated by the larger circles in Fig. 2). (left) The evolution of the ice sheet initialized on the ﬂat continent, and (right) the experiment with the Rockies. The continent is indicated by the gray triangle, the shading and solid contours depict the surface elevation (contour lines every 500 m), and the thicker outer contour indicates the edge of the topography. In the left (right) column, the dashed (solid) line on the western side of the continent shows the outer contour of the Rockies (note that the mountain range is omitted in the left column). becomes more zonally asymmetric. In the latter stages of somewhat similar to the nonlinear results in Liakka et al. the simulation, the center of mass is shifted to a more (2012) and Liakka (2012). Note, however, that the central part of the continent and the ice sheet equilibrates southern ice margin obtained here reaches much farther as a largely symmetric and continent-wide monodome. to the south than in those simulations, where the ice The symmetric structure of the equilibrium ice sheet is margin is located at approximately 408N.

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–1 FIG. 4. The ablation rate (m yr ) in (a),(b) the initial state (0%); (c),(d) 50% ice volume; and (e),(f) the equilibrium state (100% ice volume). (left) The ﬂat continent case and (right) the Rockies experiment. The gray contour shows the continental domain in the ice-sheet model. The ablation rate over the ice sheet is essentially zero, except for in a narrow region at the southern margin.

The right column in Fig. 3 shows the structural evolution in boreal summer (JJA) as it is in the warm season that of the ice sheet when accounting for the atmospheric flow most ablation occurs. However, we show the annual- interactions with the Rockies. The southern margin of the mean precipitation (Fig. 5d) since precipitation accumu- incipient/early ice sheet has a more pronounced northwest– lates and (superimposed) ice forms continuously over the southeast tilt than in the flat continent case (cf. Figs. 3c,d) year in the interior of the ice sheet (away from the mar- and the center of mass is strongly shifted to the east. This ginal ablation zone seen in Fig. 4). The symmetric sea configuration remains throughout the entire simulation surface conditions (SST and sea ice cover) and the ab- as the ablation rate is high in the continental interior sence of topography on the ice-free continent implies that (Figs. 4b,d,f) and the ice sheet equilibrates with a high the climatological atmospheric state is essentially zonally dome in the east, and a large ice-free region in the lee of the symmetric with only small zonal deviations over the Rockies. This outline is structurally similar to the geo- continent. This is particularly true for the surface tem- logically determined margin of the pre-LGM ice sheet in perature (Fig. 5a), the surface pressure (Fig. 5g), and the MIS4 (outlined in Fig. 1a). In section 4 we discuss mech- precipitation (Fig. 5d), although the latter has a more anisms and processes that are omitted in these experiments pronounced longitudinal asymmetry with higher values in that may be of importance for building the full continent- the west as the precipitation from the Pacific storm track wide Laurentide ice complex from the MIS4 ice sheet gradually decreases away from the ocean source. The when also accounting for the North American topography. summer stationary waves are typically weak and largely localized to their source region (Fig. 5j). In the absence of b. Atmospheric response topography, the only source of stationary Rossby waves is the thermal contrast between land and ocean due to ra- 1) FLAT CONTINENT diative heating of the land surface (Fig. 5m). This forcing The left column in Fig. 5 shows the climatological state is presumably weak on the ice-free continent as there are of a number of meteorological fields on the ice-free relatively small zonal variations in the surface tempera- continent. We focus primarily on the climate conditions ture field (Fig. 5a).

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FIG. 5. (left) The summer (JJA) climate on the initial and ice-free continent in the flat experiment: (a) the surface temperature (Ts, 8C) with the 08C isotherm indicated by the thicker green contour, (d) the (annual) precipitation rate (Prec, mm day–1), (g) the sea level pressure (PSL, hPa) and 850-hPa wind vectors (arrows), (j) the 300-hPa eddy streamfunction (c*, zonal mean removed, 106 m2 s–1), and (m) the net shortwave radiation at the surface (SW, W m–2). The middle and right columns illustrate how these fields differ from the initial state when the ice sheet has grown to an intermediate size (50%) and the equilibrium state (100%), respectively. Note that the full precipitation [(d)–(f)], low-level wind [(g)–(i)], and eddy streamfunction [(j)–(l)] fields are shown in all three columns.

The middle column of Fig. 5 shows deviations of the topography has been found in previous studies, in- meteorological ﬁelds with respect to the initial state cluding both linear and nonlinear atmosphere circula- when the ice sheet has grown to an intermediate size tion models (e.g., Cook and Held 1988; Roe and Lindzen (;50% of the equilibrium ice volume). To assess the 2001b; Liakka et al. 2012; Liakka 2012). temperature response due to circulation changes, we The temperature response is associated with an anti- have eliminated the elevation effect (projected the sur- cyclonic circulation (both at the surface and in the face temperature at sea level) by assuming a lapse rate of midtroposphere; Figs. 5h,k) with a slightly baroclinic 2 6.5 3 10–3 Km 1. The temperature response to the ice structure, that is, tilting westward with height. The dis- sheet is a relative warming in the west and a cooling in position of the stationary wave anomalies over the ice the east (Fig. 5b). Note that the actual surface temper- sheet has a different pattern than the streamfunction ature is below freezing in the warm region due to the response obtained in linear orographic stationary wave elevation effect. A similar disposition of the tempera- models; such models typically yield a relatively weak ture anomalies over the North American ice-sheet anticyclone over the western part of the ice sheet and a

Unauthenticated | Downloaded 09/28/21 11:21 PM UTC 9442 JOURNAL OF CLIMATE VOLUME 28 stronger downstream cyclone (e.g., Roe and Lindzen intermediate-sized ice sheet (cf. Figs. 5b,c). This rota- 2001b), and the streamfunction anomalies have an tion is consistent with the results in Cook and Held equivalent barotropic structure (i.e., proportional to the (1992), Ringler and Cook (1997, 1999), Liakka et al. temperature anomalies; Held 1983; Held et al. 2002). (2012), and Liakka (2012). They all ascribed this to an Mechanical (orographic) forcing of stationary waves increased influence of nonlinear eddy advection that requires a westerly mean flow normal to the height becomes important when the magnitude of the eddy contours (see, e.g., Held 1983). Here the low-level mean winds approach, or even exceed, the zonal-mean wind. winds over the ice-free continent (Fig. 5g) are westerly The stronger meridional deflection of the low-level on the equatorward side and easterly on the poleward winds over the ice-sheet topography (cf. Figs. 5h,i) side of the 08C surface isotherm, which is indicated by suggests that this may be the case also in our simulation. the thicker contour in Fig. 5a (note that this wind pattern The strength of nonlinear eddy advection in the sta- is somewhat artificial and is likely a result of the zonally tionary wave response is controlled by many different symmetric boundary conditions; see discussion in sec- factors—for example, surface friction, the aspect ratio of tion 4). For the intermediate-sized ice sheet, this implies the topography (latitude vs longitude extension), the that only the southernmost part of the ice-sheet topog- height of the topography, the meridional temperature raphy is exposed to a westerly mean flow; hence, it is gradient, the strength and vertical structure of the mean plausible that the presence of easterly winds weakens wind. For a detailed discussion on this topic, see Ringler the mechanical stationary wave forcing in the early part and Cook (1997). of the simulation. The wave response in Fig. 5k shares The northerly location of the ice sheet relative to the many structural similarities with the results in Ringler mean flow also implies that most precipitation falls on and Cook (1999) and Liakka (2012). They argued that the southern and southwestern slopes (Fig. 5e). This diabatic cooling and mechanical forcing couples non- contributes to the development of the fairly zonally linearly and enhances the anticyclonic response over the oriented southern ice margin in the early stages of the ice sheet; for example, Liakka (2012) found that the simulation (Fig. 3c). However, when a strong anticy- perturbation winds induced by diabatic cooling amplify clonic circulation cell is established over the ice-sheet the mechanically forced stationary waves by increasing topography, the poleward flow branch yields a pre- orographically induced vertical velocities. The contri- cipitation maximum on the western and southwestern bution from thermal forcing also helps explain why the slopes. The zonal-mean flow also advects moist air south circulation anomalies have a baroclinic structure in our of the ice margin; hence, a significant amount of pre- simulations (see Hoskins and Karoly 1981) rather than cipitation falls on the (southwestern) slopes on the ice an equivalent barotropic structure, which typically re- sheet’s equatorward outcrop on the eastern side of the sults from mechanical forcing in isolation (Held 1983; continent (Figs. 5e,h). This, together with the imposed Held et al. 2002). calving at the continental boundary, contributes to The anticyclonic circulation over the ice sheet also building the double-dome structure seen in Figs. 3e,g. influences the cloud cover and thus the surface radiation. Note that the equilibrium ice sheet expands into a region We find that the amount of low- and medium-height where the ablation rate on the ice-free continent is of the clouds decreases, whereas the amount of high-level order several tens of meters per year (Fig. 4a). clouds increases with respect to the ice-free conditions. 2) CONTINENT WITH ROCKIES This means that the downwelling shortwave radiation at the surface increases, which has a net warming effect over The left panels in Fig. 6 show the climatological state the ice-free part of the continent (west of the ice sheet; of the same meteorological fields that were presented in Figs. 5b,n). Note that the amount of downwelling short- the left column in Fig. 5, but for the initial ice-free state wave radiation also increases over the ice sheet but the in the Rockies simulation. net shortwave radiation (quantity shown) is reduced due The presence of the mountain range has a noticeable to the high surface albedo. influence on the continental climate: the summer (JJA) The right column of Fig. 5 shows the climate response surface temperature field (Fig. 6a) is considerably more to the equilibrium ice sheet (100% ice volume). The zonally asymmetric compared to the flat continent case warm anomaly on the western side of the continent is (cf. Fig. 5a), with warm temperatures extending poleward amplified and the warm temperatures also extend out along the ridgeline and over the continental interior. This over the sea ice in the Pacific Ocean. As the ice sheet temperature pattern is primarily attributed to an in- approaches equilibrium, the temperature anomalies creased net shortwave radiation at the surface due to more over the ice sheet obtain a more northwest–southeast arid conditions (less clouds) in the lee of the mountain alignment—that is, a clockwise shift relative to the range (cf. Figs. 6d,m and 5d,m, clouds are not shown). The

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FIG.6.AsinFig. 5, but for the Rockies experiment.

Rockies also influence the circulation in the free atmo- Figures 6b,c show the surface temperature (extended sphere: the midtropospheric eddy streamfunction shows a to sea level) relative to the ice-free state (Fig. 6a) for an meridionally oriented anticyclone–cyclone dipole sitting intermediate-sized ice sheet (;50% of the equilibrium over a highland region—a circulation feature known from ice volume) and the equilibrium state (100% ice vol- the real world that is generally more pronounced in winter ume), respectively. Both panels exhibit a significant than in summer (Ringler and Cook 1999; Held et al. 2002). warming in the central parts of the continent, with It is conceivable that the symmetric ocean boundary temperatures up to 108C higher than on the ice-free condition and the meridional SST gradient in the Pacific continent; the average summer temperature is about Ocean yields an anomalously strong low-level mean flow 158–208Ccomparedto108–158C in the ice-free state and impinging on the Rockies, and, consequently, a stronger the annual ablation is of the order 10–30 m yr–1 (Figs. 4d,f). mechanical stationary wave forcing than in the present- The warm anomalies coincide with the region that re- day summer. The prescribed sea surface conditions may mains ice free over the entire simulation and thereby also yield a diabatic heating distribution that is more prevent the ice sheet’s westward expansion in time, de- similar to the present-day winter, which further strengthens spite the abundant precipitation falling on the ice dome these circulation anomalies (Held et al. 2002). A discussion on the eastern side of the continent (Figs. 6e,f). on the shortcomings of the idealized background climate Thermal forcing of stationary waves is important for follows in sections 3c and 4. the regional climate over the North American continent

Unauthenticated | Downloaded 09/28/21 11:21 PM UTC 9444 JOURNAL OF CLIMATE VOLUME 28 in summer: radiative heating of the land surface investigate to what extent the simplified boundary con- creates a convergent flow in the lower troposphere as ditions influence our main conclusions, we compare our warm air is rising (low-level cyclone). The ascending air climate response with that from the more realistic MIS4 is slightly divergent away from the surface, implying experiment presented in Löfverström et al. (2014). They that a thermal surface cyclone is overlaid by an anticy- used a slab-ocean version of NCAR-CAM3.0 (same clone in the mid- and upper troposphere (White 1982; atmospheric model as used here) with insolation and Ting 1994). However, in our ice-free simulation the mid- greenhouse gas concentrations appropriate for the and upper-tropospheric circulation is dominated by MIS4 climate state and the geologically constrained mechanically forced waves induced by the westerly flow (static) surface topography shown in Fig. 1a (Kleman impinging on the Rockies; see Fig. 6j. In the glaciated et al. 2013). cases there is a low-level cyclonic response in the ice- Figure 7 shows the difference in summer climate be- free region of the continental interior and an anticy- tween the full MIS4 simulation and a sensitivity simu- clonic response over the easternmost parts of the ice lation with identical forcing but modern-day topography sheet (in the midlatitudes). At upper levels the local (i.e., no ice sheets). This comparison shows the climate response is everywhere dominated by an anticyclone. response to the ice-sheet topography and is thus similar The low-level diabatic heat field is intricate—a surface to that presented in the right column in Fig. 6. An im- warming in the ice-free region and a low-level cooling portant difference from our simulations is that the ocean over the ice surface (Figs. 6n,o)—and the wave response surface in Löfverström et al. (2014) is allowed to re- is further complicated by mechanical forcing induced by spond to changes in the atmospheric forcing; hence, the the westerly flow impinging on the ice sheet’s south- sea surface conditions are not necessarily identical in the western slopes [shown by Ringler and Cook (1999) and glaciated and nonglaciated states. This difference is Liakka (2012) to yield a nonlinear interaction]. The small, however, and is not likely a primary driver of the surface warming of the ice-free part of the continent is climate response. associated with a 30%–50% reduction of the low- and The first-order patterns are largely similar to our medium-height clouds compared to the ice-free state, simplified simulation (cf. Fig. 7 and the right column in whereas the fraction of high clouds is increased by Fig. 6): there is a warm surface temperature anomaly in almost a factor of two (not shown). A reduction of low- the region between the ice sheet and the Rockies and medium-height clouds, and an increase of high-level (though shifted somewhat toward the northwest) that is clouds, have a net warming effect on the (local) summer driven by an increased net shortwave radiation at the climate (Pierrehumbert 2010). This contributes to surface resulting from changes in the cloud cover—an strengthening the thermally driven circulation in the ice- increase of high-level clouds and a reduction of low- and free part of the continent (Figs. 6h,i,k,l). medium-height clouds. The time-mean atmospheric A consequence of the complicated structure of the circulation also has a similar structure with a surface stationary wave forcing fields is that the wave response is anticyclone over the ice-sheet topography and a somewhat different from Hoskins and Karoly (1981) westward-tilting anticyclone driven in part by diabatic and Ringler and Cook (1999), but the baroclinic vertical cooling and mechanical forcing of stationary waves. structure is similar. To investigate the relative roles of Though the radiation signal is of the same order of thermal and mechanical forcing (and the nonlinear in- magnitude as in the idealized simulation (several tens of teractions between those) requires a hierarchy of linear watts per square meter), the implied temperature signal and nonlinear stationary wave models that allows for an is somewhat weaker. This is attributed to a slightly examination of the relative contribution from these stronger northerly component of the low-level flow in wave sources in isolation. However, such investigation is the corridor between the Rockies and the ice sheet (in out of the scope of the present study. the full MIS4 simulation, not shown), which implies cold air advection that counteracts the radiative heating. c. Sensitivity experiments Reasons for the changes in the low-level flow remain The idealized boundary conditions used in these unknown but could possibly be the result of both local simulations implies that many asymmetries in the time- and remote flow interactions with topographic and mean circulation are missing. For the purpose of these thermal sources. We make no attempt to investigate this experiments, perhaps the most important difference further. from the real world is the absence of a strong surface Despite the muted temperature signal, the important anticyclone over the North Atlantic in summer, which message is that the climate response with a realistic gives rise to a low-level flow of warm and moist air from model configuration is structurally similar to what is the Gulf of Mexico over the interior of the continent. To obtained in the idealized setup. This strengthens the

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credibility of the idealized experiment and further suggests that we capture the ﬁrst-order effects that may have been responsible for the asymmetric ice-sheet development. Similar experiments, however, should be carried out in different circulation models to validate our results.

4. Summary and discussion We have used an asynchronously coupled atmosphere– ice-sheet model to examine the influence of the Rockies (Cordilleran region) on the evolution of a continental- scale ice sheet in North America. Two experiments were conducted using an aquaplanet setup with a simplified triangular continent representing North America with (i) no background topography (similar to Roe and Lindzen 2001b; Liakka et al. 2012; Liakka 2012) and (ii) a semirealistic representation of the Rockies con- sisting of a 2500-m-high Gaussian mountain range along the continent’s west coast. Our experiments present a number of improvements over the aformen- tioned studies, most notably the use of a comprehensive atmospheric general circulation model, including moisture dynamics and a prognostic cloud water parameterization (see Collins et al. 2004, 2006,andref- erences therein). We also account for the ice sheet’s diabatic cooling of the atmosphere, and we use a slightly more realistic continent with background topography. a. Flat continent The early ice-sheet development on the flat continent is qualitatively similar to Roe and Lindzen’s (2001b) equilibrium state, with a more equatorward ice expansion on the continental margins but with a slightly east- heavy disposition (cf. Figs. 3c,e with Fig. 13 in Roe and Lindzen 2001b). As time progresses the center of mass gradually migrates westward and the equilibrium state is a largely symmetric monodome, covering virtually the entire continent. The thermal and mechanical forcing of stationary waves excite an anticyclonic circulation that yields warm air advection over the western part and cold air advection over the eastern part of the ice sheet. This circulation pattern is strengthened when the ice sheet expands and it also rotates clockwise, suggesting that nonlinear interactions become more important (Cook FIG.7.AsinFig. 5, but for the sensitivity experiments with and Held 1992; Ringler and Cook 1997, 1999; Liakka a more realistic MIS4 boundary condition. We compare the sum- et al. 2012). mer climate of a full MIS4 simulation with a simulation using an The simplified and largely zonally symmetric bound- identical forcing protocol but present-day topography (no ice sheets in North America and Eurasia). ary conditions give rise to a well-defined baroclinic zone over the oceans—a low-pressure region in midlatitudes— that has no direct counterpart in the modern-day summer hemisphere. Consequently, the balance between the

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Coriolis force and the meridional pressure gradient (perhaps for as much as 75–85 kyr), and the continent- force (directed from the higher pressure in the polar wide LGM Laurentide ice sheet is thought to have ex- region) yields an artificial easterly geostrophic back- isted for a comparatively short period of time (5–15 kyr) ground flow that extends into midlatitudes (Fig. 5g). before the glacial collapse (Kleman et al. 2013). This flow and pressure configuration resembles that over The Rockies yield a warm and dry summer climate in Antarctica in the austral summer (December–February; the continental interior (cf. left columns in Fig. 5 and see, e.g., Hartmann 1994). A similar background flow Fig. 6) and the east-heavy ice sheet is found to enhance was reported by Williams and Bryan (2006), who studied these climate characteristics and thus prevent its own glacial winds in an axisymmetric aquaplanet model. westward expansion (Fig. 6). The ice sheet’s diabatic They found that low Arctic temperatures and sea ice and mechanical forcing of stationary waves induce a expanding into the midlatitudes yields an artificially westward-tilting anticyclone (baroclinic structure) that strong eddy-driven jet (baroclinicity) and polar easter- extends through the troposphere. This yields a reduction lies migrating equatorward from the Arctic region. of low- and medium-height clouds but an increase of Even though the flat continent experiment has little high-level clouds, which in turn contributes to a net relevance for the real glacial world—the ice sheet is warming of the surface climate in the interior of the expanding too far to the south, we have surface easter- continent. A sensitivity simulation with a more realistic lies in high latitudes, and we omit the Rockies that shield MIS4 boundary condition shows that the warming signal the continent from the Pacific cyclones—it nonetheless induced by the ice sheet is a robust feature in this model, presents a similar local climate response as discussed in despite the idealized zonally symmetric boundary con- previous studies using both simplified and realistic LGM ditions. However, the magnitude of the warming signal boundary conditions: that is, an anticyclone over the is muted in the realistic MIS4 simulation, suggesting that Laurentide ice sheet and a surface warming over the there might be feedback loops with importance for the northwestern parts of the continent (e.g., Manabe and real world that are omitted in our idealized experiment. Broccoli 1985; Otto-Bliesner et al. 2006; Abe-Ouchi We can only speculate on how different factors may et al. 2007; Löfverström et al. 2014; Roe 1999; Roe and have contributed to building the LGM ice sheet from the Lindzen 2001b). This is intriguing as it is known from MIS4 state; however, the problem is in essence similar to geological data that Alaska remained largely ice free that of the glacial inception. As noted earlier, a positive over the last glacial cycle (Clague 1989), whereas the mass balance (low summer ablation and an abundant rest of the North American continent poleward of about precipitation) is the key to building an ice sheet. Our 408N experienced a massive glaciation. Pollen-based results stress the importance of radiative feedbacks in reconstructions suggest that the average temperatures the continental interior; hence, a lower summer in- in Alaska were comparable or perhaps even slightly solation and possibly also lower concentrations of higher at the LGM than in the present climate (Bartlein greenhouse gases may have been essential for breaking et al. 2011). The simplified and zonally symmetric the MIS4 configuration and building the continent-wide boundary conditions used here suggest that the warm LGM ice sheet. Ice core data (Petit et al. 1999; Spahni glacial Alaska may have been the result of first-order et al. 2005) have revealed that the atmospheric green- flow–topography interactions and the ice sheet’s dia- house gas concentrations varied considerably over the batic cooling of the lower atmosphere (Roe 1999; Roe last glacial cycle, with CO2 concentrations bracketing and Lindzen 2001b). values from approximately 280 ppmv in the interglacial to 185 ppmv at the LGM. The last significant drop of b. Continent with Rockies CO2 prior to the LGM (from ;210 to 185 ppmv) oc- The ice sheet’s spatial evolution is strongly influenced curred between ;29 and 27 kyr BP, which coincides by the presence of the Rockies; ice readily forms over with a period of decreasing Northern Hemisphere the northern and northeastern parts of the continent summer insolation (from about 500 to 465 W m2 be- but a large region in the lee of the Rockies remains ice tween 30 and 20 kyr BP; see Fig. 1 in Löfverström et al. free over the entire simulation. Consequently, the ice 2014). It is in this time period that large and persistent volume in the equilibrium state is only about half as ice sheets developed in the Cordillera and the Lauren- large as on the flat continent. Interesting is that the tide expanded westward over the prairies and interior equilibrium ice sheet resembles the MIS4 glacial state plains (Clague and James 2002; Clague et al. 2005; that is also characterized by an east-heavy configuration Kleman et al. 2013). and largely ice-free conditions in the lee of the Rockies Furthermore, both the paleodata and the modeling (Fig. 1a). This general disposition is believed to have community have shown that the LGM sea surface cli- been present over the larger part of the last glacial cycle mate was quite different from the present, especially in

Unauthenticated | Downloaded 09/28/21 11:21 PM UTC 1DECEMBER 2015 L Ö FVERSTRÖ METAL. 9447 the North Atlantic region (e.g., Waelbroeck et al. 2009; serve as an explanation for the documented late ice Kucera et al. 2005a,b; Braconnot et al. 2007, 2012), and invasion in the western Laurentide area (prairies and there are also indications of significant differences in the interior plains). A similar response is also obtained in tropical ocean (Arbuszewski et al. 2013). The in- sensitivity simulations using more realistic MIS4 teraction between the ice sheet and atmosphere/ocean boundary conditions. circulation is complex, where relatively small changes in the North American ice sheet have been shown to in- Acknowledgments. We thank Rodrigo Caballero, duce large changes in the atmosphere and ocean circu- Johan Nilsson, Francesco Pausata, David Battisti, Masa lation (Jackson 2000; Zhang et al. 2014) that may feed Kageyama, and Kerim Nisancioglu for the stimulating back onto the ice-sheet development (Yin and Battisti and useful discussions. We also thank three anonymous 2001; Huybers and Molnar 2007). However, the same is reviewers for their comments, which greatly helped to not necessarily true for the Eurasian ice sheet (EIS). improve this manuscript. This work was financially sup- Although it was a large topographic obstacle, it is gen- ported by the Swedish Research Council, the Bolin Centre erally considered to have been located too far to the for Climate Research, and LOEWE of Hesse’s Ministry north to significantly influence the large-scale circula- of Higher Education. The simulations were conducted tion other than regionally in Eurasia (Löfverström on resources provided by the LOEWE Frankfurt Center et al. 2014). for Scientific Computing and the Swedish National Infra- structure for Computing (SNIC) at the National Super- computingCentre(NSC)inLinköping. 5. Conclusions Our main results and conclusions are summarized as follows: REFERENCES d We find that the ice-sheet development on a flat Abe-Ouchi, A., T. Segawa, and F. Saito, 2007: Climatic conditions representation of the North American continent for modelling the Northern Hemisphere ice sheets throughout the ice age cycle. Climate Past, 3, 423–438, doi:10.5194/ is fairly zonally symmetric. In this simulation the cp-3-423-2007. incipient/early ice sheet develops a double-dome Arbuszewski, J. A., P. B. deMenocal, C. Cléroux, L. Bradtmiller, structure due to an anticyclonic circulation that forces and A. Mix, 2013: Meridional shifts of the Atlantic in- zonal asymmetries in the precipitation field. 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