LGM Summer Climate on the Southern Margin of the Laurentide Ice Sheet: Wet or Dry?*

Abstract

Regional climate simulations are conducted using the Polar fifth-generation Pennsylvania State University (PSU)-NCAR Mesoscale Model (MM5) with a 60-km horizontal resolution domain over North America to explore the summer climate of the Last Glacial Maximum (LGM: 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). Output from a tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate is used to provide the initial and lateral boundary conditions for Polar MM5.LGM boundary conditions include continental ice sheets, appropriate orbital forcing, reduced CO 2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level.The simulated LGM summer climate is characterized by a pronounced low-level thermal gradient along the southern margin of the LIS resulting from the juxtaposition of the cold ice sheet and adjacent warm ice-free land surface. This sharp thermal gradient anchors the midtropospheric jet stream and facilitates the development of synoptic cyclones that track over the ice sheet, some of which produce copious liquid precipitation along and south of the LIS terminus. Precipitation on the southern margin is orographically enhanced as moist southerly low-level flow (resembling a contemporary Great Plains low-level jet configuration) in advance of the cyclone is drawn up the ice sheet slope. Composites of wet and dry periods on the LIS southern margin illustrate two distinctly different atmospheric flow regimes. Given the episodic nature of the summer rain events, it may be possible to reconcile the model depiction of wet conditions on the LIS southern margin during the LGM summer with the widely accepted interpretation of aridity across the Great Plains based on geological proxy evidence.

Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry

“…To test these hypotheses, we present new data on the stratigraphy, sedimentology, geochronology, mineralogy, and geochemistry of Peoria Loess in Nebraska. We also compare our results with a recent dust-cycle simulation model presented by Mahowald et al (2006) and a recent climate model presented by Bromwich et al (2005). Our results complement isotopic studies contained in a companion paper (Aleinikoff et al, 2008).…”

“…In addition, however, both Bromwich et al (2005, p. 3334) and Mahowald et al (2006, p. 17) note that frequency, magnitude, and seasonality of loess transportation may also be factors, a hypothesis also suggested by Muhs and Bettis (2000). In the Polar MM5 model presented by Bromwich et al (2005), precipitation in the central Great Plains is simulated to have been dominant in summer, a situation that would not enhance loess particle entrainment or transport during this season. Increased interannual variability may have played a role, with some summers having been drier than others (Bromwich et al, 2005).…”

“…A mesoscale model (Polar MM5) presented by Bromwich et al (2005) sheds some new light on paleowinds in midcontinental North America during the last glacial period. Polar MM5 is attractive for modeling paleowinds above and near the Laurentide ice sheet during the last glacial period, because the model has simulated modern katabatic winds over Greenland successfully (Bromwich et al, 2001).…”

“…Polar MM5 is attractive for modeling paleowinds above and near the Laurentide ice sheet during the last glacial period, because the model has simulated modern katabatic winds over Greenland successfully (Bromwich et al, 2001). Results of summer climate simulations from Polar MM5 for the last glacial period in North America indicate easterly and northeasterly katabatic winds near the ice sheet (Bromwich et al, 2005). However, the zone affected by katabatic winds is limited to an area within a few model grid points (estimated to be less than ~200 km) south of the ice sheet.…”

Northern hemisphere storm tracks during the last glacial maximum in the PMIP2 ocean-atmosphere coupled models: energetic study, seasonal cycle, precipitation

“…Concerning precipitation reconstructions over the continents, recent results using inverse vegetation modelling over Eurasia (Wu et al 2007) are consistent with the model simulations presented here, at least in its general conclusion of drier conditions. Concerning North America, the drier conditions inferred from reconstructions south of the ice-sheet (Bromwich et al 2005) is not necessarily in contradiction with PMIP2 model results, since the increased precipitation occurs farther south, South of 40°N in winter (Fig. 9).…”