Paleoclimate over the Laurentide Ice Sheet

at the Last Glacial Maximum


A collaborative research effort by the Byrd Polar Research Center Polar Meteorology Group (PMG)and the University of Maine Institute for Quaternary and Climate Studies. Supported by NSF OPP-9905381 and NSF OPP-9900477.
Overview

During the Last Glacial Maximum (LGM), at the peak of the last glacial cycle approximately 21,000 years ago, the earth's climate was much cooler than today and the Laurentide Ice Sheet covered a significant portion of North America. Over the next 10,000 years, during the transition to the present climate, the Laurentide Ice Sheet retreated and advanced periodically. This oscillation of the Laurentide Ice Sheet was marked by several massive surges of icebergs into the North Atlantic Ocean, called Heinrich events. The last of these iceberg outbursts, which occurred around 11,000 years ago, marked the irreversible collapse of the Laurentide Ice Sheet. The climate record also shows several oscillations, between relatively cold (stadial) and relatively mild (interstadial) periods, that roughly coincide with the Heinrich events. Evidence suggests that these stadial-interstadial climate transitions occurred rapidly, taking place within about 200 years.

Our paleoclimate research project uses the University of Maine Ice Sheet Model (UMISM) and the Polar MM5 regional climate model to conduct high-resolution computer simulations of the Laurentide Ice Sheet and the climate at various time periods. By improving the UMISM representation of various glacier dynamics processes and coupling the ice sheet model to the high-resolution Polar MM5, our goal is to simulate the oscillations of the Laurentide Ice Sheet and determine whether they triggered rapid changes in climate. The images and animations below highlight some of our modeling results, including a UMISM simulation of the onset, growth, and retreat of the Laurentide Ice Sheet over last glacial cycle (100,000 years ago to present). We then present aspects of the climate, simulated by Polar MM5, during a particular period in the last glacial cycle: the Last Glacial Maximum, 21,000 years ago. You can (1) view some of the constant fields used in the Polar MM5 LGM climate simulation; (2) view the annual cycle of several atmospheric fields (e.g., surface temperature and pressure) over the Laurentide Ice Sheet from Polar MM5 output; and (3) watch winter and summer storm systems evolve and move over and around the Laurentide Ice Sheet. Also, checkout the italicized keywords throughout the web page for definitions and additional information on select topics. Enjoy the presentation and feel free to send us questions about glaciological modeling or paleoclimate modeling.

Tip:  Since many of the following animations contain several model fields plotted together, you may find it helpful to first examine a single image to identify each field before looping through the animation.


The Last Glacial Cycle: Ice Sheet Dynamics

Here we present a simulation of the last glacial cycle over North America using the University of Maine Ice Sheet Model. UMISM is a multi-component ice-sheet model that uses the Finite-Element Method as its equation solver. Components include ice dynamics (continuity and momentum conservation), internal temperatures (energy conservation), a pseudo-elastic hydrostatically supported plate model of the bed isostasy, a water-conservation model of basal meltwater generated at the bed, and several simple climate models (a lapse-rate based model where accumulation rate depends on surface temperature derived from latitude and elevation, as well as climate models based on modern climate data or GCM modeling results).

These components are all coupled, so that, for instance, changes in surface temperature from the climatic component affect both the material properties of the ice dynamics (cold ice is harder), as well as the amount of meltwater generated at the bed (sliding occurs where the bed is wet and is proportional to the amount of water there). These changes in the ice-sheet configuration affect the degree of bed depression through the isostasy model, and also feedback through the climate model allowing for further surface-temperature change.

Note: You can view the "loop" animation in your browser or a faster-playing "movie" animation on a separate media player. The "loop" animation will work on most browsers, but you must have a plug-in (i.e., Quicktime or Windows Media Player) to view the "movie".

Surface Elevation shows the terrain height (meters above sea level, color shading) over the Laurentide Ice Sheet domain. In these and subsequent UMISM images, the horizontal distance scale is shown in the upper right of the image and the time in years before present in the upper left. Ice-surface elevation is the primary output of UMISM.

Early in the animation, you'll notice two main areas of glacial onset: Baffin Island (west of Greeland) and the Labrador region of eastern Canada. During the first 25,000 years of the simulation, these two small glaciers advance and retreat slightly, but remain separate and distinct. Then, around 60,000 years before present, after a period of fairly steady growth, the two glaciers merge into a single Laurentide Ice Sheet. Stepping forward in time, the Laurentide Ice Sheet continues advancing steadily, with a few fluctuations, until reaching its maximum extent approximately 21,000 years before present (the Last Glacial Maximum). At the LGM, the modeled Laurentide Ice sheet maximum elevation is about 3500 meters (11,500 feet) above sea level in central Canada and its southern limit extends to the midwestern U.S. (including parts of Ohio, Illinois, and Nebraska). The initial retreat of the Laurentide Ice Sheet after the LGM is gradual. However, between 15,000 and 7,000 years before present the modeled ice sheet retreats rapidly and is gone by 5,000 years before present.

Bed Elevation shows height of the ice sheet bed (meters above sea level, color shading) over the Laurentide Ice Sheet domain. As glaciers grow, the weight of the ice can become large enough to depress the earth's crust beneath the glacier. In these plots, warm colors (yellows, reds) are elevations above sea level, while cool colors (blues) indicate elevations below sea level. These changes in bed elevation are produced by the Isostasy component of UMISM.

Early in the simulation, the glaciers over Baffin Island and Labrador, Canada are small and cause only minor changes in the bed elevation. However, from roughly 70,000 years before present to the LGM, as the ice advances and the single Laurentide Ice Sheet forms and grows, you'll notice an expansion of cool colors as the earth's crust is increasingly depressed under the weight of the overlying ice sheet. Thus, by the LGM, a broad area of North America under the ice sheet is below sea level! From the LGM onward, as the ice sheet retreats, the burden is relieved and the earth's crust rebounds.

Ice Thickness is the simply the difference between the Surface Elevation and the Bed Elevation. Here, the ice sheet thickness is color shaded in units of meters. The animation is similar to the Surface Elevation, but gives a more direct assessment of where ice is growing or retreating.

Ice Velocity shows how fast the ice is moving. Glaciers are dynamic features and the velocity of glacial ice depends on the pressure forces in the ice, whether the underlying bed is frozen or thawed, and the amount of liquid water under the ice sheet (for a thawed bed). The ice sheet velocities show here are color shaded.Because of the very large range of velocities evident in ice sheets, we display the base-10 logarithm of the velocity, rather than its actual value. This means that a velocity in the figure of 0.0 corresponds to an actual velocity of 1 m/year, 1.0 corresponds to 10 m/year, 2.0 corresponds to 100 m/year, and 3.0 corresponds to 1000 m/year. This type of logarithmic scale shows the positions of ice streams more clearly. Ice streams are fast-flowing rivers of ice embedded in the slower-moving ice sheet. These fast-flow regions are produced within the model by the presence of water at the bed (see the next Basal Water Thickness animation).

As you watch the animation, note in particular the fast flow that frequently occurs in the Hudson Strait leading from the Hudson Bay, in the St. Lawrence between Nova Scotia and Newfoundland, and in localized regions along the southern margin of the ice sheet. Both the Hudson Strait and the St. Lawrence ice streams are well documented by geological data, and the southern margin ice streams coincide with known ice lobes. There is also a large ice stream in northwestern Canada near the border with Alaska for which there is considerable geological evidence. These types of features are recognized in the current Antarctic and Greenland Ice Sheets and are being studied now to better understand how the water lubrication mechanism works.

Basil Water Thickness shows the amount of liquid water at the base of the ice sheet where the bed is thawed. Water is generated at the bed by melting, but can move horizontally under the ice sheet through conduits and as a lubricating sheet. In the model, this basal water has a lubricating effect on the glacier, making it flow faster than if the bed is frozen to the ground. Ice streams are thought to be caused by this water lubrication.



Last Glacial Maximum Climate

The next few sections focus on the modeled climate of the Last Glacial Maximum, that period during the last glacial cycle when the Laurentide Ice Sheet reached its greatest extent.

To properly simulate climate at the LGM, several boundary conditions must be specified in the climate model, which for our study is the Polar MM5. These boundary conditions include the topography, land use (vegetation), sea level, solar forcing, trace gas (i.e., carbon dioxide) concentrations, and glacier extent, all of which were different than today. For our project, the LGM ice sheet data (elevation and extent) came from the University of Maine Ice Sheet Model. The LGM vegetation data came from contributors to the BIOME 6000 Project. The Polar MM5 was initialized using output from a global climate model simulation of the LGM climate.

The images above show the LGM and present day topography and land use categories in the Polar MM5 domain. At the LGM, the Laurentide Ice Sheet (center of the model domain), covers much of North America and is similar in size to present day Antarctica.  Greenland (which remains permanent ice today) lies just to the north of the Laurentide Ice Sheet.  A portion of the Fennoscandian Ice Sheet, which covered northern Europe during the LGM, is also included in the model domain.

The volume of water contained in the LGM continental ice sheets (like the Laurentide) equalled about 120 meters (394 feet) of global sea level. The lowered sea level meant additional exposed land surface and an altered coast line at the LGM (note the land bridge between present day Alaska and eastern Russia).

Vegetation was also different from today in reponse to the cooler LGM climate.   Note, for instance, that the southwestern U.S., which is arid today, was wetter and forested during the LGM.  What other differences do you see?



Annual Cycle

Monthly averages of atmospheric fields are commonly used to examine the evolution of the atmosphere through the year (the annual cycle).  We have selected from the Polar MM5 LGM simulation several fields at the surface and the middle of the model atmosphere.  As you loop through the annual cycle, note the following trends in these fields: 

Surface Temp, SLP shows the air temperature 2 meters above ground level (color shading) and sea level pressure (black contours).  In addition, centers of high and low pressure are box-labelled and the 0oC isotherm (freezing line) is denoted by the white contour.

As the months progess from early autumn to winter, very cold temperatures become established over North America due to the presence of the Laurentide Ice Sheet. By January, 2-meter air temperatures are below -50oC (-58oF) in the Canadian interior, over the Laurentide Ice Sheet.  The mean 0oC istotherm extends southward to the Gulf of Mexico.  This cold, dense air is associated with an area of high pressure, called a glacial anticyclone, that becomes most pronounced over the Laurentide Ice Sheet in the winter months.  Low pressure centers are located upstream near Alaska (the Aleutian Low) and downstream near Iceland (the Icelandic Low). As temperatures warm in the spring and summer, the glacial anticyclone over the ice sheet weakens.  Although the Laurentide Ice Sheet is not depicted on these plots, during the summer months the mean 0oC isotherm essentially outlines the ice sheet margins.

Surface Wind and Topography shows the terrain elevation above sea level (color shaded) and monthly mean wind vectors (arrows). The wind speed is proportional to the length of the vector.  

In the early autumn, surface winds over the Laurentide Ice Sheet blow mostly from the west and northwest and an anticyclonic (clockwise) circulation can bee seen along the southwestern margin of the ice sheet.  This anticyclonic circulation feature migrates northward to the apex of the ice sheet and becomes more pronounced in the winter months.  The distinct anticyclonic flow is the katabatic wind, which results as cold, dense air at low levels drains off the ice sheet. The Laurentide Ice Sheet katabatic winds are strongest in January (when temperatures are coldest), reaching 30 meters per second (67 miles per hour)!  Remember, these are monthly mean wind speeds.  Undoubtedly, there are much stronger peak wind events taking place during the month.  The katabatic wind signature becomes less clearly defined  in the warmer spring and summer months, although  the monthly mean winds are still fairly strong (10-15 meters per second) over portions of the Laurentide Ice Sheet in northeastern North America.

500-hPa Temp, Winds, Geo_Ht shows the air temperature (color shaded), wind speed and direction (barbs), and height above sea level (black contours) of the 500-hPa constant pressure surface.  The 500-hPa (hecto-Pascal) pressure surface is near the middle of the troposphere, approximately 4500-5500 meters (14,750-18,000 feet) above sea level.  At this level, high above the surface of the earth, the winds are geostrophic, blowing parallel to the height contours.  On such plots, we can identify the jet stream as regions where the height gradient is large and the maximum wind speeds occur.

In early autumn, the jet stream is located across northern North America.  There is a trough (area of relatively low 500-hPa heights) upstream of the Laurentide Ice Sheet, a trough downstream over Greenland, and a weak ridge (area of relatively high 500-hPa heights in between.  Beginning in November, notice that the jet stream flow (indicated by the height contours and wind barbs) coming from the Pacific splits over western North America.  One branch of the jet stream turns northward over Alaska and the Arctic, while the other branch turns southward and flows over the southern U.S.  The split flow pattern is most pronounced in January when temperatures are coldest and the low-level katabatic winds are well established (see the previous section).  The trough over the eastern part of the model domain is also strongest in the winter as indicated by the large height gradients west of Greenland and across the North Atlantic Ocean.  The split jet stream persists through the spring and becomes weaker, after which a more uniform, single jet stream configuration is established. 

Monthly Precipitation and Topography shows  the monthly total precipitation in millimeters (color shaded) and terrain elevation (black contours).  

The jet stream patterns discussed in the previous section largely determine the tracks of storm systems, and therefore, the distribution of precipitation in the model domain.  For instance, in early autumn the jet stream directs most of the synoptic-scale storms ashore along the western margin of the Laurentide Ice Sheet where precipitation maxima occur.  Further downstream, storms moving across the Atlantic result in large amounts of precipitation along the western margin of the Fennoscandian Ice Sheet over present day Great Britain.  As the split jet stream becomes established in the late autumn and winter months, precipitation maxima occur along the western coast of North America, from Alaska to California.  Precipitation also increases near the east coast of North America as the southern branch of the jet stream carries storms offshore and across the Atlantic Ocean.  The summer months bring significant amounts of precipitation to the southern margin of the Laurentide Ice Sheet as storms traverse from west to east across the ice sheet.  In fact, areas along the southern margin receive over 550 millimeters (about 22 inches) of precipitation in August!  The amount of precipitation, and whether it falls as snow or rain, are important parameters used by the glacier modelers at the University of Maine to simulate the advance and retreat of the Laurentide Ice Sheet.




Winter Synoptic Variability

While monthly mean fields are useful for describing the annual cycle of the atmosphere, more frequent model output is used to determine synoptic variability. Here we present 6-hour Polar MM5 output over an 8-day period in January of the LGM simulation to illustrate how winter storm systems move and evolve in the model domain. As you loop through the selected atmospheric fields above, notice the following: 

Surface Temp, SLP, Topography shows the air temperature 2 meters above ground level (colored, dashed contours), sea level pressure (black contours), and the model terrain elevations (color shading).  In addition, centers of high (H) and low (L) pressure are box-labelled. Temperatures above or below freezing are denoted by red or blue contours, respectively. The solid red contour is the 0oC isotherm. Note: the time label at the top of each plot is relative to the start of the model simulation and the "01" year designation should be ignored.

At the beginning of this period, there are several distinct features in the sea level pressure field. A large area of high pressure (the glacial anticyclone) is located over the Laurentide Ice Sheet and remains essentially in place throughout the period. Two strong low pressure systems are located in the upper left portion of the domain, one over the Pacific Ocean south of Alaska, and the other over the western Arctic Ocean basin north of Canada. Recall from the discussion of monthly mean fields that in January the jet stream, which steers storms systems as they move through the domain, is split around the Laurentide Ice Sheet. Thus, over the first few days of the animation the Pacific low pressure system, steered by the northern branch of the jet stream, moves northeastward and eventually comes ashore in southern Alaska. The low pressure system north of Canada crosses the Arctic and merges with an area of low pressure east of Greenland. These are just two of several storm systems that each follow a similar track during the 8-day period.

If you watch closely along the east coast of North America, you'll also notice several low pressure systems that are directed along the southern branch of the jet stream during the 8-day period. One of these storms develops in the eastern U.S. (north of Florida) on January 9th, moves offshore on January 10th, and quickly intensifies as it moves up the coast. The strong temperature gradient, between the cold air over the Laurentide Ice Sheet and the warm air over the adjacent Atlantic Ocean, aids the development of this low pressure system.

Surface Temp, Wind, Topography shows the 2-meter air temperature and terrain elevations as before, along with the surface wind vectors (arrows). The wind speed is proportional to the length of the vector. This animation illustrates the evolution of the low level winds associated with high and low pressure systems and the relationship to the surface temperature (temperature advection).

In the first frames of the animation, the two low pressure systems we discussed earlier in the northern branch of the jet stream have distinct counter-clockwise wind patterns characteristic of low pressure in the Northern Hemisphere. The storm system over the Pacific Ocean has an elongated wind circulation, with strong southerly winds ahead of the system and northerly winds behind the low. Note the inverted-V signature in the temperature field, which indicates the advection of warm air ahead of the low pressure system. The low pressure system that moves along the east coast on January 10th is also easily identified by the strong counter-clockwise surface winds. Can you also identify the katabatic wind circulation over the Laurentide Ice Sheet?

6hr Precip, Wind, Topography shows the 6-hour accumulated precipitation (color shaded), the terrain elevations (black contours), and surface wind vectors as before.

The modeled storm systems moving through the Polar MM5 domain can be clearly identified in the accumulated precipitation animation. In the first few days of the animation, the Pacific low pressure system develops a classic mid-latitude cyclone signature, with an elongated north-south band of precipitation along a front, before coming ashore on the west coast of North America. Some of the precipitation is carried south and east by the southern branch of the jet stream and eventually moves across the southwestern U.S. and northern Mexico by the end of the period. Notice also that precipitation is persistent in the Gulf of Mexico almost throughout the 8-day period. Occasionally, some of this precipitation is drawn north and east ahead of storm systems that move along the southern branch the jet stream. A very clear example of this is the storm system that develops along the east coast in the last several days of the animation. Watch carefully, beginning on January 7th, and see if you can identify the moisture (precipitation) being drawn northward from the Gulf of Mexico. What happens to the accumulated precipitation on January 10th when the storm system moves off the east coast and intensifies? By January 11th, this east coast storm has matured, as indicated by the classic comma shape in the precipitation field.



Summer Synoptic Variability

Recall from the monthly mean fields that during the summer months the Polar MM5 produces a single jet stream, moving west to east over central North America. This jet stream tends to steer storm systems over the southern portions of the Laurentide Ice Sheet. Meanwhile, the warm land surface and adjacent cold ice sheet produce a strong low-level baroclinic zone near the southern margin of the Laurentide Ice Sheet, which aids the development of storm systems. The animations above, 6-hour Polar MM5 output over an 6-day period in July, illustrate the development of a few summer storms on the Laurentide Ice Sheet.  

Surface Temp, SLP, Topography shows the air temperature 2 meters above ground level (colored, dashed contours), sea level pressure (black contours), and the model terrain elevations (color shading).  In addition, centers of high (H) and low (L) pressure are box-labelled. Temperatures above or below freezing are denoted by red or blue contours, respectively. The solid red contour is the 0oC isotherm.

At the beginning of the animation, the sea level pressure field shows a low pressure system (labelled "998" millibars) is located in west-central Canada, on the Laurentide Ice Sheet. An area of high pressure is just east of this low, and another low pressure system (995 millibars) is further east near the coast of present-day New Foundland. As the animation progresses through the first two days (July 14th-16th), note that the New Foundland low moves over the North Atlantic Ocean and intensifies. Later in the period (July 18th), this low moves north and then northwestward along the margin of Fennoscandian Ice Sheet (upper right corner of the domain), which appears to deflect the storm track.

The low pressure system over the Laurentide Ice Sheet meanders south and east during the first two days of the period. The low appears to dissipate by July 16th, but looking carefully along the southern margin of the ice sheet you'll notice a series of low pressure systems that develop and move eastward along this baroclinic zone. One such low pressure system develops along the southern margin on 12Z July 18th (999 millibars), moves east, and becomes rather intense (985 millibars) by the end of the period. Note the inverted-V shape of the 0oC isotherm in the last few frames of the animation. This indicates the strong advection of warm air from the south onto the ice sheet ahead of the low pressure system.

Surface Temp, Wind, Topography shows the 2-meter air temperature, terrain elevations, and surface wind vectors (arrows). The wind speed is proportional to the length of the vector.

Compared with the January surface wind vectors, the clockwise katabatic wind circulation over the Laurentide Ice Sheet is less pronounced in July. Also, the surface wind pattern, particularly along the southern margin of the ice sheet, is occasionally "disturbed" as low pressure systems migrate across the region. The counter-clockwise circulation associated with these low pressure systems brings southerly flow (warmer 2-meter air temperatures) ahead of the low and northerly flow (colder 2-meter air temperatures) on the backside of the low. As you watch the animation, see if you can identify the low pressure systems by the wind and temperature patterns along the southern margin of the ice sheet.

6hr Precip, Wind, Topography shows the 6-hour accumulated precipitation (color shaded), the terrain elevations (black contours), and surface wind vectors as before. Also plotted (solid red contour) is the 90% probability of liquid precipitation (rain). Thus, precipitation occurring south of the red contour is 90% likely to be rain rather than snow.

As with the animation of January accumulated precipitation, we can track summer storm systems moving through the domain with relative ease. You'll notice that from the beginning of the 6-day period, a nearly continuous series of precipitation events that develop and move along the southern margin of the Laurentide Ice Sheet. Looking more closely, you'll also notice that, beginning around 18Z July 15th, some of the precipitation falling on the ice sheet is south of the 90% probability contour (that is, most certainly rain). The rain on the ice sheet occurs in the warm sector of the low pressure system. The southerly winds encounter the ice sheet terrain and flow upslope, which further enhances the precipitation process. Also, some of this rain on the ice sheet is being generated by convection (model thunderstorms). The liquid precipitation falling on the ice sheet is very important for the behavior of the ice sheet. One important goal of our collaborative project is to model the mechanisms by which liquid water from this type of rain event can seep through the crevasses in the ice sheet, reach the bed, and act as a lubricating mechanism to affect the ice sheet flow velocity.



Thanks for visiting! We will continue to add results from our study as they become available. Again, feel free to contact us with any questions about glaciological modeling or paleoclimate modeling.