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.
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