South American Climate of the LGM: A regional modeling study

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South American Climate of the LGM A Regional
Modeling Study
Kerry H. Cook Department of Earth and Atmospheric
Sciences Cornell University Thanks to Edward
Vizy and Nancy Saltzman
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Goal Explain the Climate Dynamics behind LGM
Aridity Patterns
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. and temperature reconstructions as well
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Regional climate modeling on a large domain -
MM5 modified for climate applications in the
tropics 20 - 60 km horizontal resolution 23
vertical levels 1 min time step Year-long
integrations with climatology used for initial
and boundary conditions u, v, T, q, and surface
conditions
Regional model domain and topography shading
interval is every 500 meters.
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Observations
Climate Model
Jan
July
Validation of the present day simulation
Precipitation
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August
September
October
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The regional model represents the present day
South American climatology much more accurately
than the GCM simulations that have been used to
study paleoclimate. Using GCM lateral boundary
conditions degrades the present day
simulation significantly. This presents a
problem We cant use GCM lateral
boundary conditions for our LGM simulations.
What to do?
DJF 850 hPa geopotential heights (m) and winds
(m/s) from the (a) 1949 2002 NCAR/NCEP
reanalysis, (b) MM5 present day
simulation interpolated to NCEPs 2.5? ? 2.5?
grid, and present day PMIP simulations from (c)
ECHAM3 (fixed SSTs), (d) UGAMP (fixed SSTs), (e)
UKMO (calculated SSTs), and (f) GFDL (calculated
SSTs).
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Simulation of the LGM Climate SSTs Vegetation
(land use) Atmospheric CO2 Orbital
parameters Initial conditions Lateral
boundary conditions
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Surface Boundary conditions Vegetation
Present day USGS
LGM Crowley (2000)
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CLIMAP 1981
Core, Schäfer-Neth and Paul (2003)
We ran simulations with each of these LGM SST
anomalies, and compared the results with the
land- based proxy data. 2 of the SST
distribu- tions essentially shut down the
monsoon. Line and CLIMAP produced
similar results. We chose line.
GCM, Shin et al.(2003)
Line, Paul and Schäfer-Neth (2003)
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Back to the problem of the lateral boundary
conditions
Using GCM conditions on the lateral boundaries
seriously degrades the simulation of South
American climate. (Also found by Seth and Rojas
2004, and more generally by Pielke et al
2005). In setting the lateral boundary
conditions for the LGM simulations, we are not
too concerned about eliminating remote effects on
South American climate more concerned about the
consistency between the LGM SSTs and the
atmosphere on the boundaries and over the
nudging region.
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• Modify the present day lateral boundary
  conditions so they are dynamically consistent
  with the LGM surface boundary conditions (SSTs).
• Ran model with present day lateral boundary
  conditions
• Used interior points to develop a method to
  modify the points on the boundary
• - adjust low-level temperature, retain lapse
  rate
• - test for geostrophy (reasonable except within
  a few degrees of equator)
• - adjust surface wind based on interior
  points, and use the thermal wind
• relation to propagate the difference
  vertically
• - assume constant relative (not absolute)
  humidity
• The resulting differences in the lateral boundary
  conditions were small everywhere except over the
  tropical Atlantic. But the modeled LGM solution
  with present day lateral boundary conditions
  captured this as well.

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LGM minus Present Day Simulations
Precipitation Differences
P E Differences
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Monthly Mean Precipitation (mm/day)
Region 1
Region 2
Region 4
Region 3
Solid Present day
Dashed LGM
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Simulated Winds and Specific Humidity at 850 hPa
October
Present day
LGM
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Vertical Profile of Moist Static Energy at 5ºS
and 60ºW
dry wet
March, present day
MSE cpTLqgz
Solid MSE Dash sensible Dot/dash latent Dot
geopot
MSE increasing with height gt stability
low-level decreases in MSE stabilize the
atmosphere against convection
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Moist static energy
Sensible heat content
Latent heat content
Differences in MSE (solid), sensible (dashed) and
latent heat (dotted) terms
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P-E Difference
A region without increased aridity during the LGM
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Annual precipitation differences from the present
day simulation
LGM vegetation forcing alone (a deforestation
experiment)
LGM SST forcing alone
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Precipitation in Region 5
Present day
Full LGM
LGM SST alone
LGM veg alone
Present day
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A close up view of Region 5 in May
Surface elevation and 910 hPa winds
Precipitation differences in May
agrees with Ronis result that how
deforestation occurs is relevant to the sign of
the precipitation response
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Difference between LGM-vegetation-only and
precipitation simulations May
surface temperature differences
870 hPa height and wind anoms
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That was an overview of recent results
Vizy, E.K., and K.H. Cook, 2005 Evaluation of
LGM SST reconstructions through their influence
on South American climate. In press at J.
Geophysical Research Atmospheres. Cook, K.H.,
and E. K. Vizy, 2005 South American climate
during the Last Glacial Maximum Delayed onset of
the South American monsoon. Submitted to J.
Geophysical Research Atmospheres.
Current projects (1) Dynamical interactions
between the high Andes and the rest of South
America. What is the paleo-record in the high
Andes telling us about the Amazon and subtropical
South America? (2) Work with a PVM to explore
consistency between prescribed vegetation forcing
and the modeled climate, and translate climate
into potential vegetation.
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Present day simulation
60 km outer grid resolution with 20-km resolution
nested
LGM Simulation
Landuse categories 6 urban/cropland 7
grassland, 8 shrubland 10 savanna, 13
evergreen broadleaf forest, 14 evergreen
needleleaf forest 16 water, 19 barren or
sparsely vegetated 20 tundra 24 snow/ice
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Topography
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Precipitation is not Validating Well in the High
Andes Nested Domain in the Present Day Simulation
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We are not capturing the wetting signal in
the high Andes in the LGM simulation
20-km resolution is not fine enough for this
simulation design .
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Another project underway . using a PVM to
translate the climate produced by the regional
climate model into vegetation categories (1) To
provide a more direct comparison with some of the
geological proxy data (2) To better understand
the implications of the climate
differences simulated by the model (3) To
free ourselves from uncertainties in specifying
the vegetation distribution in simulating LGM
climate (iteration)
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Potential Vegetation Model (Oyama and Nobre 2004)
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Left initial vegetation Right vegetation after
one iteration Top Core LGM
SSTs Bottom Line LGM SSTs
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Top One iteration from Crowley initial
vegetation, LGM SSTs Bottom One iteration
from Present day initial vegetation, LGM SSTs
Different initial conditions on vegetation might
cause differ- ent solutions, as in Oyama and
Nobre 2003. Esp. note the eastern Amazon.
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Even if we use present day SSTs and this initial
vegetation, the eastern Amazon does not become
forested.

.
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Cutoffs for rainforest TC ? 15.5C
h ? 0.8 s ? 0.81
s seasonality index h wetness index
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Conclusions
A somewhat different modeling approach to
studying paleoclimate, using regional models and
our knowledge of how the present day climate
works to evaluate the quality of the paleoclimate
simulation. (We are working on LGM South America
and the AHP.)
The approach has plusses and minus as do other
approaches to understanding paleoclimate such as
GCM modeling and the analysis of various
proxies. Compared with GCMs Better
simulation of South American climate, finer
resolution, able to resolve
interactions across space scales (relatively
large domain with a relatively fine
resolution). - global teleconnections are not
considered
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• Compared with proxy data
• constrained by physics (Navier-Stokes eqns),
  produces fields
• that are internally consistent
• - model dependent results
• We need all of these approaches.
• We find that
• There is large-scale drying in the Amazon basin
  during the LGM,
• delivered in the form of annual precipitation
  reductions on the order
• of 30.
• In the Southern Hemisphere, this drying is due
  primarily to a
• shortening of the rainy season, and a lengthening
  of the dry season.
• The shortening of the dry season is caused by a
  delay in the onset
• of the monsoon. The monsoon starts later because
  the tropical
• Atlantic is cool, so the buildup of moisture/MSE
  is delayed.

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Cooling of more than about 2K in the tropical
Atlantic shuts downs the monsoon completely in
this model. In the Northern Hemisphere, in which
summer precipitation is more ITCZ-like in its
circulation, drying is also due to the fact that
the low-level convergence is dryer again
associated with cool tropical Atlantic.
We find that the first-order forcing is from the
SSTs, but there are interesting and important
regional responses related to vegetation forcing.
For example, the region of increased
precipitation along the Equator when the
large-scale circulation anomaly interacts with
a Regional orographic feature.