Role of Land SurfaceAtmosphere Interactions in Abrupt Climate Change over Northern Africa

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Role of Land Surface/Atmosphere Interactions in
Abrupt Climate Change over Northern Africa
Kerry H. Cook and Christina Patricola Departmen
t of Earth and Atmospheric Sciences Cornell
University
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The African Humid Period (AHP)
Geological reconstruction of precipitation minus
evaporation for 6,000 years ago.
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Ultimate cause of the AHP Incoming shortwave
radiation during Northern Hemisphere summer was
greater.
Contour interval 20 W/m2.
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Ultimate cause of the AHP Incoming shortwave
radiation during Northern Hemisphere summer was
greater.
1 July 15N
Contour interval 20 W/m2.
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Vegetation for present day and African Humid
Period according to Hoelzmann et al. (1998)
with grassland - 7, shrubland - 8, savanna - 10,
evergreen broadleaf forest - 13, and desert -19.
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GREEN SAHARA CLIMATE STATE
Vegetation for present day and African Humid
Period according to Hoelzmann et al. (1998)
with grassland - 7, shrubland - 8, savanna - 10,
evergreen broadleaf forest - 13, and desert -19.
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Paleoclimate evidence suggests the climate of
northern Africa can change rapidly. Example
Despite gradual solar forcing, the onset and
termination of green Sahara conditions of the
African Humid Period occurred within centuries.
(deMenocal et al. 2000) Concern for the future
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Part I. The African Humid Period Climate How
were the high precipitation rates maintained?
How was the dynamics of the monsoon system
different? Patricola, C.M., and K.H. Cook, 2007
Dynamics of the West African monsoon under
mid-Holocene precessional forcing. J. Climate,
20, 694-716. Part II. Rapid transitions
between the wet (e.g., AHP) and dry (e.g., today)
climate states Patricola and Cook, 2008
Interactions Between Vegetation and the Dynamics
of the West African Monsoon A Mechanism for
Abrupt Climate Change in the Sahel and Sahara.
JGR-Atmospheres
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Part I. The African Humid Period Climate How
were the high precipitation rates maintained?
How was the dynamics of the monsoon system
different? Patricola, C.M., and K.H. Cook, 2007
Dynamics of the West African monsoon under
mid-Holocene precessional forcing. J. Climate,
20, 694-716. Part II. Rapid transitions
between the wet (e.g., AHP) and dry (e.g., today)
climate states Patricola and Cook, 2008
Interactions Between Vegetation and the Dynamics
of the West African Monsoon A Mechanism for
Abrupt Climate Change in the Sahel and Sahara.
JGR-Atmospheres
Vegetation feedbacks play an important role in
both.
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Regional Climate Model Simulations of the AHP
Climate

• MM5 modified to a regional climate model (RCM)
• climatological, but seasonally varying, surface
  and lateral boundary conditions updated every 12
  hours using a linear interpolation between
  monthly means from the 1949-2002 NCEP/NCAR
  reanalysis

90 km resolution 90 s time step Run May 15
through September 30, with the first 17 days
disregarded as model spin-up.
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925 hPa flow and geopotential heights
RCM
NCEP
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925 hPa flow and geopotential heights
sources of moisture for the continental rainfall
at this level
westerly jet
RCM
NCEP
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925 hPa flow and geopotential heights
sources of moisture for the continental rainfall
at this level
westerly jet
monsoon inflow
RCM
NCEP
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NCEP
RCM
African easterly jet sink of moisture
sources of moisture for the continental rainfall
at this level
675 hPa flow and geopotential heights
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Mass-weighted vertically-integrated moisture
transport present day simulation, JJAS
Vector scale is 500 kg m-1 s-1.
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Mass-weighted vertically-integrated moisture
transport present day simulation, JJAS
Vector scale is 500 kg m-1 s-1.
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Mass-weighted vertically-integrated moisture
transport present day simulation, JJAS
Vector scale is 500 kg m-1 s-1.
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• Simulation of the AHP Climate In the Regional
  Climate Model
• Changes in orbital parameters (insolation)
  imposed
• CO2 set to preindustrial values
• AHP vegetation imposed
• Three Simulations
• Full AHP forcing
• Radiative AHP forcing alone (insolation CO2)
• AHP vegetation forcing alone

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Response of the RCM under Full African Humid
Period Forcing In agreement with the
paleodata, the model simulates increased summer
Saharan and Sahelian precipitation during the
AHP.
Regional Climate Model Simulations The
West African Monsoon is simulated from June
through September (JJAS) using the Penn State
University/ National Center for Atmospheric
Research Mesoscale Model version 3.6 (PSU/NCAR
MM5) modified to a regional climate model (RCM),
which utilizes climatological, but seasonally
varying, surface and lateral boundary conditions
updated every 12 hours using a linear
interpolation between monthly means from the
1949-2002 NCEP/NCAR reanalysis. The simulations
use a 90 km resolution and a 90 second time step
and are run from May 15 through September 30,
with the first 17 days disregarded as model
spin-up. No land surface model is used. The RCM
produces an excellent present day simulation.
The focus is to understand the effects of
increased solar insolation on the monsoon system.
Understanding the changes of the AHP climate
provides insight into the operation of todays
climate, and builds confidence in our ability to
simulate future climate change in this important
and vulnerable region.
Response of the West African Summer Monsoon
In agreement with the paleodata, the model
simulates increased summer Saharan and Sahelian
precipitation during the AHP with the largest
increase between 15N and 20N. One concern with
the simulation is a precipitation decrease in the
Guinean coast region.
(a)
(b)
(c)
Figure 4. Simulated precipitation (mm/day)
averaged JJAS for the (a) present day, (b) AHP,
and (c) AHP present day. Shading is every 2
mm/day, 0.5 and 1.0 mm/day are contoured in (a)
and (b) and 0 mm/day is contoured in (c).
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Vertically-integrated moisture transport anomaly,
August
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Analysis of the vertical stability properties of
the atmosphere show that this added moisture
transport destabilizes the vertical column
MSE cpT Lq gz
Low-level cooling stabilizes
Added moisture transport destabilizes
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• 2 differences in the flow are responsible for the
  added moisture transport onto the west coast
• (1) Increase in the low-level westerly jet
• (2) Disappearance of the African easterly jet

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675 hPa Geopotential Heights and Winds
Present day
Present day
Present day
Present Day
AHP
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• What are the individual roles of radiative
  forcing and vegetation forcing for the AHP Green
  Sahara solution?

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JJAS precipitation differences
Radiation and vegetation together
Radiative forcing alone
vegetation forcing is important
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Monthly Precipitation Differences Radiative
Forcing Alone
June
July
Sept
Aug
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• Solar (Milankovich) forcing alone increases
  rainfall in August and September, and rainfall
  amounts for the season are not as large as
  indicated by the geological proxy data
• With vegetation forcing added, rainfall rates
  increase in June and July as well, and are more
  consistent with the proxy data.
• Vegetation supplies a memory of the late summer
  enhanced rainfall that carries into the following
  spring.

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• Now investigate the transition between the green
  and desert Sahara climate states
• Is there a feedback with vegetation that may
  instigate rapid climate change over Northern
  Africa?

Method Idealized regional climate model
simulations with both static prescribed and
interactive vegetation distributions are
analyzed.
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• Regional climate model (RCM) simulations
• Prescribed vegetation distributions
• Choose a latitude that is the boundary between
  desert and grasslands.
• North of the boundary, extending to 35.8ºN,
  impose desert surface conditions.
• South of the boundary to 0.34ºN, impose mixed
  shrubland/grassland conditions over regions of
  desert.

desert
savanna
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Albedo and moisture availability prescribed in
the RCM based on vegetation category.
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desert
grass/shrub
savanna
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desert
Rainfall is north of desert boundary
savanna
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Rainfall is same as desert boundary
desert
grass/shrub
savanna
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Rainfall is south of desert boundary
desert
grass/shrub
savanna
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desert
grass/shrub
savanna
Rainfall extends far north of desert boundary
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desert
grass/shrub
savanna
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Results Two climate states determined by
location of desert border
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MSE moisture anomaly (8-12E, 17-20N)
Why does this threshold exist? When vegetation
is prescribed to a sufficient northward extent,
positive soil moisture anomalies superimposed
beneath the region of vertical velocity
associated with the thermal low promote increases
in low level moist static energy ? increased
convection.
Desert border 10.0N 15.5N 17.9N 19.4N 20.9N
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Stability of vegetation distributions?
Asynchronously couple the RCM with a simple
vegetation model. Two simulations Initialize
with desert border at 10.0N and 20.9N.
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Asynchronous coupling of the RCM and a Potential
Vegetation Model (PVM)
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 10.0N
desert
savanna
evergreen broadleaf forest
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 10.0N
desert
grass/shrub
savanna
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 10.0N
desert
grass/shrub
savanna
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 10.0N
desert
grass/shrub
savanna
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 10.0N
desert
grass/shrub
savanna
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 10.0N
Equilibrium resembles present day
desert
grass/shrub
savanna
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desert
grass/shrub
savanna
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• New equilibrium when initialized with desert
  border at 10.0N
• Suggests that the atmospheric conditions would
  allow vegetation to grow back if there were
  deforestation in the present day Sahel.

desert
desert
grass/shrub
grass/shrub
savanna
savanna
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 20.9N

desert
grass/shrub
grass/shrubs
savanna
evergreen broadleaf forest
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 20.9N
desert
grass/shrub
savanna
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 20.9N
desert
grass/shrub
savanna
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Asynchronous coupling of the RCM with a simple
vegetation model initialized at 20.9N
Green Sahara equilibrium
desert
grass/shrub
savanna
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• New equilibrium when initialized with desert
  border at 20.9N
• When vegetation reaches the threshold latitude,
  interactions between the atmosphere and
  vegetation can produce abrupt climate change.
• Initial northward vegetation growth (prescribed
  in the RCM) may be related to combinations of
• SST forcing
• Interannual variability
• Solar forcing (as in AHP)

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• Conclusions
• When vegetation is prescribed at/north of the
  threshold latitude (19.4N) in the RCM...
• a mechanism for abrupt climate change,
  interactions between atmosphere and vegetation,
  is enabled. Possible explanation for abrupt
  onset/termination of AHP
• AEJ weakens by 50
• low-level westerly jet strengthens by 20
• intense monsoon
• shallow thermal low/Saharan high replaced by
  deep moist low
• equilibrium vegetation grasslands in the
  Sahara
• When vegetation is prescribed south of the
  threshold latitude
• changes in the monsoon are relatively small.
• equilibrium vegetation resembles present day
• more work needed with a more complex vegetation
  model
• Interactions between vegetation and the monsoon
  flow are not a mechanism for rapid climate change.

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Mass-weighted vertically-integrated moisture
transport for the AHP_RV simulation in (a) June,
(b) July, (c) August, and (d) September. Vector
scale is 500 kg m-1 s-1.
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• Motivation
• African easterly jet does not form in a
  regional climate model simulation of 6 ka.
• Vegetation feedbacks are very strong in the
  model.

(Patricola and Cook, 2007)
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• Method Idealized regional climate model
  simulations with both static prescribed and
  interactive vegetation distributions are
  analyzed.
• Regional Climate Model (RCM) - Pennsylvania State
  University/ National Center for Atmospheric
  Research Mesoscale Model 5 (version 3.6)
• 90 km horizontal resolution
• 90 second time step
• May 15 - September 30

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desert
grass/shrub
savanna
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desert
grass/shrubs
savanna
evergreen broadleaf forest