Causes of Reduced North Atlantic
Storminess During the Last Glacial
Maximum from CCSM3
Aaron Donohoe
UW COGS talk
April 12, 2007
Why Study North Atlantic Storms
During the LGM?
d18O Temperature Proxy from GISP 2, Greenland Summit
12 C
TIME
(Stuiver andWarmer
Grooves, 2000)
(Stuiver and Grooves, 2001)
The North Atlantic climate system underwent
repeated instances of abrupt climate change
What do we expect the mid-latitude
glacial atmosphere to be like?
More Ice
Stronger Equator to pole
Temperature Gradient
More potential energy in the midlatitudes
(stronger jets and more baroclinicity)
Stronger Storms
First Generations of Climate Models did have
stronger mid-latitude storms in the LGM
Eddy Activity in CCSM3
Baroclinicity
Eddy Activity
Modern
LGM
LGM – Modern
Eddy Activity
Overview
• Sensitivity of LGM storms and jets to GCM
boundary conditions (review of work by
Camille Li)
• A closer look at North Atlantic eddy
structures, statistics and energetics
• A linear model of eddy growth in the North
Atlantic
Section 1: When does CCSM/CAM3 give
a strong LGM jet with weak eddies?
Cool Colors = LGM
Warm Colors = Modern
Atmosphere only with ICE 4g
And Climap SSTs
Fully Coupled with ICE 5g
Generates its own SSTs
Atmosphere only with ICE 5g
SSTs from coupled run
Courtesy of Camille Li
Land Ice Topography
Courtesy of Camille Li
The Land Ice fixes the jet
•Top panel is ICE 4g
runs with different
SSTs/ sea ice
•The Bottom panel is
the same runs with
ICE 5g
•Ice 5g = strong jet
and narrow jet
Courtesy of Camille Li
Reduced Storms Require ICE 5g
and appropriate SST/sea ice
Present Day
ICE 5G and SSTs from Coupled Run
ICE 5G and Climap SSTs/Ice
ICE 4G SSTs from Coupled run
Neither Land Ice or SSTs/Sea Ice Alone
Gives weak Storms
Summary Thus Far
• CCSM 3 gives a picture of the Atlantic
Circulation with a strong, narrow jet and
weak eddies
• The Atmosphere only component of the
model reproduces a strong, narrow jet if
ICE 5G Land ice is used
• Weak storms are produced in the
atmosphere only model only if both ICE
5G and SSTs/Sea Ice from the coupled
run are used
Section II: North Atlantic Eddy
Statistics
• LGM statistics are a composite of 25 years of
uncoupled CAM3 L26 T42 with ice 5g and
Climap SSTs
• Control run is a composite of 25 years of
uncoupled CAM3 L26 T42 with observed SSTs
from 1950 to 1975 (Ensemble Run 5 of NCAR)
• Records are decomposed into eddy and mean
state components with a double pass
Butterworth filter with cutoff period of 10 days
• Fields shown are JFM (winter)
Atlantic Sector Definition
Vertically Integrated Spatial Map of
JFM Meridional Eddy Heat Transport
LGM
Modern
K m/s
LGM- Modern
Contours are the jet speed at 400 hPa
Storm Heat
Transport
Temperature
Gradient
3.0
3
Vertical
Average
V’T’ 2
(k*m/s)
dT/dy
(K/m)
X 10^-6
1
Nov
LGM
MODERN
LGM
MODERN
1.5
Jan
Mar
Day of year
Nov
Jan
Mar
Day of year
Conclusion: In the modern, the storms transport more heat
when the temperature gradient is largest… In the LGM,
something inhibits storm growth in the middle of winter
Viewed Another Way: Baroclinicity
Doesn’t Determine Eddy Heat Transport
4
LGM
MODERN
Vertical 3
Average
V’T’
2
(k*m/s)
1
1x10^-6
2x10^-6
3x10^-6
Meridional Temperature Gradient (k/m)
Jet Core Cross Sections
Contours = Zonal
Velocity
10 m/s intervals
Colors = Temperature
Traditional Eddy Energy Cycle
Plumb (1983)
Mean
Kinetic
Eddy
Kinetic
Mean
Potential
Eddy
Potential
To Simplify:
Neglect Sources
And Energy Fluxes
Decrease Eddy Kinetic by:
Less Baroclinic Conv. OR more barotropic conv.
Hypothesis: LGM eddies are
weaker because the strong narrow
jet causes more barotropic decay
• We can test this hypothesis by looking at the
eddy energy budget
Eddy Energy Budget
LGM
-120
-60
0
60
Conclusion: Changes in baroclinic, not barotropic
Conv. account for weak LGM Eddies
In other terms: storms are big
when they grow baroclinically
4
Vertical 3
Average
V’T’
(k*m/s) 2
LGM
Modern
1
1
2
3
4
Baroclinic Conversion (m^2/3^3) X 10^-4
Eddy Structures: 1 point regression maps
Regression point is the black dot at 700 hPa
.
LGM
Modern
Colors =
850 hPa
Heights (m)
.
Vertical Tilt for
Contours =
550 hPa
Heights (m)
LGM = 50 longitude/450 hPa
Modern = 70 longitude/450 hPa
Static Stability at 750 hPa
LGM
Brunt-Vaisala
Frequency
(1/s)
MODERN
LGM-MOD
Conclusion: LGM has much larger static stability
North of Jet
Summary
• Eddies are suppressed from what we
would ‘expect’ based on baroclinicity in the
mid- winter
• Eddy momentum fluxes into the narrow jet
can NOT explain the suprression
• Despite the enhanced baroclinicity, LGM
eddies do not exhibit stronger baroclinic
growth
• There are differences in the vertical eddy
structure between LGM and modern
Section III: Linear Stability Analysis
• Simple analogy: How fast will a ball resting on
top of a hill move away from it resting position
when perturbed
Strategy: Take mean states from the
wintertime GCM WINTER climatology
and access their linear stability
How do Storms Grow?
Perturbations (Storms) extract energy from the
mean state via two different mechanisms
Meridional Temperature Gradient
(Baroclinic)
Velocity Shear
(Barotropic)
High Energy Isotherms
High Energy
Hot
Cold
Low Energy
Not too hot
Not too cold
Low Energy
Wind Vectors
1D Baroclinic GrowthEady Growth Rate =
LGM
Contours =
450 hPA
Zonal Wind
MODERN
LGM - MOD
Eady
Growth
Rate
(1/day)
Atlantic Jet 1D Barotropic Normal Modes
Zonal Velocity Profiles at Jet Maximum
Modern-E folds 14 days
LGM - E folds 3 days
Upper Level Basic States – 250 hPa
LGM Most Unstable Mode
Quadrature phases: Oscillate between solutions with a period of 24 days
E folds in 8 days
Modern Most Unstable Mode
Stationary Mode
E folds in 10 days
Stability Summary Thus Far
• The Glacial is more unstable baroclinically
There is a stronger temperature gradient
• The Glacial is more unstable barotropically
The jets are narrower
• Does this mean that the glacial mean state
is more unstable?
Not necessarily, there are good dynamical
reasons to think that a narrow jet will
inhibit baroclinic growth
Explore the stability of the mean state using a
linear 2 layer beta channel quasigeostrophic model
Level 1- Tropopause- no vertical motion
Level 2 – 450 hPa
Barotropic Vorticity Equation
Layers 2 and 4
‘communicate’
Through vortex stretching
Use spatial average Static Stability
From GCM
Level 3- Thermodynamic Equation
Level 4 – 900 hPa
Barotropic Vorticity Equation
Level 5- Ground- no vertical motion
Linearized about the DJF climatology from CCSM3
Jet Core Cross Sections
Contours = Zonal
Velocity
10 m/s intervals
Colors = Temperature
LGM Zonally Invariant Stability
Atlantic Jet Core Cross Section
Spatial Structure
Contours = geopotential height
Colors = height tendency
Meridional
Location (m)
Zonal Location (m)
Temporal Growth of
Most unstable mode
Ln
(Storm
Magnitude)
Time (seconds)
The Optimal storm structure DOUBLES IN MAGNITUDE EVERY 1.4 DAYS
Modern Zonally Invariant Stability
Atlantic Jet Core Cross Section
Spatial Structure
Contours = geopotential height
Colors = height tendency
Meridional
Location (m)
Zonal Location (m)
Temporal Growth of
Most unstable mode
Ln
(Storm
Magnitude)
Time (seconds)
The Optimal storm structure DOUBLES IN MAGNITUDE EVERY 2.2 DAYS
Compared to 1.4 days for the LGM, the Glacial is more unstable
Stability Summary Again
• Height-Latitude cross section stability of jet
core predicts Glacial storms should grow
much more rapidly than the modern
• Other cross section (i.e. max barotropic
shear) give similar results (not shown)
• Does the story change for the 3d mean
state?
3D Linear Stability
1.) Define a domain over which the thermal wind
between 900 hPa and 450 hPa exceeds a
threshold
LGM
MODERN
Thermal wind between 900 hPa and 450 hPa (m/s)
3D Linear Stability - cont.
2.) Smoothly transition from the jet in the domain to the zonal mean jet
3.) Make the domain periodic- damp the storm growth outside the
‘Atlantic’
LGM
Zonal Velocity (m/s)
Zonal Velocity (m/s)
MODERN
Zonal Velocity (m/s)
Zonal Velocity (m/s)
LGM
LGM – Storm Growth
MODERN
LGM and Control– Storm Growth
Stability Summary once again
• 3D mean states predict the glacial storms
double in magnitude every 2.2 days versus 2.8 days for the
modern
• Seems like we struck out… BUT remember the static
stability
LGM with Spatially Variant Static Stability
LGM- LGM with Static Stability and
Control Storm Growth
Conclusion: Spatial Pattern of Static Stability has a
profound affect on LGM storms, hardly affects the
modern (not shown)
Conclusions
• General Circulations Models predict a
strong LGM jet with weak storms if ICE5G
and appropriate SSTs/ Sea Ice are used
• LGM storms are depressed in the middle
of the winter because their structure
doesn’t allow them to efficiently grow
baroclinically
• A three dimensional linear model of storm
growth with the LGM spatial structure of
static stability begins to explain the
reduced eddy activity despite enhanced
baroclinicity
Thanks To:
•
•
•
•
•
David Battisti
Camille Li, Jeff Yin
Gerard Roe
Joe Barsugli, Ceci Bitz
Friends, Family,
Neighbors
• NSF, ARCS Foundation,
UW PCC, Comer
Foundation, Department
of Atmospheric Sciences
Thanks To:
•
•
•
•
•
•
David Battisti
Camille Li, Jeff Yin
Gerard Roe
Joe Barsugli, Ceci Bitz
Friends, Family
NSF, ARCS Foundation,
UW PCC, Comer
Foundation, Department
of Atmospheric Sciences
How do you change Greenland
temperature by 12 C in a decade ?
Most likely reflects a change in heat transport
Heat Transport by the Climate System
The atmosphere does the lion share of the heat transport
It probably makes sense to understand how it works in the Glacial
There are concurrent changes in
other parts of the North Atlantic
warmer
Time
(Sachs and Lehman, 1999)
• SST at Bermuda Rise
deduced from alkenones
(red)
• Correlates with
Greenland record (blue)
with one third the
magnitude
Direct evidence of reorganization of the
atmospheric circulation during DO events
Stadial (Cold)
Data Points
Interstadial
(Warm) Data
Atmospheric
Dust Load
Temperature
[Fuhrer,1999]
Temperature (oxygen isotopes) overlap between regimes but calcium does not
•CLIMATE REGIME IS BEST CHARACTERIZED BY ATMOSPHERC
DUST LOAD – FACTOR OF 10 change
Zonally Asymmetric Barotropic
Vorticity Equation

   (U ,V ) a  F
t
Basic State with Zonal and Meriodional Winds
(U ,V )  (U ( , ), V ( , ))
Linearized Equation Becomes

t
'
'
d'
d
d
d

 U
V
u'
v'
 '   (U ,V )
ad
a cos( )d
ad
a cos( )d
The assumed forcing maintains the basic state
F    (U ,V ) a
And we assume a spherical harmonic basis
N
mn
P
n  0 m  n
m
n
im
(sin  )e
Eddy Energy Budgets
Orlanski, Katzfey (1991)
Barotropic Conversion
Baroclinic Conversion
Composite of 13 abrupt climate
change events (DO events)
12 C
Composite annual mean warming of 12 C
Half point of transition reached in 2 years
Jet Speed and Width
Strong
Narrow
LGM Jet is Fast and Narrow
LGM
LGM with spatially variant static stability
LGM with Static Stability
MODERN