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?

(Stuiver and Grooves, 2000)

(Stuiver and Grooves, 2001)TIME

Warmer

The North Atlantic climate system underwent repeated instances of abrupt climate change

18O Temperature Proxy from GISP 2, Greenland Summit

12 C

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 StormsFirst Generations of Climate Models did have

stronger mid-latitude storms in the LGM

Eddy Activity in CCSM3Baroclinicity

Eddy ActivityModern 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 = LGMWarm Colors = Modern

Atmosphere only with ICE 4gAnd Climap SSTs

Fully Coupled with ICE 5gGenerates its own SSTs

Atmosphere only with ICE 5gSSTs 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 5gand 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 AloneGives 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

Contours are the jet speed at 400 hPa

K m/s

LGM

Modern

LGM- Modern

Storm Heat Transport

TemperatureGradient

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

Day of year Day of yearJan JanMar MarNov Nov

VerticalAverage

V’T’(k*m/s)

3

2

1

dT/dy(K/m)

X 10^-6

1.5

3.0

LGMMODERN

LGMMODERN

Viewed Another Way: Baroclinicity Doesn’t Determine Eddy Heat Transport

Meridional Temperature Gradient (k/m)1x10^-6 2x10^-6 3x10^-6

VerticalAverage

V’T’(k*m/s)

4

3

2

1

LGMMODERN

Jet Core Cross Sections

Contours = Zonal Velocity

10 m/s intervals

Colors = Temperature

Traditional Eddy Energy Cycle

MeanKinetic

Mean Potential

Eddy Potential

EddyKinetic

To Simplify:Neglect SourcesAnd Energy Fluxes

Decrease Eddy Kinetic by: Less Baroclinic Conv. OR more barotropic conv.

Plumb (1983)

Hypothesis: LGM eddies areweaker because the strong narrow jet causes more barotropic decay

• We can test this hypothesis by looking at the eddy energy budget

Eddy Energy Budget

Conclusion: Changes in baroclinic, not barotropic Conv. account for weak LGM Eddies

LGM

0 60-60-120

In other terms: storms are bigwhen they grow baroclinically

Baroclinic Conversion (m^2/3^3) X 10^-4

VerticalAverage

V’T’(k*m/s)

4

3

2

1

1 3 42

LGM Modern

Eddy Structures: 1 point regression mapsRegression point is the black dot at 700 hPa

LGM

Modern

.

.Colors =850 hPa

Heights (m)

Contours =550 hPa

Heights (m)

Vertical Tilt for LGM = 50 longitude/450 hPa Modern = 70 longitude/450 hPa

Static Stability at 750 hPa

Brunt-Vaisala Frequency

(1/s)

LGM

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

Low Energy

High Energy

Low Energy

Wind VectorsIsotherms

Hot

Cold

Not too hot

Not too cold

1D Baroclinic Growth- Eady Growth Rate =

LGM

MODERN

LGM - MOD

EadyGrowth

Rate(1/day)

Contours =450 hPA

Zonal Wind

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 ModeQuadrature phases: Oscillate between solutions with a period of 24 daysE folds in 8 days

Modern Most Unstable ModeStationary 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

Linearized about the DJF climatology from CCSM3

Level 1- Tropopause- no vertical motion

Level 2 – 450 hPaBarotropic Vorticity Equation

Level 4 – 900 hPaBarotropic Vorticity Equation

Level 3- Thermodynamic Equation

Level 5- Ground- no vertical motion

Layers 2 and 4 ‘communicate’

Through vortex stretching

Use spatial average Static StabilityFrom GCM

Jet Core Cross Sections

Contours = Zonal Velocity

10 m/s intervals

Colors = Temperature

LGM Zonally Invariant StabilityAtlantic Jet Core Cross Section

Ln(Storm

Magnitude)

Time (seconds)Zonal Location (m)

MeridionalLocation (m)

Spatial StructureContours = geopotential height

Colors = height tendency

Temporal Growth of Most unstable mode

The Optimal storm structure DOUBLES IN MAGNITUDE EVERY 1.4 DAYS

Modern Zonally Invariant StabilityAtlantic Jet Core Cross Section

Ln(Storm

Magnitude)

Time (seconds)Zonal Location (m)

MeridionalLocation (m)

Spatial StructureContours = geopotential height

Colors = height tendency

Temporal Growth of Most unstable mode

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 Stability1.) 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 MODERN

Zonal Velocity (m/s)

Zonal Velocity (m/s)

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 transportIt probably makes sense to understand how it works in the Glacial

There are concurrent changes in other parts of the North Atlantic

• SST at Bermuda Rise

deduced from alkenones

(red)

• Correlates with Greenland record (blue) with one third the magnitude

(Sachs and Lehman, 1999)

warmer

Time

Stadial (Cold)Data Points

Interstadial (Warm) Data

Temperature (oxygen isotopes) overlap between regimes but calcium does not

•CLIMATE REGIME IS BEST CHARACTERIZED BY ATMOSPHERC

DUST LOAD – FACTOR OF 10 change

[Fuhrer,1999]

Direct evidence of reorganization of the atmospheric circulation during DO events

Atmospheric Dust Load

Temperature

Zonally Asymmetric Barotropic Vorticity Equation

( , ) ( ( , ), ( , ))U V U V

Basic State with Zonal and Meriodional Winds

Linearized Equation Becomes

The assumed forcing maintains the basic state

( , ) aF U V And we assume a spherical harmonic basis

0

(sin )N m n

mn

n m n

imP e

( , ) aU V Ft

'

'''

' ' ( , )cos( )cos( )

d d ddU V u v U Vt ad a d ada d

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 CHalf point of transition reached in 2 years

Jet Speed and Width

LGM Jet is Fast and Narrow

Narrow

Strong

LGM

LGM with Static StabilityLGM with spatially variant static stability

MODERN