Diagnosing stratospheric contribution to climatechange in the CMIP5 models

Alexey Karpechko &Elisa Manzini

22.04.2023 2

Contributions from many people, most importantly:

James Anstey,Seok-Woo Son,Paolo Davini,Giuseppe Zappa,Natalia Calvo,Steven Hardiman,and others…

22.04.2023 3

Stratosphere and climate change

The atmosphere will change in response to GHG emissions

The stratosphere will change too

How will the stratosphere change?

And what are the implications of stratospheric changes for tropospheric climate change?

22.04.2023 4

Arctic polar vortex changes -> ?In the Northern Hemisphere future changes in the Arctic polar vortex remain poorly understood, although a convergence of model results is appearing.

Sigmond et al (2004)

Equatorward shift of the polar night jet Seems to be the most common response to doubling CO2 in stratosphere-resolving models

but …not reproduced by all models Large decadal variability may mask the forced signal (Butchart et al. 2000)

22.04.2023 5

Arctic polar vortex changes

Scaife et al (2011)

• There seems to be a systematic difference between high-top (stratosphere-resolving) and low-top models

• Low-top models typically simulate strengthening of zonal winds throughout the polar stratosphere but…• Not necessarily → a high-top is not needed

to simulate weakening of the polar winds:

ECHAM5, U, 2xCO2, JFM

6

Arctic polar vortex changes: Impacts on the troposphere

Positive vs negative response of the Northern Annular ModeSpread of the results (Shindell et al. 1999; Fyfe et al 1999; Gillett et al. 2003;

Sigmond et al. 2008, 2010; Scaife et al. 2011, Karpechko and Manzini 2012)Likely related to changes in polar stratospheric winds (weaken or strengthen)Future weakening of the polar stratospheric winds drives the NAM towards

negative phase(But there are other factor influencing future NAM changes)

Karpechko and Manzini (2012)

22.04.2023 7

Future Arctic polar vortex changes

Large interannual and decadal variability

Lack of understanding of the mechanisms

Up-down or down-up influence?

Impacts on future surface climate remain unclear

Equatorward shift of the Arctic polar night jet seems to be the most common response to GHG increases in stratosphere-resolving models

22.04.2023 8

What do CMIP5 climate simulations say aboutfuture changes in the Arctic wintertime polar vortex

andits implications for surface climate change?

CMIP5 models

22.04.2023 9

CMIP3/IPCC AR4 models

High top models:the lid is above the stratopauseLow top models:the lid is below the stratopause

based on Cordero and Forster (2007)

Low tops dominate

22.04.2023 10

CMIP5/IPCC AR5 models

from Charlton-Perez et al (2013)

High tops dominate

22.04.2023 1122.04.2023 11

Data24 CMIP5 models10 Low-tops, 13 High-tops, 1 intermediate (1 hPa)Historical and rcp 8.5 simulationsMonthly mean sea level pressure (SLP), zonal mean U and T 42 simulations for SLP and U

7 models with more than 1 simulation24 simulations for TOne simulation per model onlyFocus on DJF mean (unless otherwise mentioned)Difference between 2060-2099 mean and 1961-2000 mean

22.04.2023 12

Multi-model mean change

CMIP5 zonal wind changes~70% models simulate weakening

polar stratospheric winds

Large spread among individual models

Largest spread is around 70°N and 10hPa

Why spread?

What are its implications for surface climate?

Intermodel standard

deviation (σ)

Define stratospheric winds (SUA) index:

(-1∙ΔU)

22.04.2023 1322.04.2023 1313

CMIP5 vs CMIP3 models

1%CO2 experiment in CMIP3&CMIP5 the period 101-140 minus the period 1-40 DJF CMIP5: Zonal winds weaken north 70N CMIP3: Zonal winds strengthen through the stratosphere

22.04.2023 14

CMIP5 zonal temeparture changes

Largest intermodel spreads in:Polar stratosphere (SUA index)Upper tropical troposphere

(tropical warming)Lower high latitude

troposphere (Arctic amplification) Is the stratospheric spread

related to those in the troposphere?

Or is it not?

Multi-model mean change

Intermodel standard

deviation (σ)

22.04.2023 15

Impact of tropical warming on tropospheric dynamics:

90oN60oN30oN30oS EQ

1.

22.04.2023 16

90oN60oN30oN30oS EQ

STRONGER SUBTROPICAL WESTERLY JET

Impact of tropical warming on tropospheric dynamics:

1.

22.04.2023 17

60oN30oN30oS EQ 90oN

2. Impact of polar amplification on tropospheric dynamics:

22.04.2023 18

60oN30oN30oS EQ 90oN

WEAKER WINDS AT HIGH LATS

2. Impact of polar amplification on tropospheric dynamics:

1990oN60oN30oN30oS EQ

3. A stratospheric pathway?

22.04.2023 2090oN60oN30oN30oS EQ

3. A stratospheric pathway?

22.04.2023 2190oN60oN30oN30oS EQ

3. A stratospheric pathway?

22.04.2023 2290oN60oN30oN30oS EQ

3. A stratospheric pathway?

1. Separating a stratosphere-congruent signal

22.04.2023 24

Approach),(),(),(),(),(),( 3322110 pXpaXpaXpapapY jjjjj

Yj (φ,p) – T, U, or SLP change in j-th model between 2060-2099 and 1961-2000X1

j – tropical warming index ( change between 2060-2099 and 1961-2000)

X2j – polar amplification index ( change between 2060-2099 and 1961-2000)

X3j – stratospheric change index ( change between 2060-2099 and 1961-2000)

All indices are normalized and have zero mean and unity standard deviation.

NShPaT

3030300

NNhPaT

9070850

NNhPaU

807010)1(

Index correlation matrix:

116.004.016.0157.004.057.01

333231

232221

131211

rrrrrrrrr

R Indices correlateTropicsArcticSUA

Trop

ics

Arc

tic

S

UA

22.04.2023 25

Approach

),(),(),(

),(),(),(

),(),(),(),(

332

2221

1110

pXpap

pXpap

pXpapapY

jjj

jjj

jjj

A multiple regression is not used because of correlation between indices. A step-by-step regression is applied instead:

X2j – is calculated on residuals after the 1st regression

X3j– is calculated on residuals after the 2nd regression

The stratosphere-congruent part (a3 regression coefficient) does not change between the approaches because the SUA index does not correlate with the other indices.

1.

2.

3.

26

Zonal temperatures

σ(Y)

Multi-model mean change (Y)

σ2(ε1)/σ 2(Y) σ2(ε2)/σ 2(Y) σ2(ε)/σ 2(Y)

Tropical warming (X1)

Regressions

Polar amplification (X2)

SUA index(X3)

27

Zonal winds

Regressions

σ(Y)

Tropical warming (X1)

σ2(ε1)/σ 2(Y) σ2(ε2)/σ 2(Y)

Polar amplification (X2)

SUA index(X3)

σ2(ε)/σ 2(Y)

Multi-model mean change (Y)

22.04.2023 28

Sea level pressure

Regressions

Tropical warming (X1)

σ(Y) σ2(ε1)/σ 2(Y) σ2(ε2)/σ 2(Y)

Polar amplification (X2)

SUA index(X3)

σ2(ε)/σ 2(Y)

Multi-model mean change (Y)

22.04.2023 29

High tops vs low tops

Different climate sensitivity in high-tops and low tops

=> different Arctic SLP responseThe reason is not clearNo evidences that different

climate sensitivity is related to different stratospheric parts

There are evidences that the difference is NOT related to startospheric parts: Similar climate sensitivity in

high/low versions of the same modelfigure by S.-W. Son

22.04.2023 30

2. Internal variability?

22.04.2023 31

Intra- vs intermodel spread

ANOVA test for SUA index:Intermodel variance: 18.2 m2/s2

Mean intramodel variance: 3.6 m2/s2

F-statistics: 5.1

Intermodel spread dominates

The intermodel spread in SUA index is unlikely explainable by internal variability

(-1)∙

22.04.2023 32

3. Up-down or down-up influence?

33

Lagged correlation between changes at different altitudes

NAM changes at the end of winter (February)

Correlated with zonal wind changes at different altitudes and in different months

Remove the tropical warming signal

22.04.2023 34

Lagged correlations

NAM in February correlates better with stratospheric wind changes in previous months, rather than with tropospheric ones

Downward influence

35

Summary (1) ~70% of CMIP5 models simulate weakening of the Arctic stratospheric

winds in winter (equatorward shift of the polar night jet)

(Almost all CMIP3 models simulate strengthening of the stratospheric winds)

Spread of the polar stratospheric wind changes among individual models is large

Multiple realizations available for some models show much smaller spread around their respective means

• This suggests that internal variability unlikely explains the intermodel spread in the Arctic stratospheric wind changes

36

Summary (2) The intermodel spread in stratospheric wind changes is not related to

model climate sensitivity (i.e. tropical warming or polar amplification)

There is a strong coupling between zonal wind changes and sea level pressure changes:• A considerable inter-model spread in sea level pressure change is

associated with the inter-model spread in the Arctic winter stratospheric change

Weakening of the Arctic stratospheric winds corresponds to a shift towards more negative NAM

(But overall future NAM change depends also on climate sensitivity)

Lagged correlations suggest that changes in stratospheric winds influence tropospheric changes (downward influence)