Role of Baroclinic Transport on Seasonal Rectifier Effect

Misa Ishizawa1,2, Douglas Chan1, Kaz Higuchi1, Shamil Maksyutov3 and Jing Chen2

1 Atmospheric Science and Technology Directorate, Environment Canada 2 University of Toronto, Canada3 National Institute for Environmental Studies, Japan

Fig 3 from Gurney et al., (Tellus, 2003, T3L2 Experiment)

The annual mean north-south CO2 gradient as a result of the covariance of the transport and the neutral biospheric fluxes.

Seasonal Rectifier Effect

In CO2 flux inversion estimate, background CO2 field is an important factor, especially one produced by the biospheric fluxes.

Rectifier Effect - Impact on CO2 Inversion -

Large Carbon Sink Required in Northern High and Mid Latitudesto balance the carbon budget with observations

Fig 1 from Denning et al., (Tellus, 1999)

Rectifier EffectBiosphere

Observations

Required Sink

Annual mean: Accumulation of CO2 near the ground

Dilution of photosynthesis signal through deep mixing

Transport of low-CO2 air into upper troposphere

Summer

Deep PBLMixing

Low CO2

Concentration

Photosynthesis

StrongConvection

Accumulation of respiration signal near the surface

Elevated CO2 in lower troposphere

Autumn

Shallow PBLMixing

High CO2

Concentration

Decomposition

Weak Cumulus Convection

One Suggested Mechanism: Seasonal covariation of PBL and NEP fluxes (Denning).

(From S. Denning)

A Rectifier Mechanism

• with less difference between Fraserdale(50ºN) and Alert in winter than in summer

• with more synoptic variability in winter than in summer

Another Rectifier Mechanism?observational evidence

Continuous CO2 measurements

at Alert (82ºN) 400

390

380

370

360

350

340

200520042003200220012000

Alert Fraserdale

2.5

2.0

1.5

1.0

0.5

0.0

CO

2 fo

r A

lert

[pp

m]

5

4

3

2

1

0

CO

2 for Fraserdale [ppm

]

J F M A M J J A S O N D

Alert Fraserdale

Mean deviations of obs to fitted curves

Another Rectifier Mechanism?

• Motivation: – In the extra-tropics, north-south sensible heat and

humidity transports are dominated by the baroclinic meridional circulations.

– Meridional transport by LSE (Large Scale Eddies or Synoptic Systems) ~dT/dy. dT/dy has seasonal variations.

• Potential Mechanism of Rectifier Effect: – Seasonal variation of baroclinic meridional transport

has covariance with NEP fluxes?

Summer : T=~25

Wnter : T=~50

Annual : T=~35

Horizontal Temperature Gradient : T

Eq NP

Relationship between LSE and TFrom 2-D diffusive transport by Stone (1974)

LSE is a function of T

Large Seasonal Variation of T Seasonal Variation of LSE

Another Rectifier MechanismWhy Baroclinic Process (Large Scale Eddies)?

Vertical Cross-SectionWinter (DJF)Summer (JJA)

More CO2 air transported to Northern High Latitudes

Weaker transport results in stronger CO2 gradient from mid-latitude to the pole

Winter (DJF)

Annual mean

Summer (JJA)

Simulated Fossil Fuel-CO2 with constant PBL

Seasonality of Meridional Transport7

6

5

4

3

2

1

0

CO

2 [p

pn]

-50 0 50

500

400

300

200

100

0

FF

Em

ission [TgC

/grid/yr]

Summer• Equator-Pole T~(30-5)=25• Weak Meridional Transport (of

photosynthesis signals to the polar region)

Winter• Equator-Pole T~(30-(-20))=50• Strong Meridional Transport (of

respiration signals to the polar region)

NET RESULT: In winter, higher CO2 concentration in polar region, hence ‘RECTIFIER EFFECT’

NPEQNPEQ

5°C

30°C 30°C

-20°C

T=~25 T=~50

Low CO2High CO2

Another Rectifier Mechanism Baroclinic Process and NEP in Northern Hemisphere

• To study the effect horizontal transport has on the CO2 distribution

• To study the effect PBL variation has on the CO2 distribution

LocallyInteraction between atmospheric transport/mixing and biospheric CO2 flux

GloballyNorth-South atmospheric CO2 gradient

Biosphere

Atmosphere

CO2

Horizontal Transport

Vertical Mixing

CO2 flux

Objectives

• Transport Model– NIES Transport Model – NCEP Reanalysis– PBL monthly

• Surface Fluxes– Biospheric Flux (annually balanced monthly mean NEP

from Biome-BGC)

• Model Integration– 10 years’ (1990-1999) integration: – First two years for spinup, – Averaged over year 3 to year 10

Method

• Reference

• Case 1 : Constant PBL No Rectifier ?– Fix the PBL annually

• Case 2 : PBL Shifted Negative Rectifier ?– Shift the PBL seasonality by 6 months

• Case 3 : Constant PBL & CO2 Flux Shifted ??– Fix the PBL annually– Shift the CO2 biospheric fluxes by 6 months

Test Proposed Mechanisms

NIES model with Biome-BGC has mean gradient of ~3-4 ppm

Reference Rectifier

4

3

2

1

0

CO

2 [p

pm]

-90 -60 -30 0 30 60 90

Latitude [degree]

Annual mean PBL

Case 1: Constant PBL

• Fix the PBL height annually, no seasonal variation.• PBL varies spatially but not temporally.• NEP normal.• No PBL-NEP covariation• Covariation of the meridional transport and NEP

Seasonal variation of PBL has ~30% effect on the north-south CO2 gradient.

Case 1: Constant PBL

4

3

2

1

0C

O2

[ppm

]

-90 -60 -30 0 30 60 90Latitude

Case1 Reference

W

S

W

S

W

S

W

S

700mb

Surface

Zonal Mean CO2

Zonal Mean Biospheric Fluxes

Summer

Winter

Transport N-S Gradient

Weak/Slow Small

Strong/Fast Large

In annual mean, positive CO2 anomaly in N-S Gradient

Differential CO2 transport between Summer and Winter

Covariation of Meridional Transport and CO2 fluxes

Case 1: Constant PBL

Mid NP Mid

700mb

Low CO2 High CO2

NPSurface

Summer Winter Tsummer< Twinter

vcsummer < vcwinter

In Winter, High CO2 is transported to polar region from mid latitudes along isentropic surface faster than in summer

Case 1: Constant PBL Faster Northward Transport in Winter

EQ NPMidZonal-mean cross section of equivalent potential temperature

• Shift the PBL seasonality by 6 months• Seasonal covariation of PBL and

NEP.

Reduction of rectifier effect (Negative amplification), but still North-South CO2 gradient (high CO2 in North)

PBL effect is locally strong.

Away from Lands, PBL effect is not so significant.

Case 2: PBL Shifted

4

3

2

1

0

-90 -60 -30 0 30 60 90

Case 2 (180E) Reference

4

3

2

1

0

-90 -60 -30 0 30 60 90

Case 2 (zonal mean) Reference

- Opposite North-South CO2 Gradient, but weaker

- Strong winter North-South transport with low CO2 signal resulting in negative Rectifier Effect

Case 3:Constant PBL& CO2 Flux Shifted• Fix the PBL height annually, no seasonal variation.• Shift the NEP seasonality by 6 months.• No PBL-NEP covariation• Seasonal Covariation of the meridional transport and NEP

4

3

2

1

0

-1C

O2

[ppm

]

-90 -60 -30 0 30 60 90Latitude

Case 3 Reference

• Seasonal variation of PBL acts as an amplification mechanism, it has ~30% effect on the north-south CO2 gradient.

• The seasonal covariation of the meridional transport and CO2 flux appears to be the dominant factor in the annual mean north-south CO2 gradient (seasonal rectifier effect).

The effects of seasonal PBL variation (or vertical mixing) and seasonal variation of meridional transport on the mean annual north-south CO2 gradient were examined.

Summary

• Seasonal variation of meridional atmospheric transport can be understood in terms of the seasonal variation of the meridional temperature gradient (baroclinic forcing). The dominant baroclinic atmospheric processes are cyclones and anticyclones (synoptic processes).

North-South CO2 Gradient

CO2 fluxMeridional Transport

Vertical Mixing

Rectifier Effect

• To improve the simulation of the rectifier effect for CO2 inversion, the results of this study suggest improving the simulation of synoptic scale transport processes in transport models.

• The results also showed that PBL and CO2 covariations have stronger effects near the source regions, this is an important factor if continental sites are used in the CO2 inversion. However, synoptic processes are also important factors for the atmospheric PBL dynamics (PBL tends to be higher in the cyclonic low pressure area than under the stronger subsidence in the anticyclonic high pressure area, synoptic variations are as large as seasonal variations).

• Synoptic processes also affect the CO2 fluxes through temperature, humidity, precipitation and radiation; hence the CO2 flux and atmospheric transport covariation.

• These indicate the importance of the proper simulation of the synoptic coupling between the atmosphere and biosphere in CO2 inversion.

Impact on CO2 inversion