Role of Baroclinic Transport on Seasonal Rectifier Effect

1. 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, Canada 3 National Institute for Environmental Studies, Japan

2. Seasonal Rectifier Effect The annual mean north-south CO2 gradient as a result of the covariance of the transport and the neutral biospheric fluxes. Fig 3 from Gurney et al., (Tellus, 2003, T3L2 Experiment)

3. Rectifier Effect - Impact on CO2 Inversion - In CO2 flux inversion estimate, background CO2 field is an important factor, especially one produced by the biospheric fluxes. Observations Rectifier Effect Biosphere Fig 1 from Denning et al., (Tellus, 1999) Required Sink Large Carbon Sink Required in Northern High and Mid Latitudes to balance the carbon budget with observations

4. Summer Autumn Weak Cumulus Convection StrongConvection High CO2Concentration Low CO2Concentration Deep PBLMixing Shallow PBLMixing Decomposition Photosynthesis A Rectifier Mechanism • One Suggested Mechanism: • Seasonal covariation of PBL and NEP fluxes (Denning). Dilution of photosynthesis signal through deep mixing Transport of low-CO2 air into upper troposphere Accumulation of respiration signal near the surface Elevated CO2 in lower troposphere Annual mean: Accumulation of CO2 near the ground (From S. Denning)

5. Another Rectifier Mechanism?observational evidence Continuous CO2 measurements at Alert (82ºN) • with less difference between Fraserdale(50ºN) and Alert in winter than in summer • with more synoptic variability in winter than in summer Mean deviations of obs to fitted curves

6. 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?

7. Another Rectifier MechanismWhy Baroclinic Process (Large Scale Eddies)? Horizontal Temperature Gradient : T Relationship between LSE and T Eq From 2-D diffusive transport by Stone (1974) NP Annual : T=~35 Wnter : T=~50 Summer : T=~25 LSE is a function of T Large Seasonal Variation of T Seasonal Variation of LSE

8. Seasonality of Meridional Transport Winter (DJF) Simulated Fossil Fuel-CO2 with constant PBL Annual mean Summer (JJA) Vertical Cross-Section Summer (JJA) Winter (DJF) Weaker transport results in stronger CO2 gradient from mid-latitude to the pole More CO2 air transported to Northern High Latitudes

9. Another Rectifier MechanismBaroclinic Process and NEP in Northern Hemisphere 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) 30°C 30°C T=~25 T=~50 5°C EQ NP EQ NP High CO2 Low CO2 -20°C NET RESULT: In winter, higher CO2 concentration in polar region, hence ‘RECTIFIER EFFECT’

10. Objectives • To study the effect horizontal transport has on the CO2 distribution • To study the effect PBL variation has on the CO2 distribution Atmosphere Vertical Mixing CO2 Horizontal Transport Biosphere CO2 flux Locally Interaction between atmospheric transport/mixing and biospheric CO2 flux Globally North-South atmospheric CO2gradient

11. Method • 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

12. Test Proposed Mechanisms • 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

13. Reference Rectifier NIES model with Biome-BGC has mean gradient of ~3-4 ppm

14. Case 1: Constant PBL Annual mean PBL

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

16. Case 1: Constant PBL Covariation of Meridional Transport and CO2 fluxes Zonal Mean CO2 Differential CO2 transport between Summer and Winter 700mb S Transport N-S Gradient Summer Weak/Slow Small W Winter Strong/Fast Large S In annual mean, positive CO2 anomaly in N-S Gradient W Surface S W S Zonal Mean Biospheric Fluxes W

17. Case 1: Constant PBL Faster Northward Transport in Winter Summer Winter Tsummer< Twinter 700mb vcsummer < vcwinter Surface NP Mid Mid NP Low CO2 High CO2 In Winter, High CO2 is transported to polar region from mid latitudes along isentropic surface faster than in summer EQ Mid NP Zonal-mean cross section of equivalent potential temperature

18. Case 2: PBL Shifted • Shift the PBL seasonality by 6 months • Seasonal covariation of PBL and NEP. PBL effect is locally strong. Away from Lands, PBL effect is not so significant. Reduction of rectifier effect (Negative amplification), but still North-South CO2 gradient (high CO2 in North)

19. 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 - Opposite North-South CO2 Gradient, but weaker - Strong winter North-South transport with low CO2 signal resulting in negative Rectifier Effect

20. Summary 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. • 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). Meridional Transport Vertical Mixing CO2 flux Rectifier Effect North-SouthCO2 Gradient • 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).

21. Impact on CO2 inversion • 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.