Trends and Anomalies in Southern Hemisphere OH Inferred from 12 Years of 14 CO Data

1. Trends and Anomalies in Southern Hemisphere OH Inferred from 12 Years of 14CO Data Martin Manning, Dave Lowe, Rowena Moss, Gordon Brailsford National Institute of Water & Atmospheric Research (NIWA)New Zealand with acknowledgements to: Bill Allan (NIWA) Rodger Sparks, Institute of Geological and Nuclear Sciences, New Zealand Carl Brenninkmeijer, Max Planck Institute fuer Chemie, Mainz, Germany Research supported by the New Zealand Foundation for Research Science and Technology under contract C01X0204.

2. 14CO sources Total source ~ 570 Mole yr-1 Lifetime ~ 10 to 12 weeks Inventory ~ 120 Mole CO sources CO vs 14CO sources Total source ~ 1014 Mole yr-1 Lifetime ~ 8 to 10 weeks Inventory ~ 1.7 x 1013 Mole

3. OH 14CO2 transport OH 14CO2 14CO (13%) recycled Carbon uptake by biosphere and oceans cosmic rayneutrons 14CO cycling in the atmosphere direct production Stratosphere 14CO (52%) Troposphere direct production 14CO (35%) In the extra-tropical southern hemisphere, recycled 14CO accounts for 15 to 20% of the total in the troposphere.

4. Model derived 14CO distributions Tropospheric distributions of 14CO are not very sensitive to details of production distribution pattern…. … but are sensitive to the location and strength of cross-tropopause transport, and… …show large latitudinal gradients in the winter hemisphere But observed gradients in the high latitudes are less than simulated in models! Source: Joeckel, P.; “Cosmogenic 14CO as tracerfor atmospheric chemistry and transport”. PhD Thesis, Mainz, 2000.

5. The role of the variable Sun Electrically charged material ejected from the Sun interacts with the magnetic field around the solar system. During periods of high solar activity a larger proportion of cosmic rays are deflected away from the solar system. Changes in sunspot numbers track the variation in solar activity - but observed neutron fluxes are a more direct indicator of 14C production.

6. Cycles in 14C production Monthly sun spot numbers.NOAA NGDC web site. 14C production rates (molec cm-2 s-1) derived from neutron count rate data. Lowe and Allan (in press).

7. New Zealand14CO measurements Antarctic14CO measurements Storage correction for 14CO production in sample cylindersLowe et al (in press) Inverse model CO sources(Bergamaschi et al) Expected 14C/C ratios for recycled CO Observed CO concentration Subtract recycled 14CO Direct 14CO compare sites 14CO data analysis - I

8. Direct 14CO data - New Zealand and Antarctica Note: Large annual cycle relative to mean concentration. No strong gradient between New Zealand and Antarctica.

9. Direct 14CO data - New Zealand and Antarctica Note: Secular trend following solar cycle of estimated 14C production rates.

10. New Zealand14CO measurements Antarctic14CO measurements Storage correction for 14CO production in sample cylinders Lowe et al (in press) Inverse model CO sources(Bergamaschi et al) Expected 14C/C ratios for recycled CO Observed CO concentration Subtract recycled 14CO merge Scale by production rate(2, 3 or 4 month lag) 14C production rate Normalized direct 14CO data series 14CO data analysis – I (continued)

11. Normalized 14CO data Normalized direct 14CO concentrations have a fairly regular cycle over 12 years (using 3-month lag from production). However, there is some residual inter-annual variability.

12. Smooth and calculate derivatives with error analysis Extratropical southern hemisphere uniformity suggests behavior as a well mixed box Calculate apparent net removal rate R (for constant source) estimate S0 so that mean value for R = 6 yr-1 Where is this Tropospheric production + transport from stratosphere Removal ratek [OH] Determine average annual cycle in R. NB this includes seasonality in OH and cross tropopause transport Hypothesis: Variations in R about climatological average values are most likely due to [OH] variations Analysis of 14CO dynamics(with minimal reliance on models) Normalized direct 14CO data series

13. Derived apparent net removal rates Net removal rate determined with a mean value of 6 yr-1. Varies by a factor of ~3 from winter to summer. Apparent net removal rate R: monthly values (red band) and average seasonal cycle (blue line)

14. Apparent removal rate vs calculated [OH] values Phasing and seasonal amplitude agree closely with [OH] values derived by Spivakovsky et al for southern hemisphere mid latitudes. Lower apparent removal rates in September to December period may reflect higher stratosphere troposphere exchange at this time.

15. Kalimantan fires? Pinatubo eruption? Variations in Effective Removal Rates Monthly ratios of apparent net removal rate to climatological average value. Upper and lower estimates based on data errors (purple lines) plus smoothed values (12 month window). No significant trend in removal rate from 1990, but two “events” with variations of > 20% over time scales of 3 to 6 months.

16. Anomalies in cross-tropopause transport ? Variations in cross-tropopause transport may be related to QBO in the stratosphere or other dynamical effects. Schauffler and Daniel proposed that Pinatubo eruption caused an increase in stratosphere troposphere exchange. However, various indicators suggest that such effects are small. Daily anomalies in potential vorticity at 350 K isentropic surface over 30 to 70oS Daily anomalies in total column ozone at 45oS

17. Summary • Tropospheric 14CO concentrations appear to scale linearly with our estimates of 14C production rate • The lag between production and concentration appears consistent with model estimates of 3 months • The apparent net removal rate derived from the data is very similar in phase and seasonality to that expected for mid latitude OH • There appears to be no significant trend in southern hemisphere OH over the 1989 – 2001 period (agrees with the AGAGE analysis of methyl chloroform data) • However, two “events” show major anomalies in apparent removal rates of > 20% over 3 to 6 month time scales • We propose that the main cause of these variations is change in OH concentrations