Analysis of a 39-yr continuous atmospheric CO₂ record from Baring Head, New Zealand

We present an analysis of a 39-yr record of continuous atmospheric CO₂ observations made at Baring Head, New Zealand, filtered for steady CO₂ mole fractions during southerly wind conditions. We discuss relationships between variability in the filtered CO₂ time series and regional to global carbon cycling. Baring Head is well situated to sample air that has been isolated from terrestrial influences over the Southern Ocean, and experiences extended periods of strong southerly winds with low CO₂ variability. The filtered Baring Head CO₂ record reveals an average seasonal cycle with amplitude of 0.95 ppm that is 13% smaller and 3 weeks earlier in phase than that at the South Pole. Seasonal variations in a given year are sensitive to the timing and magnitude of the combined influences of Southern Ocean CO₂ fluxes and terrestrial fluxes from both hemispheres. The amplitude of the seasonal cycle varies throughout the record, but we find no significant long-term seasonal changes with respect to the South Pole. Interannual variations in CO₂ growth rate in the Baring Head record closely match the El Niño/Southern Oscillation, reflecting the global reach of CO₂ mole fraction anomalies associated with this cycle. We use atmospheric transport model results to investigate contributions to seasonal and annual-mean components of the observed CO₂ record. Long-term trends in mean gradients between Baring Head and other stations are predominately due to increases in Northern-Hemisphere fossil-fuel burning and Southern Ocean CO₂ uptake, for which there remains a wide range of future estimates. We find that the postulated recent reduction in the efficiency of Southern Ocean anthropogenic CO₂ uptake as a result of increased zonal winds is too small to be detectable as significant differences in atmospheric CO₂ between mid- to high-latitude Southern Hemisphere observing stations.