Analysis of a 39-year continuous atmospheric CO<sub>2</sub> record from Baring Head, New Zealand

Abstract

Abstract. We present an analysis of a 39-year record of continuous atmospheric CO2 observations made at Baring Head, New Zealand, filtered for steady background 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 episodes 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.

Sea–air CO<sub>2</sub> fluxes in the Southern Ocean for the period 1990–2009

“…This enhanced uptake in ocean biogeochemical models is consistent with the expected uptake of −0.05 Pg C yr −1 decade −1 of the Southern Ocean sink (Le Quéré et al, 2007), due to the increasing atmospheric CO 2 concentration. Conversely, the inversions are more consistent with a reduction in the strength of the Southern Ocean sink reported by Le Quéré et al (2007), but the range is 4050 A. Lenton et al: Sea-air CO 2 fluxes in the Southern Ocean large across inversions (−0.06 to 0.08 Pg C yr −1 decade −1 ) and previous work has shown that the inversion trends are likely quite sensitive to atmospheric CO 2 data quality, with atmospheric CO 2 gradients being close to measurement uncertainty (Stephens et al, 2013). This suggests that linear trends in model output over periods less than 20 yr are unlikely to provide a statistically meaningful statement about the very small changing rate of Southern Ocean CO 2 uptake.…”

“…Law et al, 2008). Analyses of long-running atmospheric CO 2 time series in the Southern Hemisphere and model simulations suggest that it may not be possible to robustly detect a slowdown in the Southern Ocean sink 4040 A. Lenton et al: Sea-air CO 2 fluxes in the Southern Ocean from atmospheric CO 2 measurements at present (Stephens et al, 2013). Furthermore, ocean observations also suggest the changes in the sink efficiency may not be zonally uniform for the Southern Ocean .…”

Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

“…A much larger fraction of the variability in CO 2 is caused by exchanges with the land biosphere; this is true even in regions remote from direct land biospheric influences, such as the high latitudes of the Southern Hemisphere (e.g., Stephens et al, 2013). An atmospheric constraint on oceanic processes can be derived by combining measurements of atmospheric CO 2 and O 2 / N 2 ratios to compute atmospheric potential oxygen (APO).…”

Adaptive Baseline Finder, a statistical data selection strategy to identify atmospheric CO<sub>2</sub> baseline levels and its application to European elevated mountain stations

“…The first method named SI is based on "steady intervals" (Lowe et al, 1979;Stephens et al, 2013). Steady intervals which are considered as baseline conditions are defined by a standard deviation being lower or equal than 0.3 ppm for 6 or more consecutive hours.…”