Evidence for Oceanic Control of Interannual Carbon Cycle Feedbacks

Large-scale carbon-cycle feedbacks within Earth's climate system can be inferred from the statistical correlation of atmospheric CO2 and other climate observations. These statistical relationships can serve as validation targets for global carbon-cycle models. Fourier-transform coherence between atmospheric CO2 measured at Mauna Loa, Hawaii, and Hadley Centre global-average temperatures changed in the late 20th century at interannual frequencies, from a 6-month time lag to a 90° phase lag that scaled CO2 fluctuations to a time-integral of the global-average temperature anomaly. Wavelet coherence estimates argue that this change occurred with a recognized ocean-circulation climate transition during the late 1970s. General features of these CO2-temperature correlations are confirmed using global-average temperature from other sources and atmospheric CO2 measured at other locations, though only the Mauna Loa CO2 record is long enough to resolve well the coherence properties before the 1970s transition. The CO2-coherence phase for the global-average surface-air temperature time series from NASA-GISS and the lower-troposphere temperature series from the MSU satellite is more complex than for the Hadley-Centre dataset, the only estimate that incorporates sea-surface temperature (SST) observations. Near f = 0.25 cyc/year, 4-year oscillation period, the CO2-coherence is particularly strong for the Hadley-Centre gridpoint temperature-anomaly time series from low-latitude oceans. This suggests that sea-surface temperature is a primary driver of the correlation, at least for the 0.2 < f < 0.5 cyc/yr bandpass where the El-Nino/Southern-Oscillation (ENSO) climate process dominates. Outside the ENSO bandpass coherence is significant between 14 long-running GLOBALVIEW CO2-observing sites and the sea-level-pressure-based Southern Oscillation Index (SOI) and North Atlantic Oscillation (NAO) time series, consistent with wind stress and mixed-layer-thickness influences on ocean-atmosphere CO2 flux, independent of temperature fluctuations. Evidence for terrestrial biosphere influence is strongest in the leading principal component of GLOBALVIEW CO2-variability at f = 0.25 cpy, where a larger amplitude and a 4-month phase shift distinguish the mid- and high-latitude Northern Hemisphere CO2 fluctuations from those of the tropics and the Southern Hemisphere. The terrestrial signal we infer, however, coheres more strongly with oceanic-gridpoint temperatures than to continental-gridpoint temperatures.