Evidence for Oceanic Control of Interannual Carbon Cycle Feedbacks

Abstract

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.