Variability in a four-network composite of atmospheric CO 2 differences between three primary baseline sites

Abstract. Spatial differences in the monthly baseline CO 2 since 1992 from Mauna Loa, (mlo, 19.5° N, 155.6° W, 3379 m), Cape Grim (cgo, 40.7° S, 144.7° E, 94 m) and South Pole (spo, 90° S, 2810 m), are examined for consistency between four monitoring networks. For each site pair, a composite based on the average of NOAA, CSIRO and two independent SIO analysis methods is presented. Averages of the monthly standard deviations are 0.25, 0.23 and 0.16 ppm for mlo-cgo, mlo-spo and cgo-spo respectively. This high degree of consistency and near-monthly temporal differentiation (compared to CO 2 growth rates) provides an opportunity to use the composite differences for verification of global carbon cycle model simulations. Interhemispheric CO 2 variation is predominantly imparted by the mlo data. The peaks and dips of the seasonal variation in interhemispheric difference act largely independently. The peaks mainly occur in May, near the peak of Northern Hemisphereterrestrial respiration. Borealspring is when interhemispheric exchange via eddy processes dominates, with increasing contributions from mean transport into borealsummer. The dips occur in September, when the CO 2 partial pressure difference is near zero, just after the peak in the mean interhemispheric exchange via the Hadley circulation. Surface-air terrestrial flux anomalies would need to be up to an order of magnitude larger than found in order to explain the peak and dip CO 2 variations (large enough to significantly influence short-term northern hemispheregrowth rate variations). Recent features in the composite records, inconsistent in timing and amplitude with air-surface fluxes, are largely consistent with interhemispheric transport variations. These include the remarkable stability in annual CO 2 inter-hemispheric difference in the 5-year relatively ENSO- quiet period2010–2014, and the 2017 recovery in the CO 2 interhemispheric gradient from the unprecedented ENSO event in 2015–16.