Intermittent behaviour in the seasonal inter-hemispheric exchange of 12CO2 and 13CO2

Recent changes in the annual average CO2 inter-hemispheric difference frequently coincide with decreased winds in the westerly duct[1], a region above ~6 km over the equatorial Pacific. This is caused by winds impinging the Himalayas that episodically penetrate equatorial tropical convection.The coincidence of an ‘open duct’ (with associated increased turbulent kinetic energy) and near maximum interhemispheric CO2 difference in the boreal winter expedites CO2exchange. Intermittently through previous decades, the westerly winds have been abnormally low or missing during the boreal winter. These occasions coincide with anomalies in CO2 measured in baseline conditions at Mauna Loa and Cape Grim and the interhemispheric difference using 25-yea rCSIRO GASLAB data (with the extreme cases visible throughout most of the iconic Keeling 5-decadeMauna Loa-South Pole CO2 records). When the duct is open, the strength of westerly winds exhibits an anti-correlation with the Pacific Decadal Oscillation that is reflected in Mauna Loa CO2 seasonality. Isotopic signals in the interhemispheric difference and seasonal behaviour imply that a significant terrestrial contribution to the Southern Hemisphere CO2 seasonality is Northern Hemisphere CO2 transferred rapidly in brief periods around February each year. Atmospheric CO2 inversions without the duct do not allow for this this CO2 exchange process, instead generally attributing spatial changes to terrestrial source/sinks.