Unprecedented strength of Hadley circulation in 2015–2016 impacts on CO ₂ interhemispheric difference

The extreme El Niño of 2015 and 2016 coincided with record global warming and unprecedented strength of the Hadley circulation with significant impact on mean interhemispheric (IH) transport of inline-formulaCO₂. The relative roles of eddy transport and mean advective transport on interannual differences in inline-formulaCO₂ concentration between Mauna Loa and Cape Grim (inline-formulaC_mlo−cgo), from 1992 through to 2016, are explored. Eddy transport processes occur mainly in boreal winter–spring when inline-formulaC_mlo−cgo is large; an important component is due to Rossby wave generation by the Himalayas and propagation through the equatorial Pacific westerly duct generating and transmitting turbulent kinetic energy. Mean transport occurs mainly in boreal summer–autumn and varies with the strength of the Hadley circulation. The timing of annual changes in inline-formulaC_mlo−cgo is found to coincide well with dynamical indices that we introduce to characterize the transport. During the unrivalled 2009–2010 step in inline-formulaC_mlo−cgo, the effects of the eddy and mean transport were reinforced. In contrast, for the 2015 to 2016 change in inline-formulaC_mlo−cgo, the mean transport counteracts the eddy transport and the record strength of the Hadley circulation determines the annual IH inline-formulaCO₂ difference. The interaction of increasing global warming and extreme El Niños may have important implications for altering the balance between eddy and mean IH inline-formulaCO₂ transfer. The effects of interannual changes in mean and eddy transport on interhemispheric gradients in other trace gases are also examined.