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

The extreme El Nino 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 CO 2 and on the difference in CO 2 concentration between Mauna Loa and Cape Grim (C mlo-cgo ). The relative roles of eddy transport and mean advective transport on IH CO 2 annual differences from 1992 through to 2016 is explored. Eddy transport processes occur mainly in boreal winter-spring when C 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 C mlo-cgo is found to coincide well with dynamical indices that we introduce to characterize the transports. During the unrivalled 2009–2010 step in C mlo-cgo indices of eddy and mean transport reinforce. In contrast for the 2015 to 2016 change in C mlo-cgo the mean transport counteracts the eddy transport and the record strength of the Hadley circulation determines the annual IH CO 2 difference. The interaction of increasing global warming and extreme El Ninos may have important implications for altering the balance between eddy and mean IH CO 2 transfer.