Abstract
Abstract. 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 CO2. The relative roles of eddy transport and mean advective transport on interannual differences in CO2 concentration between Mauna Loa and Cape Grim (Cmlo−cgo), from 1992 through to 2016, are explored. Eddy transport processes occur mainly in boreal winter–spring when Cmlo−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 Cmlo−cgo is found to coincide well with dynamical indices that we introduce to characterize the transport. During the unrivalled 2009–2010 step in Cmlo−cgo, the effects of the eddy and mean transport were reinforced. In contrast, for the 2015 to 2016 change in Cmlo−cgo, the mean transport counteracts the eddy transport and the record strength of the Hadley circulation determines the annual IH CO2 difference. The interaction of increasing global warming and extreme El Niños may have important implications for altering the balance between eddy and mean IH CO2 transfer. The effects of interannual changes in mean and eddy transport on interhemispheric gradients in other trace gases are also examined.
Highlights
Interhemispheric (IH) exchange of CO2 occurs mainly by eddy transport in the boreal winter–spring and by mean convective and advective exchange in the boreal summer–autumn (Bowman and Cohen, 1997; Lintner et al, 2004; Miyazaki et al, 2008; and references therein).On the basis of long-term (1949–2011) correlations of the upper tropospheric zonal wind with the Southern Oscillation Index (SOI), Francey and Frederiksen (2016; hereafter FF16) defined an index for the Pacific westerly duct, uduct, as a measure of IH eddy transport of CO2
The strength of the Hadley circulation increased to unprecedented levels during 2015–2016 and had a major impact on the mean interhemispheric (IH) transport of CO2 and on the difference in CO2 concentration between Mauna Loa and Cape Grim (Cmlo−cgo)
This study has focussed on the roles of IH transient eddy and mean transport of CO2 on interannual variations in Cmlo−cgo and has established dynamical indices that characterize the broad features of this transfer (Table 1)
Summary
Interhemispheric (IH) exchange of CO2 occurs mainly by eddy transport in the boreal winter–spring and by mean convective and advective exchange in the boreal summer–. On the basis of long-term (1949–2011) correlations of the upper tropospheric zonal wind with the Southern Oscillation Index (SOI), Francey and Frederiksen (2016; hereafter FF16) defined an index for the Pacific westerly duct, uduct, as a measure of IH eddy transport of CO2. The focus here is on IH CO2 difference, anomalies in the mean convective and advective mode of IH CO2 exchange, and changes in the relative importance of the mean and eddy transport modes
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