Inter-annual variability in the interhemispheric atmospheric CO2 gradient: contributions from transport and the seasonal rectifier

The observed interhemispheric gradient in atmospheric carbon dioxide (CO2) indicates the distribution of CO2 sources and sinks, and for recent decades is evidence of a Northern mid-latitudes sink, a tropical source and southern hemisphere sink. As such, the variability in the gradient also reflects how these fluxes vary with time. However, the variability in the gradient is sensitive to the network of stations used to calculate the gradient. Also, an important consideration when dealing with variability in atmospheric measurements is the contribution due to the variability in the atmospheric transport. Most previous studies have ignored transport variability. Using an atmospheric tracer transport model driven with analysed circulation products, we demonstrate here that the interannual variability in the interhemispheric gradient due to transport alone is significant when compared with the observations. Model experiments show that interannually varying transport combined with both cyclostationary terrestrial biosphere fluxes and time-constant fossil CO2 fluxes generates significant interannual variability, but that the component due to the interannually varying transport and the ocean CO2 fluxes is small. The key contributor to the transport generated interannual variability is due to the variability in the seasonal rectifier (the covariance between the seasonality in the terrestrial biosphere fluxes and atmospheric transport, which results in non-zero surface CO2 concentrations despite the fluxes balancing at each grid-point). This study shows that the rectifier variability is complex, with different regions displaying different modes of variability. We also investigate the role of the Pearman Pump (gradient due to the seasonal covariance in the fluxes and cross-hemispheric transport) and show that while it appears to be a process occurring in the atmosphere, it is of second-order importance in forcing the interhemispheric gradient