Unprecedented strength of Hadley circulation in 2015–2016 impacts on CO<sub>2</sub> interhemispheric difference

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.

Revised records of atmospheric trace gases CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O, and <i>δ</i><sup>13</sup>C-CO<sub>2</sub> over the last 2000 years from Law Dome, Antarctica

“…Predicting how GHG concentrations will vary in the future requires a clear understanding of the biogeochemical processes responsible for their variations. However, models of future long-term climate changes predict a large range in GHG concentrations for a given scenario of emissions (Friedlingstein et al, 2014), and one of the key uncertainties is associated with feedbacks in the coupled carbon-climate system (Arora et al, 2013). Climate modellers have analysed and compared results from state-of-theart climate model simulations to gain insights into the processes of climate variability, change, and feedbacks through the Coupled Model Inter-comparison Project (CMIP).…”

Variability in a four-network composite of atmospheric CO<sub>2</sub> differences between three primary baseline sites

“…There is a linear relationship between u duct and equatorial upper troposphere transient kinetic energy shown in Fig. 6 of Frederiksen and Webster (1988) and discussed in FF16 and FF18. Note that in Fig.…”

“…FF18 developed wind indices that characterize both types of IH transport based on reanalysis data sets focussing on the National Center for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCAR) reanalysis (NRR) data (Kalnay et al, 1996). Eddy transport is described by u duct , the average 300 hPa zonal velocity in the Pacific westerly duct region (Frederiksen and Webster, 1988)…”

“…The top panel in Fig. 6a shows a 3-decade time series of the u duct index which characterizes cross-equatorial Rossby wave dispersion, Rossby wave breaking, and corresponding increases in transient kinetic energy and eddy transport in the near-equatorial upper troposphere (Webster and Holton, 1982, Frederiksen and Webster, 1988, Ortega et al, 2018. The large-scale Rossby waves are generated by thermal anomalies and topographic features including the Himalayan Figure 5.…”

“…Yue et al, 2017). However, Frederiksen and Francey (2018;FF18), argued that the unprecedented strength in the Hadley circulation increased IH exchange (reduced IH CO 2 difference) late in 2016, overwhelming the earlier reduced eddy exchange linked to the strong 2015-2016 El Niño. They also indicated dynamical contributions to IH CO 2 during both El Niño and La Niña periods (e.g.…”