MLS measurements of stratospheric hydrogen cyanide during the 2015-16 El Niño event

Abstract. It is known from ground-based measurements made during the 1982-83 and 1997-98 El Niño events that atmospheric
HCN tends to be higher than usual during such years. The Microwave Limb Sounder (MLS) on Aura has been measuring HCN
mixing ratios since launch in 2004; the measurements are ongoing at the time of writing. The winter of 2015-16 has seen the
largest El Niño event since 1997-98. We present MLS measurements of HCN in the lower stratosphere for the Aura mission
to date, comparing the 2015-16 5 El Niño period to the rest of the mission. HCN in 2015-16 is higher than at any other time
during the mission, but ground based measurements suggest that it may have been even more elevated in 1997-98. As the MLS
HCN data are essentially un-validated, we show them alongside data from the MIPAS and ACE-FTS instruments; the three
instruments agree reasonably well in the tropical lower stratosphere. Global HCN emissions calculated from the GFED (V4.1)
database are very much greater during large El Niño events and are greater in 1997-98 than in 2015-16, thereby showing good
10 qualitative agreement with the measurements. Correlation between ENSO indices, measured HCN and GFED HCN emissions
is less clear away from the 2015-16 event. In particular, the 2009-10 winter had fairly strong El Niño conditions and fairly
large GFED HCN emissions, but very little effect is observed in the MLS HCN.