Tropical tape recorder observed in HCN

The tropical tape recorder is an effect in which a seasonal cycle in the mixing ratio of a gas is imposed on air entering the stratosphere and is then transported upwards by the Brewer‐Dobson circulation. We report the observation of this effect in the mixing ratio of HCN, as measured by two satellite instruments: MLS on Aura and ACE‐FTS on SCISAT‐1. The cycle does not repeat every year; the data so far suggest that the dominant period is about two years. The reason for this periodicity is not yet fully understood. Comparisons with MODIS fire‐count data suggest that it may be connected to inter‐annual variations in biomass burning in Indonesia and the surrounding region.