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.