The tropical tape recorder observed in satellite measurements of HCN

The tropical tape recorder is a well-known 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 upwelling branch of the Brewer-Dobson circulation. We report the observation of this effect in the mixing ratio of HCN, as observed by two satellite instruments: MLS on Aura and ACE-FTS. 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 understood.