On detecting a trend in the residual circulation from observations of column HCl

Abstract

Ground‐based measurements show that the HCl column at middle latitudes varies seasonally with a late spring maximum and a minimum in late summer. HALOE measurements of HCl profiles show that this seasonal cycle in the column is controlled by the seasonal variations of the mass and composition of the lowermost stratosphere. The seasonal maxima are produced by wintertime downward transport of stratospheric air. We hypothesize that the amplitude of the seasonal cycle is quantitatively related to the strength of the stratospheric residual circulation and stratosphere‐troposphere exchange (STE). We investigate this relationship using a multi‐decadal simulation from the GSFC Chemistry and Transport Model driven by output from a general circulation model in which an STE trend exists. We also examine the ground‐based record of HCl measurements at Jungfraujoch for evidence of a trend in the HCl seasonal amplitude that would be a signature of a long‐term change in the stratospheric circulation.