The semi‐annual oscillation and equatorial tracer distributions

Some interesting low latitude features of tracer distributions revealed by recent satellite experiments have been studied using a two‐dimensional atmospheric model. It is found that the equatorial semi‐annual oscillation, forced in the model by simply prescribing a suitable momentum convergence, is important in determining the tracer behaviour. In particular, the meridional circulation associated with the semi‐annual oscillation leads to a successful model simulation of the ‘double peak’ in N 2 O and CH 4 mixing ratios, observations of which show, at certain times of the year, maxima on a fixed pressure surface in the subtropics with a local minimum at the equator.