Laminae in the tropical middle stratosphere: Origin and age estimation

In situ measurements of N2O and O-3 aboard the Observations from the Middle Stratosphere (OMS) balloon payload in the tropics are presented. In Novemb er 1997 at 7 degrees S very distinct laminated structures in the N2O profil e were observed in contrast to earlier studies based solely on O-3 profiles . The laminae probably are an observation of Rossby wave-breaking events th at lead to transport of midlatitude air into the tropics. Using a photochem ical model of the ozone response within those air parcels, their residence time in the tropics is estimated to be 4 to 6 weeks. In February 1997 at 7 degrees S a similar vertical profile of N2O revealed no mid-latitude lamina e. We present dynamical arguments to support the existence of such laminae in November, but not in February.