Laminae in the tropical middle stratosphere: Origin and age estimation

In situ measurements of N 2 O and O 3 aboard the Observations from the Middle Stratosphere (OMS) balloon payload in the tropics are presented. In November 1997 at 7°S very distinct laminated structures in the N 2 O profile were observed in contrast to earlier studies based solely on O 3 profiles. The laminae probably are an observation of Rossby wave‐breaking events that lead to transport of mid‐latitude air into the tropics. Using a photochemical 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°S a similar vertical profile of N 2 O revealed no mid‐latitude laminae. We present dynamical arguments to support the existence of such laminae in November, but not in February.