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Laminae in the tropical middle stratosphere: Origin and age estimation

In situ measurements of N2O and O3 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 N2O profile were observed in contrast to earlier studies based solely on O3 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 N2O revealed no mid‐latitude laminae. We present dynamical arguments to support the existence of such laminae in November, but not in February.