Annual cycle of horizontal in‐mixing into the lower tropical stratosphere

Based on the HALOE and SHADOZ observations of ozone (O 3 ) and on a simple conceptual model of transport and photochemistry, the seasonality of O 3 within the stratospheric part of the tropical tropopause layer (TTL) extending between 360 and 420 K potential temperature is discussed. We show that the seasonality of O 3 diagnosed on pressure ( p ) surfaces has a significantly larger annual cycle compared with the same kind of analysis on surfaces with constant potential temperature (θ), in particular around p = 80 hPa, where the strongest annual variation in tropical temperature occurs. Thus by using θ instead of p as the vertical coordinate, the (seasonal) adiabatic variability is removed, and consequently, a much smaller seasonal cycle of O 3 remains, which can be understood as a consequence of chemistry, cross‐isentropic transport (upwelling), and horizontal, i.e., quasi‐isentropic, transport (in‐mixing). Furthermore, we show that the observed, θ‐related seasonality of O 3 , with highest values during boreal summer, cannot be understood only by photolytical O 3 production in slowly rising air masses, which are well isolated from the extratropics. By using the SHADOZ climatology at θ = 360 K and quantifying the photochemical production of O 3 in ascending air above θ = 360 K, we determine the residual variability between the observations (SHADOZ, HALOE) and the calculated O 3 values and, consequently, interpret this residuum as horizontal in‐mixing from the extratropical stratosphere. We find that between 380 and 420 K, in‐mixing contributes to about 40% of the observed O 3 mixing ratios during boreal summer.