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 (theta), in particular around p = 80 hPa, where the strongest annual variation in tropical temperature occurs. Thus by using theta 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, theta-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 theta = 360 K and quantifying the photochemical production of O-3 in ascending air above theta = 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.