Horizontal transport affecting trace gas seasonality in the Tropical Tropopause Layer (TTL)

Abstract

We analyze horizontal transport from midlatitudes into the tropics (in‐mixing) and its impact on seasonal variations of ozone, carbon monoxide and water vapor in the Tropical Tropopause Layer (TTL). For this purpose, we use three‐dimensional backward trajectories, driven by ECMWF ERA‐Interim winds, and a conceptual one‐dimensional model of the chemical composition of the TTL. We find that the fraction of in‐mixed midlatitude air shows an annual cycle with maximum during NH summer, resulting from the superposition of two inversely phased annual cycles for in‐mixing from the NH and SH, respectively. In‐mixing is driven by the monsoonal upper‐level anticyclonic circulations. This circulation pattern is dominated by the Southeast Asian summer monsoon and, correspondingly, in‐mixing shows an annual cycle. The impact of in‐mixing on TTL mixing ratios depends on the in‐mixed fraction of midlatitude air and on the meridional gradient of the particular species. For CO the meridional gradient and consequently the effect of in‐mixing is weak. For water vapor, in‐mixing effects are negligible. For ozone, the meridional gradient is large and the contribution of in‐mixing to the ozone maximum during NH summer is about 50%. This in‐mixing contribution is not sensitive to the tropical ascent velocity, which is about 40% too fast in ERA‐Interim. As photochemically produced ozone in the TTL shows no distinct summer maximum, the ozone annual anomaly in the upper TTL turns out to be mainly forced by in‐mixing of ozone‐rich extratropical air during NH summer.