Low-ozone bubbles observed in the tropical tropopause layer during the TC4 campaign in 2007

Abstract

In the summer of 2007, the NASA DC-8 aircraft took part in the Tropical Composition, Cloud and Climate Coupling campaign based in San Jose, Costa Rica. During this campaign, multiple in situ and remote-sensing instruments aboard the aircraft measured the atmospheric composition of the tropical tropopause layer (TTL) in the equatorial region around Central and South America. During the 17 July flight off the Ecuadorian coast, well-defined “bubbles” of anomalously low-ozone concentration (less than 75 ppbv) were detected above the aircraft in the TTL at the altitude near 365 K (between 14 and 16 km) and at ∼3°S and ∼82°W. Backward trajectories from meteorological analyses and the aircraft in situ measurements suggest that the ozone-depleted air mass originated from deep convection in the equatorial eastern Pacific and/or Panama Bight regions at least 5 days before observation by the DC-8; this was not a feature produced by local convection. Given uncertainties known in regard to trajectories calculated from global reanalysis, it is not possible to identify the exact convective system that produced this particular low-ozone anomaly, but only the general origin from a region of high convective activity. However, the fact that the feature apparently maintained its coherency for at least 5 days suggests a significant contribution to the chemical composition of the tropical upper troposphere portion of the TTL from convective systems followed by quasi-horizontal transport. It also suggests that mixing time scales for these relatively small spatial features are greater than 5 days.