Tropical Tropopause Layer Evolution During QBO Disruptions

The tropical tropopause layer (TTL) is studied during 2015/16 and 2019/20 quasi-biennial oscillation (QBO) disruption events using GNSS-RO and SWOOSH satellite observations and the ERA5 reanalysis. By compositing temperature, water vapor, and tropical upwelling during different phases of QBO and QBO disruptions (XQBO), we show that XQBO events lead to cold anomalies (relative to WQBO winters) of as much as −4.6 K during April 2016 at 60 hPa and −3.9 K during December 2019 at 50 hPa. These cold anomalies initiate around 50 hPa and propagate downward, cooling the cold point tropopause (CPT). We investigate regional CPT temperature anomalies in relation to different modes of natural variability such as the QBO, the El Niño-Southern Oscillation (ENSO), and the Indian Ocean Dipole (IOD) using multiple linear regression. The warm-then-cold evolution of the CPT during November 2015–October 2016 and November 2019–October 2020 can be explained in part by the modulation of convection and Walker circulation by ENSO and the IOD and by the impact of QBO on the stratospheric upwelling as all play major roles in regulating regional CPT. CPT temperatures during the boreal summer following the two disruptions are both colder than almost all summers following WQBO winters; CPT temperatures during summer 2016 are the coldest since 1979, suggesting that any increased frequency of QBO disruptions is likely to impact atmospheric stability near the tropopause.