The Tropical Tropopause Layer – Detailed Thermal Structure, Decadal Variability and Recent Trends

Abstract

The tropical tropopause layer (TTL) is a key region for troposphere-stratosphere exchange and acts as a “gate” for trace gases entering the stratosphere. In particular, tropical tropopause temperatures (TPTs) control the content of stratospheric water vapour, which influences stratospheric chemistry, radiation and circulation and is also an important driver of surface climate. Decadal variability or even long-term trends in TPTs and stratospheric water vapour are of great interest but are still not well understood. A comprehensive analysis of the TTL, including its detailed thermal structure, recent variability and dominant processes spanning time scales of years to decades, is conducted in this thesis using the recently available decade of high accuracy and high vertical resolution Global Positioning System Radio Occultation (GPS-RO) data, the Modern Era Retrospective-Analysis for Research and Applications (MERRA) reanalysis data, and a series of model simulations with NCAR's fully-coupled CESM model, which employs the chemistry climate model WACCM as its atmospheric component. The GPS-RO data measures a significant warming of TPTs and a weakening of the strength of the tropopause inversion layer (TIL) since 2001. Based on a series of model simulations, which switch on/off the corresponding factors, this recent warming in the TTL is mainly due to internal variability, i.e. a decrease in sea surface temperatures (SSTs) and a strengthening in Quasi-Biennial Oscillation (QBO) associated westerlies. A version of WACCM with higher vertical resolution (~ 300 m) reproduces this recent temperature variability better than with the standard vertical resolution (~ 1 km). This thesis provides the first evidence for a connection between TPTs and the Pacific Decadal Oscillation (PDO), from both observations and model simulations. The phase of the PDO, and in particular the change from positive to negative phases around the year 2000, can very well explain the recently observed TPT (multi-) decadal variability. This connection between SSTs and TPTs has consequences for stratospheric water vapour and may provide an important feedback on the Earth's global surface temperatures. Additionally, the hotly debated (multi-) decadal variability in lower stratospheric (LS) water vapour between 1979 and 2014, can be well understood with the 11-year solar cycle, the decadal El-Nino Southern Oscillation (ENSO) and the PDO. LS water vapour lags the solar cycle by 2-3 years and can be explained using a link between the solar cycle, decadal ENSO variations and tropopause temperature variability. This thesis highlights the importance of a fine vertical resolution for climate models and improves the understanding of the TTL temperature and LS water vapour variability over the recent decades. In particular it opens up a debate of the connection between stratospheric decadal to multidecadal variability and modes of SST variability, such as the PDO.