Widening of the Cold Point Tropopause and Implications for Stratospheric Composition

Abstract:

The tropical belt has expanded over the past few decades with potentially important implications for tropospheric circulation systems and precipitation patterns. While most existing studies focus on the tropical widening of the upper troposphere, possible changes of the upper tropical tropopause layer (TTL) and in particular the cold point tropopause (CPT) are still unclear. Since the CPT describes the entrance region for trace gases travelling from the troposphere into the stratosphere, an expansion of this region and possible consequences for stratospheric composition need to be explored.

We investigate changes of the upper TTL based on the Lagrangian approach and determine the Lagrangian cold point (LCP) for 1979-2011 from ERA-Interim based trajectory calculations. The LCP distribution shows strong longitudinal variations with the main entrance region to the stratosphere above the tropical West Pacific. In addition, pronounced changes in the latitudinal LCP distribution with season and year occur. We define the width of the cold point (or final dehydration) belt as the latitudinal extent of the LCP distribution. The width of the cold point belt is on average 28° and thus relatively narrow compared to the latitudinal distribution of the air parcels above and below this level. Based on the long-term time series of the LCPs, we detect a significant broadening of the cold point belt of 1.2°/decade for the period 1979-2011. We investigate how changes in the width of the cold point belt correspond to changes in LCP temperature and show that a positive trend in the first can drive a positive trend in the latter. Based on this relationship implications for stratospheric composition and in particular for stratospheric water vapour are discussed.

Our analysis results in a measure of the latitudinal extent of the upward mass transport at the inner-tropical cold point. It is quite different from existing diagnostics focusing on lower parts of the atmosphere and encompassing the complete width of the circulation cell. Nevertheless, the diagnosed expansion of the cold point belt of 1.2°/decade is in good agreement with most other diagnostics confirming that the tropical widening is a phenomenon also apparent in the upper TTL impacting cold point and dehydration patterns and thus stratospheric water vapour.