Over the past ~5 years, the confusion regarding conflicting estimates of recent tropical widening based on different observational datasets (e.g. satellites, meteorological reanalyses) and climate model simulations has largely been resolved through the efforts of groups such as the ISSI Tropical Width Diagnostics Intercomparison Project and US CLIVAR Working Group on Tropical Width.

Despite this recent progress, many new questions have emerged. One question in particular relates to the finding that there are two broad categories of tropical width metrics, referred to as “lower” and “upper” atmospheric metrics. While the “lower” atmospheric metrics co-vary with the Hadley cell edge, the “upper” metrics based on the subtropical jet and tropopause do not. The reason for the disconnect between “lower” and “upper” tropical edge metrics is currently unknown and is surprising given the expected connection between the tropospheric overturning circulation and subtropical jet.

The “upper” atmospheric category of metrics has been inadequately studied despite it being important for climate, since variability in the subtropical jet and tropopause characteristics affects transport and mixing of climatically important trace gases in the upper troposphere and lower stratosphere (UTLS), including water vapor and ozone. Indeed, satellite measurements indicate recent unexplained trends in lower stratospheric ozone and other trace gases, and changes in the regions of turbulent mixing near the subtropical jet have been implicated as a possible cause. The causes of such shifts in mixing zones are largely unexplained, and a major deficiency remains in our knowledge of the interrelationships between stratospheric transport and mixing, jet/tropopause properties, UTLS trace gas variability, and their relation to the tropospheric Hadley cells. The need to advance the fundamental understanding of these interrelationships motivates this new ISSI team.

What we’re doing

The goals of the ISSI TWIST (Tropical Width Impacts on the STratosphere) team are to: 1) identify robust satellite-observed metrics of tropical width in the UTLS region based on temperature structure and atmospheric composition, 2) characterize relationships between UTLS tropical width and tropospheric circulation, and 3) identify how these tropical width variations in the UTLS relate to variability and trends in trace gas concentrations and their impact on climate.