The tropopause region thermal structure and tropical cyclones

Abstract

This thesis is an exploration of two seemingly unrelated questions: First, how do water vapor and ozone variations radiatively influence the thermal structure of the tropopause region? Second, what sets the thermodynamic limits of tropical cyclone intensity across the seasonal cycle? The link between these subjects is tropical cyclone outflow, which often reaches into the tropopause region, allowing the thermal structure there to impact tropical cyclone potential intensity. A radiative transfer model is employed to calculate the radiative effects of the 2000 and 2011 tropopause region abrupt drops -- events in which temperatures, water vapor, and ozone plunge suddenly to anomalously low levels. Results show that radiative effects partially offset in the region above the tropopause, but nonlocally combine to cool the layers below the tropopause. Persistently low water vapor concentrations associated with the abrupt drops spread to extratropical latitudes, and produce a total negative radiative forcing that offsets <12% of the carbon dioxide forcing over 1990-2013. Next, the importance of local and nonlocal radiative heating/cooling for tropopause region temperature seasonal cycles is examined. The radiative effects of water vapor seasonality are weak and local to the tropopause, whereas ozone radiatively amplifies temperature seasonality in the tropopause region by 30%, in part because stratospheric ozone seasonality nonlocally affects the tropopause region thermal structure. To determine how the tropopause region thermal structure affects thermodynamic limits on tropical cyclone intensity, this study presents the first comprehensive seasonal cycle climatology of potential intensity. Perennially warm sea surface temperatures in the Western Pacific result in outflow altitudes that are near the tropical tropopause region throughout the seasonal cycle, whereas the seasonalities of other ocean basins are less influenced by the tropopause region. Probing the potential intensity environmental drivers reveals that the seasonality of near-tropopause temperatures in the Western Pacific damps potential intensity seasonal variability by <30%. Incorporating a best track tropical cyclone archive shows that this result is relevant for real-world tropical cyclones: the tropopause region thermal structure permits intense Western Pacific tropical cyclones in every month of the year, which may have critical consequences for coastal societies.