UC Santa Barbara

Stratospheric ozone protects Earth’s surface from harmful ultraviolet radiation. Hence, understanding variability responsible for stratospheric ozone depletion is vital to protect human health and the environment. Despite low ozone concentration at 100 hPa in the upper troposphere-lower stratosphere (UTLS), ozone variability at this level plays an important role in regulating air temperatures, which in turn regulates troposphere to stratosphere exchanges and stratospheric chemistry. This work investigates the spatial and temporal ozone variability in the UTLS over South America, with an emphasis on the La Plata Basin (LPB). This variability is investigated to understand the influence of teleconnections originating in the Pacific Ocean, the El Niño-Southern Oscillation (ENSO) on interannual time scales, and the Pacific Decadal Oscillation (PDO) on interdecadal time scales. Variability in the UTLS and mechanisms driving stratospheric ozone variability over South America are not well understood. This work fills this knowledge gap with two overarching goals, 1) by identifying primary patterns of ozone variably related to large-scale processes (e.g. ENSO and PDO) with reanalysis (Chapters 1 and 2), and 2) by investigating local UTLS variability related to deep convection with an atmospheric numerical model, to simulate deep convection in the LPB, and to quantify lower stratospheric hydration (Chapters 3 and 4). Results from the first goal shows that the primary patterns of UTLS ozone variability is strongly modulated by teleconnections with the tropical Pacific Ocean (e.g. El Niño) via Rossby wave trains interacting with South America. This teleconnection is further modulated by PDO phases. The strongest connections between the ENSO and UTLS ozone occur during the cool PDO phase and are dependent upon the location of sea-surface temperature anomalies in the tropical Pacific, especially the presence of Modoki-type El Niño events. Negative ozone anomalies are shown in South America during the wet season over the LPB and connected to El Niño during the cool PDO phase; and in the East and parts of the Southeast negative ozone anomalies are shown during the austral spring. For the second goal of this work, we investigated double tropopause events during three types of deep convection (discrete convective cells, mesoscale convective complex and squall line) to identity lower stratospheric hydration using the Weather Research and Forecasting model. Double tropopause events occurred during all convective types. The discrete convective cells did not produce stratospheric hydration due to the mixing of ice and water vapor in the tropopause, inhibiting net positive buoyancy and preventing the transport of material aloft. In contrast, the mesoscale convective complex and squall line both exhibit a dry layer in the tropopause, collocated with an ice layer, where net positive buoyancy contributed to stratospheric hydration as high as 20 km. Additional research is vital to understand how UTLS variability can affect surface processes, especially in a warming world.