Impact of Upper versus Lower Stratospheric Ozone Loss on Long-term Circulation Changes

Abstract:

Anthropogenic halocarbon emissions in the latter half of the twentieth century has led to a global decline in stratospheric ozone, particularly over the polar regions. The radiative induced cooling from ozone depletion results in a strengthened springtime polar vortex, which has been linked through dynamical coupling to trends in the dominant modes of tropospheric circulation variability, the Northern and Southern annular modes, and surface climate changes. The chemical mechanism for ozone depletion is dependent on the vertical location in the stratosphere. Heterogeneous chemical processes involving chlorine drive local chemical ozone depletion in the polar lower stratosphere (12-25 km), while gas-phase ozone photochemistry drives substantial depletion globally in the upper stratosphere (peak near 40 km). Trends in ozone depletion in the Artic are larger in the upper stratosphere than in the lower stratosphere. Whether the evolution of changes in ozone in the upper stratosphere could couple to long-term changes in the structure of the annular modes in the troposphere, particularly in winter and spring, is the primary focus of this study. We will present results utilizing the Whole Atmosphere Community Climate Model (WACCM), which is a fully coupled state-of-the-art interactive chemistry-climate model. WACCM is a high-top atmospheric model well suited to our study that extends from the surface to 140km with fully interactive chemistry and dynamics. Two free-running historic simulations were run from 1955 to 2015, with the sole variation being the inclusion of heterogeneous halogen chemistry. Analysis of the trends in circulation between the two simulations reveal the effect to which gas-phase ozone loss of the upper stratosphere dynamically influences responses at lower altitudes in a fully coupled framework.