From Surface to Stratosphere: Understanding Interannual Climate Variability and Decadal Changes

Abstract

This thesis investigates key processes governing interannual and decadal variability in the stratosphere and troposphere and emphasizes their implications for climate projections. Arctic Amplification (AA), the disproportionate warming of the Arctic relative to global mean temperatures, is a robust feature of climate change. Using machine learning and CMIP6 models, this work demonstrates that internal variability has amplified AA by 38% since 1980, reconciling discrepancies between observed and simulated AA. These inflated values of AA are made possible by a unique pattern of multi-decadal internal variability which warms the Barents and Kara Sea, while cooling the Tropical Eastern Pacific and Southern Ocean. These results highlight the critical role of internal variability in shaping observed climate trends. Focusing on the tropical tropopause layer (TTL), this thesis quantifies the drivers of interannual variability in temperature, water vapor, and cirrus clouds, revealing the central role of stratospheric processes in determining variability of this region. The QBO's impact on tropical clouds is further explored, identifying a seasonally synchronized response in cloud fraction, temperature, and even the radiative budget of the tropics. Finally, this work addresses the rapid warming of the Southern Hemisphere subtropical lower stratosphere, linked to changes in the SH BDC. These circulation changes reconcile observed temperature and ozone trends with simulations, shedding light on the dynamical processes influencing stratospheric variability and ozone recovery. Together, these studies advance understanding of how internal variability contributes to observed changes from the surface to the stratosphere, informing model validation and projections of future climate.