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Anomalous atmospheric circulations forced by volcanic aerosols Young, Alyssa Michelle


During a strong volcanic eruption, the energy balance of the Earth is temporarily disturbed due to the addition of sulphate aerosols to the stratosphere. The selective absorption and reflection of radiation by these aerosols lead to both radiative and thermodynamical effects in the atmosphere. The goal of our study is to identify the anomalous atmospheric circulations generated under these volcanic conditions. A Principal Component Analysis (PCA) and Combined Principal Component Analysis (CPCA) are first applied to the height and temperature data from the stratosphere at 30 mbar. While PCA extracts the Arctic Oscillation (AO) as a weak volcanic mode, the CPCA performs better and is found to extract a definite volcanic signal with a distinct zonal pattern of circulation. However, the stratospheric volcanic modes are different patterns from the linear PCA modes suggesting that the system's response to the volcanic forcing is not obvious. The raw data are analysed for comparison. Application of PCA and CPCA to tropospheric height and temperature revealed only a weak (not statistically significant) volcanic signal, but did extract a strong El Nino signal. Through an Analysis of Variance (ANOVA) technique it is possible to separate the response to volcanic forcing from that due to El Nino. Combining the 30 mbar and 300 mbar height fields in a vertical CPCA, it is finally possible to extract the volcanic modes at both levels. The resulting tropospheric volcanic mode is shown to have similarities to a composite map of 500 mbar heights representing the difference between years with a strong polar vortex and years with a weak one.

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