High-Latitude Southern Hemisphere Climate Variability, Trends, and Tropical Forcing.

This thesis investigates several inter-related aspects of climate variability in the SouthernHemisphere (SH) high-latitudes, with a focus on the time-varying behaviour and long-termtrends in Antarctic sea ice and atmospheric circulation, as diagnosed from observations,reanalyses, and simulations using an Atmospheric General Circulation Model (AGCM).Three components are examined:Antarctic sea ice concentration (SIC) variability is investigated in relation to the leadingmodes of SH climate variability, the Southern Annular Mode (SAM) and the El Niño-Southern Oscillation (ENSO). On interannual time-scales, a dipolar pattern of SIC anomaliesemerges in association with both the SAM and ENSO, the structure and amplitude ofwhich varies as a function of season, consistent with changes in the overlying atmosphericcirculation. ENSO-related SIC variability exhibits marked phase-dependence, and relativeto the SAM, is weaker in amplitude and more di↵use in structure. Despite strong interannualassociations, SIC trends cannot be attributed to changes in the SAM/ENSO.The spatio-temporal evolution of Antarctic sea ice trends is examined over 1979-2012.Negative trends in the Bellingshausen Sea are dominated by changes over 1979-1989, suchthat virtually no trend is detectable when this period is excluded from calculations. In theRoss Sea, positive trends are more consistent over decadal and multi-decadal time-scaleswhen evaluated over 1979-2012. This dependence on the time-period examined indicatesthat long-term trends are confounded by short-term variability, suggesting that observedchanges may be, in part, a consequence of natural fluctuations.The connections between sea surface temperature (SST) trends and changes to the SHatmospheric circulation are assessed in idealised AGCM simulations. Teleconnections fromthe Atlantic Ocean, driven by warmer SSTs, changes to the zonal Walker Circulation, anintensification of the Hadley Cell, and the subsequent initiation of atmospheric Rossbywaves, are found to be key components forcing spring-time circulation changes in the SH.Via the impacts on the atmospheric circulation, Atlantic teleconnections also promote apattern of warming across the Antarctic Peninsula that resembles changes seen in observations.The little-explored Atlantic teleconnections should thus be considered whenassessing the mechanisms driving Antarctic climate variability