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

• Main Author: Simpkins, Graham
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: UNSW Sydney 2014
• Subjects: Sea ice; Climate variability; Antarctica; Atmospheric Circulation; Atmosphere-ice interactions; Antarctic; The Antarctic; Antarctic Peninsula; Ross Sea; Bellingshausen Sea; Antarc*
• Online Access: https://dx.doi.org/10.26190/unsworks/16759; http://hdl.handle.net/1959.4/53441

This thesis investigates several inter-related aspects of climate variability in the Southern Hemisphere (SH) high-latitudes, with a focus on the time-varying behaviour and long-term trends 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 leading modes 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 anomalies emerges in association with both the SAM and ENSO, the structure and amplitude of which varies as a function of season, consistent with changes in the overlying atmospheric circulation. ENSO-related SIC variability exhibits marked phase-dependence, and relative to the SAM, is weaker in amplitude and more di↵use in structure. Despite strong interannual associations, 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, such that virtually no trend is detectable when this period is excluded from calculations. In the Ross Sea, positive trends are more consistent over decadal and multi-decadal time-scales when evaluated over 1979-2012. This dependence on the time-period examined indicates that long-term trends are confounded by short-term variability, suggesting that observed changes may be, in part, a consequence of natural fluctuations. The connections between sea surface temperature (SST) trends and changes to the SH atmospheric circulation are assessed in idealised AGCM simulations. Teleconnections from the Atlantic Ocean, driven by warmer SSTs, changes to the zonal Walker Circulation, an intensification of the Hadley Cell, and the subsequent initiation of atmospheric Rossby waves, 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 a pattern of warming across the Antarctic Peninsula that resembles changes seen in observations. The little-explored Atlantic teleconnections should thus be considered when assessing the mechanisms driving Antarctic climate variability.