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\section{Data}  \subsection{Overview} Data from the European Centre for Medium-Range Weather Forecasts Interim Reanalysis \citep[ERA-Interim;][]{Dee2011} were used in this study. In particular, we utilized the six-hourly 500 hPa zonal and meridional wind, surface air temperature, sea ice fraction, sea surface temperature and mean sea level pressure analysis fields, from which daily mean time series were calculated for the 36-year period 1 January 1979 to 31 December 2014. For precipitation, the `total precipitation' forecast fields were used. Each forecast field represents the accumulated precipitation since initialization, so the daily rainfall total was calculated as the sum of the two 12 hours post initialization accumulation fields for each day. The horizontal resolution of the ERA-Interim data was 0.75$^{\circ}$ latitude by 0.75$^{\circ}$ longitude.  The series of reliable, spatially complete atmospheric data available for the mid-to-high southern latitudes is relatively short. The reanalysis projects have produced sequences of surface and upper air fields that in some cases date back to the 1950s \citep{Kistler2001,Uppala2005,Kobayashi2015}, however it is generally accepted that these have limited value prior to 1979 at high southern latitudes, due Relative  toa lack of satellite sounder data for use in  the assimilation process \citep{Hines2000}.  The other  latest generation reanalysis datasets (which all date back to at least 1979) are the European Centre for Medium-Range Weather Forecasts Interim Reanalysis \citep[ERA-Interim;][]{Dee2011}, Modern Era Retrospective-analysis for Research and Applications \citep[Merra;][]{Rienecker2011}, Climate Forecast System Reanalysis \citep[CFSR;][]{Saha2010} and Japanese 55-year Reanalysis \citep[JRA-55;][]{Kobayashi2015}. While assessments of the validity of these datasets in the mid-to-high southern latitudes have only just begun to emerge, the available evidence suggests that datasets,  ERA-Interim may be the superior product. In comparison is thought  toits peers, ERA-Interim  best reproduces reproduce  the precipitation variability \citep{Bromwich2011,Nicolas2011},  vertical temperature structure \citep{Screen2012}, precipitation variability \citep{Bromwich2011,Nicolas2011} \citep{Screen2012}  and mean sea level pressure and 500 hPa geopotential height at station locations \citep{Bracegirdle2012} around Antarctica.As such, daily timescale ERA-Interim data for the 36-year period 1 January 1979 to 31 December 2014 was used in this study.  While ERA-Interim may be considered the superior reanalysis product, these results are encouraging,  it should be said is worth noting  that all reanalysis datasets need to be treated with caution the sparsity of observational data  in the mid-to-high southern latitudes due means that ERA-Interim (like all reanalysis data) still needs  to the sparsity of observational data. be interpreted with caution.  There are also well-known difficulties with the representation of low-frequency variability and trends in reanalysis data, due to factors such as changes in the observing system, transitions between multiple production streams, and/or various other errors that can occur in a complex reanalysis production system over time  \citep{Dee2014}. These issues are highly relevant to the PSA pattern trends discussed in this study, but are somewhat less critical for the results pertaining to seasonal and interannual variability.\subsection{ERA-Interim reanalysis}  Reanalysis projects typically provide both analysis and forecast fields for download. The analysis fields are the output of the data assimilation cycle at each time interval, which for ERA-Interim is every six hours. They represent arguably the most accurate possible depiction of the atmospheric state for several dozen variables that are all coherent on the calculation grid. These analysis fields are then used to initialize weather forecasts for the coming hours/days. ERA-Interim forecasts are initialized twice daily at 0000 UTC and 1200 UTC and forecast fields are available for 3, 6, 9 and 12 hours post initialization.   In this study we utilize the six-hourly 500 hPa zonal and meridional wind, surface air temperature, sea ice fraction, sea surface temperature and mean sea level pressure analysis fields, from which a daily mean time series was calculated for each variable. For precipitation, the `total precipitation' forecast fields were used (i.e. the sum of the convective and large-scale precipitation, which are also provided separately). Each forecast field represents the accumulated precipitation since initialization, so the daily rainfall total was calculated as the sum of the two 12 hours post initialization accumulation fields for each day. The horizontal resolution of the ERA-Interim data used here was 0.75$^{\circ}$ latitude by 0.75$^{\circ}$ longitude.