This dataset documents the trends and variability in the latitude and strength of the belt of lower-atmosphere westerly winds over the Southern Ocean, referred to as the 'westerly jet'. Time series of annual mean and seasonal diagnostics are available for the period 1979-present, specifically time series of seasonal and annual mean jet latitude and strength. The diagnostics are derived from the European Centre for Medium Range Weather Forecasts (ECMWF) ERA-Interim reanalysis (for more information see www.ecmwf.int and Dee et al. (2011)), which is an observationally-constrained reconstruction of atmospheric conditions. The broad characterisation of the westerly winds into these simple diagnostics has been found to be useful for understanding long-term climate change due to contrasting drivers of change and impacts on other aspects of the climate system. This is an index of winds around the full circumference of all longitudes at Southern Hemisphere middle latitudes. The exact latitude depends on the position of the jet at any given time, but on average the jet (the core of the westerlies) is located at approximately 52 deg S.

The file contains Southern Hemisphere winter (September) sea ice concentration (sic) from a simulation performed using the isotope-enabled HadCM3 climate model forced with early last interglacial boundary conditions, centred approximately 128,000 years ago. The resulting sic represents a reduction in winter sea ice area of approximately 54% relative to pre-industrial and is proposed as the best explanation for the Antarctic ice core data from 128,000 years ago. The spatial pattern of sea ice retreat was determined using a large ensemble of model experiments and a pattern search optimization approach to match the last interglacial ice core isotope peak. Further details can be found in the published manuscript (https://doi.org/10.1002/2017GL074594). This work was funded by NERC grants NE/P009271/1, NE/P013279/1, and NE/K004514/1.