Methodology:

Seasonal covariates describing the timing of the seasonal sea ice cycle and annual sea ice extent and area in relation with inter-annual variation in the sea ice edge, as estimated by a 15% sea ice concentration threshold. Sea ice concentration was derived from satellite measurements of percent sea ice concentration between 1982 and 2024, from the NOAA/NESDIS/NCEI Daily Optimum Interpolation Sea Surface Temperature (SST), version 2.1, dataset (DOISST v2.1). Selected daily data has a resolution of 1/4 degrees, from a polygon of longitude range [-44 degrees,-47 degrees], latitude range [-60 degrees,-62 degrees], and total area of 37,966km2, with the South Orkney Islands at approximately -46.875 degrees and -61.875 degrees (dataset ncdcOisst21Agg_LonPM180 URL: https://coastwatch.pfeg.noaa.gov/erddap/ ).

The raw data consisted of sea ice concentration (from 0 to 100%) which was the relative amount of a grid cell of 1/4° covered by ice. For a larger area it was calculated as the sum of the area of each cell multiplied by the fractional concentration for that cell. Sea ice extent defined a region as either ice-covered or uncovered as a binary term; a grid cell had ice (1) or no ice (0), as defined by the 15% concentration threshold. For each year from 1982 to 2024, we considered an annual period starting approximately around the date of mean sea ice extent minimum on February 16th; i.e., day 46 in year 1 to day 410 -or 411 in leap years- of year 2. We then calculated the timing of the seasonal sea ice cycle and annual sea ice extent and area. The derived covariates were: 1) day of advance: first day when sea ice concentration exceeded 15% for at least five consecutive days; 2) day of retreat: first day when sea ice concentration remained < 15% until the end of period; 3) ice season duration: total number of sea-ice days, between day of advance and retreat; 4) sea-ice persistence: percent time sea ice was present between day of advance and retreat; 5) day of minimum sea ice extent; 6) day of maximum extent; 7) day of minimum sea ice area; 8) day of maximum sea ice area; 9) minimum area of the ocean in km2 with at least 15% sea ice concentration; and 10) maximum area of the ocean with at least 15% sea ice concentration. If the sea ice concentration never decreased below this threshold, the days of advance and retreat were set to the lower and upper limits, respectively.

The R scripts used to derive this data can be found at: www.doi.org/10.5281/zenodo.15544181

Data collection:

x1 .csv

Data quality:

Data quality control is guaranteed by the satellite data providers, NOAA/NESDIS/NCEI; and the quality of the derived covariates used in the analysis responds to the peer review process of the associated manuscript, where the data was used. For further quality control purposes, the code used to download the raw data and analysis can be found at: www.doi.org/10.5281/zenodo.15544181.