Extended Data Figure 4: Temporal inhomogeneities in the NSIDC satellite sea-ice drift data.

a, Annual mean meridional sea-ice drift speed averaged over the entire sea-ice area (sea-ice concentration >50%). The thick orange lines show the spurious trends due to changes in the underlying data. The black lines shown the data corrected for inconsistencies and used in this study (1982–2008). b, Low-pass filtered (91 d running mean) sea-ice drift speed averaged over the entire sea-ice area (sea-ice concentration >50%). The grey lines show the reduced wind speed from ERA-Interim using a reduction factor from the period 1988–2008. The uncorrected data for each satellite instrument combination are shown in colour (dashed lines show the mean over the respective period). The black vertical lines show the periods of the channels. The coloured text denotes the sensors and the frequency of the microwave radiometer channels used. c, The fraction of sea-ice covered grid boxes with at least one drift vector observation in a 21-d window and a 75 km × 75 km grid box using the non-gridded NSIDC drift data. The colours indicate the contribution of each sensor and channel. d, Different combinations of instruments and passive microwave sensor channels and the related periods underlying the NSIDC sea-ice drift data. See Methods for details.