Satellite input changes step the spatial detail of DOISST v2.1 and ESA CCI v3 daily SST and cause most 1982-2021 trends of SST front fraction and small-scale variance (60S-60N)

Question: Earlier studies found that NOAA DOISST v2.1 lost day-to-day variance in steps at satellite input changes. Do the input changes also change the spatial detail of daily SST analyses, and how much of the 1982-2021 change of SST fronts and gradients do they cause? Approach: We calculated gradient, front-fraction and small-scale-variance statistics for every daily DOISST field in 1982-2025 on the full 0.25-degree grid (465,873 ice-free pixels, 60S-60N). We tested steps at input-change dates against all other dates. We repeated the statistics on the same pixels for the satellite-only ESA SST CCI Analysis v3.0.1 (1,020 sampled days, 1982-2024) and compared both with 148 pairs of moorings 100-400 km apart. Results: DOISST spatial detail steps at its input changes, and the sign depends on the region. At 2007-01-01 the small-scale variance changed by x2.8 in the Gulf Stream box and by x0.74 in 20S-20N; at 2021-11-01 by x2.2 and x0.71. The steps decrease with spatial scale. CCI has its own steps at its dual-view sensor dates: ATSR-1 start (x1.29), end of Envisat in April 2012 (x0.83) and SLSTR-A start (x1.24). Moorings confirm both: relative to the buoys, the DOISST 1-day change of the 100-400 km SST difference fell by 0.32 ln units at 2007 (113 of 129 pairs) and by 0.75 at 2022, and the CCI difference variance at mid-latitude buoys rose by 0.22 and 0.18 when ATSR and SLSTR data started. The steps cause most trends: the CCI global front-fraction trend fell from +9.7% to +3.8% per decade with three a-priori sensor step terms (+1.3% inside 1992-2011), and DOISST boundary-current gradient trends of +4% to +25% per decade became -7% to -1% with one 2007 step term. After step terms, both products still show a decrease of the mean gradient, mainly in the tropics; its cause is not known. Limitations: one threshold front definition on a 0.25-degree grid; CCI sampled on 2 days per month and as point samples; approximate CCI sensor dates; buoys are DOISST inputs and no mooring pair is in a boundary-current front; 60S-60N only.