A segmented-breakpoint sea-surface-temperature upwelling index for the Benguela Upwelling System

Abstract. Satellite sea-surface-temperature (SST) upwelling indices usually contrast coastal temperature with a fixed offshore reference, although the offshore extent of the surface cool tongue varies across cells and days. We replace the fixed reference with a daily fitted thermal breakpoint from a continuous two-segment regression on cross-shore SST profiles, accepted only when model-evidence, slope-ratio, offshore-baseline, edge-buffer, and residual-autocorrelation criteria are met. We apply the method to five Benguela upwelling cells (UCs) from 2015 to 2025 using two fine-resolution Level-4 SST products, namely the Operational Sea Surface Temperature and Ice Analysis and the Geo-Polar Blended analysis from the US National Oceanic and Atmospheric Administration; coarser Advanced Very High Resolution Radiometer and microwave optimum interpolation products serve as resolution comparisons. Median fitted breakpoints span 114 to 270 km across UCs, so 50 to 100 km reference points usually sample inside the fitted inshore segment rather than offshore water. In the Operational Sea Surface Temperature and Ice Analysis product, the daily correlation between inshore-segment SST drawdown and the conventional fixed-distance contrast rises from r = 0.06 at 50 km to r = 0.93 at 500 km, showing that reference distance changes the diagnostic itself. The Lüderitz annual r₅₀ point estimate is negative (-0.16), but its block-bootstrap interval includes zero. A paired comparison with a Bakun-type offshore-Ekman-transport flag shows that southern Benguela cells retain SST structure without the local wind flag on ca. one fifth of matched days. The method therefore positions fixed-reference SST indices as structural surface-temperature indicators whose interpretation depends on cell-scale geometry and SST-wind co-activity.