Multidecadal loss of surface thermal structure in the largest marine upwelling ecosystems

Step horizontal gradients of sea surface temperature (SST) occur around upwelling cores, eddies, meanders, current boundaries, island-effect mixing areas, among many other oceanographic features. These thermally structured areas provide ideal turbulence for phytoplankton growth and biomass aggregation, triggering complex and abundant food webs, in turn exploited by populations of marine megafauna and fisheries. How the distribution and degree of this surface heterogeneity has varied at climate-change scales is unknown. In this study, we prove that the horizontal surface thermal structure of the ocean has declined steadily in the most important upwelling ecosystems during the last 41 years (1982-2022: -0.1°C of SST standard deviation within 0.25x0.25-dregree cells at the seasonal upwelling peak). Years with low thermal structure showed remaining hotspots towards upwelling cores, close to shore. The mechanisms of this long-term decline remain unclear. The correlation with the mean SST itself was negative but non-consistent among upwelling regions, while the negative correlation with the steadily increasing absolute dynamic topography was strong in all regions. This points to a multidecadal heat content gain along the water column as a potential cause of the homogenization process. Arguably, this loss in sea surface thermal structure could be related to described declines and redistributions of some marine megafauna species.