Transport of warm bias from Indian Ocean subsurface to Southern Ocean surface in Coupled Model Intercomparison Project phase 6 models

Models from the Coupled Model Intercomparison Project phase 6 exhibit a persistent warm bias in the Southern Ocean, which hinders their ability to accurately capture local and global climate variability. Here we demonstrate that the subsurface Indian Ocean warm bias—averaging 1.80 °C in the multi-model ensemble mean and present in all 39 models—accounts for approximately 37% of sea surface temperature warm bias in the Southern Ocean. Both inter-model regression analyses and numerical sensitivity experiments indicate that this subsurface Indian Ocean warm bias is transported southward by the Agulhas Current and eastward by the Agulhas Return Current and the Antarctic Circumpolar Current. This transport, coupled with vertical mixing and upwelling, exacerbates the sea surface temperature bias in the Southern Ocean. Our findings underscore the necessity for improved representation of Indian Ocean subsurface processes to enhance sea surface temperature predictions in the Southern Ocean and, by extension, global climate projections. Modelled subsurface Indian Ocean warm bias accounts for approximately 37% of sea surface temperature warm bias in the Southern Ocean, according to analysis of Coupled Model Intercomparison Project results.