The role of atmospheric correction errors in the underestimation of satellite-derived chlorophyll-a concentration in the Southern Ocean

Previous studies have consistently reported underestimation of satellite-derived chlorophyll- a concentration (CHL) in the Southern Ocean (SO). While this bias is often attributed to the region’s unique bio-optical properties, the potential contribution of atmospheric correction errors has been comparatively underexplored. This study evaluated the performance of three mainstream CHL retrieval algorithms (OC2, OC3, OCI) in high-latitude waters of the SO, mainly poleward of 63°S, using three types of remote-sensing reflectance ( R rs ) data: in-situ R rs , NASA-distributed standard VIIRS R rs , and VIIRS R rs obtained from a recently developed Cross-Satellite Atmospheric Correction (CSAC) algorithm. Our results show that when in-situ R rs were used as inputs, the mean absolute percent difference (MAPD) values between measured and estimated CHL by OC2, OC3, and OCI were 24.4%, 23.0%, and 30.2%, respectively. Using NASA-distributed VIIRS R rs increased the MAPD values to 41.7%, 50.4%, and 55.4%, respectively. In contrast, when CSAC-derived R rs were used, the MAPD values reduced to 25.4%, 22.5%, and 21.4%, respectively. This is because CSAC-derived R rs showed much stronger agreement with in-situ R rs than NASA-distributed R rs , with MAPD values reduced by approximately 50% across the visible bands. These results suggest that imperfect atmospheric correction plays a big role in the systematic underestimation of CHL in the SO from ocean color remote sensing. Additionally, CSAC was able to recover CHL retrievals under challenging observation conditions, such as moderate sunglint and straylight, which were previously masked in the standard VIIRS product, thereby substantially improving data coverage. Therefore, we anticipate that incorporating CSAC into the production of ocean color products will yield more accurate, better spatially covered products in this critical region.