Mapping coastal upwelling in the Baltic Sea from 2002 to 2020 using remote sensing data

Coastal upwelling (CU) is an elementary phenomenon in coastal waters. CU brings up deep, often cold, saline water rich of nutrients and carbon, and plays a vital role in local air-sea exchange of gases and heat, marine ecosystem maintenance, and ocean physical dynamics. In this study, regional CU in the Baltic Sea was mapped on the daily MODIS SST from 2002 to 2020, using a method modified developed by Lehmann et al. (2012). Based on the individual CU event detected, the spatiotemporal distribution of CU in the Baltic Sea was depicted, the CUwind relationship and potential CU drivers in the Baltic Sea on different temporal scale were analyzed. The results found that: 1) The modified approach can effectively delineate the CUs formed by upwelled cold water; 2) The 19 zones delineated with frequent CU occurrences aligned well with previous study and the overall CU occurrence spatial heterogeneity was casted by the different directional relationship between the local coastline and wind; 3) The occurrences of the CU detected in this study showed strong seasonality and primarily driven by SST seasonality and then intensified by local wind in fall; 4) The interannual difference of CU occurrences were affected by heatwaves and its monthly timing. The CUs detected in this study have a high potential for facilitating investigations with respect to oceanic modeling, air-sea exchange of heat and greenhouse gases, and physical dynamics in the Baltic Sea.