The 2023 Major Baltic Inflow Event Observed by SWOT Altimetry

The Baltic Sea is an intercontinental marginal sea that is vertically stratified with a strong halocline isolating the saline bottom layer from the brackish surface layer. The surface layer is eutrophic and abiotic zones lacking oxygen are common in the deeper regions. While freshwater is constantly flowing into the North Sea, oxygen-rich bottom waters can only occasionally enter the Baltic following a special sequence of transient weather conditions. These so-called Major Baltic Inflow events can be monitored via the sea level gradients between the Kattegat and the western Baltic Sea. Innovative interferometric altimetry from the Surface Water and Ocean Topography (SWOT) mission give us the first opportunity to directly observe the sea level signal associated with the inflow in December 2023. In addition, we use observations from recent high-rate multi-mission nadir altimetry. For scales larger than 50 km, SWOT and nadir altimetry are in very good agreement. The SWOT observations are compared to the simulations with the regional 3D HBMnoku ocean circulation model operated by the German Federal Maritime and Hydrographic Agency (BSH). Both agree very well for most aspects. The north-south gradients of the two data sets differ by about 10% of the total value. Comparison with tide gauges suggests that there may be model deficiencies on daily to sub-daily time scales. In addition, the SWOT data have many fine scale structures such as eddies and fronts that cannot be modelled adequately.