A new conceptual framework for assessing the physical state of the Baltic Sea

Abstract

Abstract. Climate change is placing growing pressure on all parts of the ocean, increasing the need for regular information to support regional assessments and inform policy and decision-making. Understanding not only what is changing and where but also why is essential for effective response and meaningful action. To answer this need a new conceptual framework for the assessment of the physical state of the general natural water basin was introduced and then tested for the Baltic Sea. The approach is based on major process characteristics of the Baltic Sea and includes the analysis of mutual variability of well-established climate indicators such as ocean heat content (OHC), freshwater content (FWC), subsurface temperature, and salinity, combined with atmospheric forcing functions along with salt transport across the open boundaries as well as river runoff. A random forest model is used as the main analysis tool to enable statistical dependencies between state variables and potential forcing factors. Results reveal a clear 30-year warming trend in the Baltic Sea, closely linked on an interannual scale to 2 m air temperature, evaporation, and wind stress magnitude. The study highlights that interannual variations in temperature and salinity within the vertically extended halocline are key drivers of changes in OHC and FWC in the Baltic Sea. Interannual changes of FWC are explained by large-volume saline water inflows, net precipitation, and zonal wind stress. This framework also offers a new perspective of the potential impact of a shallowing mixed layer depth, resulting from sustained sensible heat flux changes at the air–sea interface, on salt export and the overall reduction of FWC in the Baltic Sea. This new framework could be applied to other geographical regions or future datasets, providing consistent information for a basin-wide monitoring tool that tracks the state and variability of the sea. Such a tool could be integrated into regional climate and environmental assessments.