The significance of global warming on thermodynamic characteristics of the Baltic sea

Abstract The impact of global warming on the heat content of the Baltic Sea was explored during the years 2000–2009 in terms of entropy and enthalpy parameters derived using a validated 3-D hydrodynamic model and the Gibbs thermodynamic potential of seawater. The Finite-Time Lyapunov Exponent method was also employed to detect Lagrangian coherent structures (LCSs) in vertical planes. At depths less than 50 m, the enthalpy increased by a factor of 3.2 at a rate of 2.73 J/kg per day. The rate of 0.97 J/kg per day was substantially lower in the surface waters. The mean monthly enthalpy in the upper water layers showed strong seasonal characteristics, which differed from the accumulation pattern in the deeper layers. The 37-year in situ water temperature records revealed an increase in the surface layers and layers below 80 m of 0.38–1.34 °C and 0.32–2.05 °C, respectively. Between 1957 and 2021, the extent of ice decreased significantly because of rising temperatures. New findings were that the LCSs correlated with regions of vertical advective heat transfer and that a plausible mechanism for the transfer of excess surface heat to the deeper water layer has been developed. The structures transferred 40–90% of the surface water heat to the deeper water layers. Unexpectedly rising temperatures and heat content in deeper water layers may be a key sign of global warming’s effects, with significant environmental implications for the ecology and marine habitats of the Baltic Sea.