Vertical temperature dynamics in the Northern Baltic Sea based on 3D modelling and data from shallow-water Argo floats

• NEMO Nordic hydrodynamic model was used for the Bothnian Sea, Baltic Sea.
• The model was able to reproduce large-scale seasonal variations in temperature.
• The k − ε and k − ω mixing schemes were studied.
• The schemes showed clear differences in simulating the surface layer dynamics.
• The new shallow-water Argo floats are valuable for model validation.

3D hydrodynamic models often produce errors in the depth of the mixed layer and the vertical density structure. We used the 3D hydrodynamic model NEMO to investigate the effect of vertical turbulence parameterisations on seasonal temperature dynamics in the Bothnian Sea, Baltic Sea for the years 2012 and 2013. We used vertical profiles from new shallow-water Argo floats, operational in the area since 2012, to validate our model. We found that NEMO was able to reproduce the general features of the seasonal temperature variations in the study area, when meteorological forcing was accurate. The k-ε and k-ω schemes were selected for a more detailed analysis. Both schemes showed clear differences, but neither proved superior. While sea surface temperature was better simulated with the k-ω scheme, thermocline depth was clearly better with the k-ε scheme. We investigated the effect of wave-breaking on the mixing of the surface layer. The Craig and Banner parameterisation clearly improved the representation of thermocline depth. However, further tuning of the mixing parameterisations for the Baltic Sea is needed to better simulate the vertical temperature structure. We found the autonomous Baltic Sea Argo floats valuable for model validation and performance evaluation.