Analysis of Vertical Structure of Temperature in the Baltic Sea based on 3D
Modelling and Data from Shallow-Water Argo Floats
Antti Westerlund1 and Laura Tuomi1
1
Finnish Meteorological Institute, Helsinki, Finland
Vertical mixing is a challenging issue in ocean modelling. Several studies have shown
that 3D hydrodynamic models often produce considerable errors in mixed-layer depths
regardless of chosen vertical turbulence parameterisations. These errors can be especially
pronounced in areas with complex hydrography. For example, in the Baltic Sea there are high
horizontal and vertical salinity gradients. Furthermore thermocline and halocline are located at
different depths. This produces stratification conditions challenging for all ocean models, see e.g.
[1].
In this work vertical structure of temperature is studied in the Baltic Sea with modelling
experiments and new observational data. NEMO 3D ocean model has been set up at Finnish
Meteorological Institute (FMI) for the Baltic Sea, based on the NEMO NORDIC configuration
[2], using FMI-HIRLAM atmospheric forcing. The model has been run for a Baltic Sea - North
Sea grid with 2 nautical mile resolution and 56 vertical layers.
The observational data for Baltic Sea off-shore areas is very sparse and new methods are
needed to collect data for model validation and development. FMI has been testing Argo floats in
the Baltic Sea since 2011 in order to increase the amount of observed vertical profiles of salinity
and temperature. This is the first time Argo floats have been successfully used in the brackish,
shallow waters of the Baltic Sea. This new data set is well suited for evaluating the capability of
hydrodynamic models to produce the vertical structure of temperature. It provides a time series
of profiles from the area of interest with good temporal resolution, showing the structure of
temperature in the water column throughout the summer.
Preliminary comparison of model results to Argo measurements shows that the model is
able to describe the basic features of the vertical structure of temperature in the Bothnian Sea,
Baltic Sea. Thermocline depths are reproduced with good accuracy, although measurements
show steeper gradients in temperatures than the model. The old winter water or dicothermal layer
is not produced correctly. Spring warming and autumn cooling in the mixed-layer seems to be
slightly delayed in the model. Necessary model improvements and parameterisations of vertical
mixing available in the model are discussed in the light of the results.
[1] Tuomi, L., Myrberg, K., & Lehmann, A. Continental Shelf Research, 50, 64-79 (2012)
[2] Hordoir, R., Dieterich, C., Basu, C., Dietze, H., & Meier, H. E. M. Continental Shelf
Research, 64, 1-9 (2013)