by Antti Westerlund1 and Laura Tuomi1
1Finnish Meteorological Institute, MArine Research, Helsinki, Finland
Westerlund A., Tuomi, L. : Vertical temperature dynamics in the Northern Baltic Sea based on 3D modelling and data from shallow-water Argo floats. Journal of Marine Systems, 158 (2016), 34-44. https://doi.org/10.1016/j.jmarsys.2016.01.006
The Bothnian Sea (BS, Figure 1) is a semi-enclosed basin in the Gulf of Bothnia, where the hydrography differs considerably from that of the other basins of the Baltic Sea and more globally of the World Ocean. The density stratification in the Baltic Sea is mostly determined by salinity, and the location of halocline and thermocline at different depths are challenging for the present hydrodynac models.
In this study, the 3D hydrodynamic model NEMO is used to simulate vertical temperature dynamics in the Bothnian Sea for the summers 2012 and 2013. Argo floats deployed in shallow and low salinity waters of the Bothnian Sea by FMI in 2012-2013 are used to evaluate the model results.
DATA AND METHOD
NEMO 3D ocean model version 3.6 has been set up at FMI for the Baltic Sea, based on the NEMO Nordic configuration by Hordoir et al. (2013a,b, 2015). The model was run for the years 2012 and 2013. Temperature data from the model was saved as 3-h averages for surface fields and daily averages for full profiles. Initial conditions for both runs were taken from the FMI operational ocean model HBM-FMI (Berg and Poulsen, 2012), which provides a daily model output for roughly the same domain as the NEMO Nordic configuration.
Since 2012, the FMI has deployed sixteen Argo floats into the shallow and low salinity Baltic Sea. Observations from those autonomous Argo floats, together with other in-situ observations collected from moored buoys or monitoring data, were used to study the accuracy of the model results, especially in estimating mixed layer depths and vertical temperature structure.
Data from Argo floats were collected during two separate missions in 2012 and 2013. The first mission lasted approximately six months from 17 May 2012 to 5 Dec. 2012, during which around 200 vertical profiles of temperature and salinity were collected. The second mission lasted approximately four months from 13 Jun. 2012 to 2 Oct. 2013 with over 100 acquired profiles. Argo tracks and locations of the profiles are shown in Figure 1. In the Baltic Sea, where the average depth is 54 m and the deepest spot - the Landsort deep - is 459 m, the Argo floats are equipped with a two-way satellite communication in order to be able to change the diving parameters of the float during its mission.
Figure 2a and 2b show measured and modelled temperature profiles during Argo measurement campaigns in the Bothnian Sea in 2012 and 2013. In both years, seasonal changes in the water masses - warming of the surface layer during the summer time and mixing of the water column in fall - can be easily detected.
NEMO was able to reproduce the vertical structure of temperature near the surface, when the atmospheric forcing was sufficiently accurate. The deepening of the thermocline as well as the temperature gradient was well represented by the model. However, the warming of the surface layer in spring was slower in the modelled than in the measured profiles. Moreover, the layers below the thermocline had a warm bias, and the dicothermal layer or the old winter water layer was not as pronounced as in the measurements. The most probable reason behind this is the combination of initial conditions, limited vertical resolution, and over-mixing in the deeper layers. Overall, temperature gradients in the model were gentler than measured and some fine-scale features were not correctly reproduced.
Three vertical turbulence parameterisations have been tested (not shown) in order to determine their accuracy in reproducing the measured dynamics with new shallow-water Argo floats, operational in the Bothnian Sea since 2012: k-ε and k-ω with CB (Craig and Banner) parametrization and k-ε without CB parameterisation.
The k-ε and k-ω schemes showed clear differences in simulating the surface layer dynamics in the study area, but neither proved superior. While SST was better simulated with the k-ω scheme, the k-ε scheme was clearly better at simulating the thermocline depth. As the use of CB parameterisations improved the description of thermocline depth, including the wave effects is clearly important in the Bothnian Sea. However, the use of CB parameterisations led to excessively cool SST at the beginning of the summer. Thus, more sophisticated methods of including surface wave effects in NEMO Nordic are recommended.
The Gulf of Bothnia has been one of the least investigated sub-basins of the Baltic Sea. There has been a renewed interest in the Gulf of Bothnia, as ecosystem health indicators and eutrophication-related parameters have shown a decline in the eutrophication status of the area. Warming trends in surface air and sea temperatures seem stronger in the Gulf of Bothnia than elsewhere in the Baltic Sea, but relatively large uncertainties remain in terms of the impact of climate change on the biogeochemistry of the area.
In addition to monitoring by in-situ observations, evaluations based on modelling studies are needed. In this study, the autonomous Argo floats have proved their usefulness in the development of numerical models for the Baltic Sea.
Overall, the model was able to reproduce the large-scale seasonal variations in temperature in the area, provided that good quality meteorological forcing was available. The seasonal thermocline gradients and depths are depicted by the model with sufficient accuracy to provide a good basis for coupling it with a biogeochemical model. The new dataset produced by the autonomous shallow-water Argo floats in the Baltic Sea is also valuable for model validation and performance evaluation.