Effects of lateral processes on the seasonal water stratification of the Gulf of Finland: 3-D NEMO-based model study

Roman E Vankevich,Vladimir A Ryabchenko,Alexey V Isaev,Mikhail S Molchanov,Ekaterina V Sofina,Tatiana E Eremina

doi:10.5194/os-12-987-2016

Abstract

Abstract. This paper aims to fill the gaps in knowledge of processes affecting the seasonal water stratification in the Gulf of Finland (GOF). We used a state-of-the-art modelling framework NEMO (Nucleus for European Modelling of the Ocean) designed for oceanographic research, operational oceanography, seasonal forecasting, and climate studies to build an eddy-resolving model of the GOF. To evaluate the model skill and performance, two different solutions were obtained on 0.5 km eddy-resolving and commonly used 2 km grids for a 1-year simulation. We also explore the efficacy of non-hydrostatic effect (convection) parameterizations available in NEMO for coastal application. It is found that the solutions resolving submesoscales have a more complex mixed layer structure in the regions of the GOF directly affected by the upwelling/downwelling and intrusions from the open Baltic Sea. Presented model estimations of the upper mixed layer depth are in good agreement with in situ CTD (BED) data. A number of model sensitivity tests to the vertical mixing parameterization confirm the model's robustness. Further progress in the submesoscale process simulation and understanding is apparently not connected mainly with the finer resolution of the grids, but with the use of non-hydrostatic models because of the failure of the hydrostatic approach at submesoscale.

Highlights

The Gulf of Finland (GOF) is a 400 km long and 48–135 km wide sub-basin of the Baltic Sea with a mean depth of 37 m and complex bathymetry
During the summer season the water column in the deeper areas of the GOF consists of three layers – the upper mixed layer (UML), the cold intermediate layer, and a saltier and slightly warmer near-bottom layer, separated by two pycnoclines – the thermocline at the depths of 10–20 m and the permanent halocline at the depths of 60–70 m
This study shows that the most advanced 3-D circulation models are able to simulate the major features of the hydro-physical fields of the GOF

Summary

Introduction

The Gulf of Finland (GOF) is a 400 km long and 48–135 km wide sub-basin of the Baltic Sea with a mean depth of 37 m and complex bathymetry (see Fig. 1). The vertical stratification in the GOF as well as in the Baltic Sea is unusual (the thermocline and halocline are usually separated) with a pronounced and relatively stable halocline, whereas the temperature is largely controlled by the seasonal variability of the surface heat fluxes (see, e.g., Hankimo, 1964). During the summer season the water column in the deeper areas of the GOF consists of three layers – the upper mixed layer (UML), the cold intermediate layer, and a saltier and slightly warmer near-bottom layer (see Liblik and Lips, 2012), separated by two pycnoclines – the thermocline at the depths of 10–20 m and the permanent halocline at the depths of 60–70 m. The surface mixed layer reaches a maximum depth of 15–20 m by midsummer and an erosion of the thermocline starts in late August due to wind mixing and ther-

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Conclusion