Germo Väli

Optical properties of north-eastern Baltic Sea in spring and summer 2007

Underwater irradiation profiles in the north-eastern Baltic Sea near the Estonian northern and north- western coast, near Helsinki, and in the central part of Gulf of Finland were measured in spring and summer 2007. The vertical profiles of downwelling and scalar irradiance in the PAR region were measured in situ using a frame completed with two planar and a spherical PAR sensor. The measuring system allows the calculation of the mean attenuation coefficients of a water column of scalar irradiance (Ko). Optical density of the sea water was varying from 0.13 m-1 (Tallinn-Helsinki line, May) up to 0.85 m-1 (Tallinn-Helsinki line, April). The concentrations of optically active substances were highly variable; the chlorophyll a concentration varied from 0.7-21.4 mg m-3, the suspended particulate matter concentration from 1-7 mg l-1 and the concentration of dissolved organic matter from 2.1-5.8 mg l-1. The water transparency was much better in May compared to April 2007. Also, it is seen, that excepting the stations immediately near the port and ship line, the Gulf of Finland was quite clear, especially in May. Correlations between the water depth and optical density were calculated for the shallows near Hiiumaa. Water quality based on measurements conducted in summer 2007 at North-Western Estonian coastal waters indicates that the water quality was on average satisfactory; associating values for diffuse attenuation coefficient was 0.5 m-1, Secchi depth 3.7 m and chlorophyll a concentration 8 mg m-3.

Pathways of suspended particles released in the bottom boundary layer of the Bornholm Deep, Baltic Sea (numerical simulations)

A model system consisting of a circulation model and a random-walk model is developed to simulate suspended particulate matter transport in the bottom boundary layer (BBL) of the southern Baltic Sea. The circulation model is based on POM, the Princeton Ocean Model, in which the vertical grid size is refined towards the bottom in order to resolve BBL properly. 3D fields of velocity, vertical and lateral apparent diffusivities generated by the circulation model are used as an input for the random walk model to simulate transport and dispersion of particles with prescribed settling velocity. The random-walk scheme allows for non-uniform vertical profiles of the vertical apparent diffusivity, and test runs have been done to make sure that the model does not display unrealistic removal of particles from highly turbulent BBL and further accumulation in low-diffusivity above-lying layers. A number of numerical experiments have been performed to study pathways of suspended particles released in the BBL in the centre of the Bornholm Deep at different wind conditions. At northerly and easterly winds the particles initially move westward and then get involved into either northern or southern detours around the Deep. The particles from the northern detour are finally absorbed into the Slupsk Furrow while those of the southern detour do not enter the Furrow keeping on cyclonic rotation within the Bornholm Basin. To the contrast, for the westerly and southerly wind conditions the particles move to the northeast for some 20 km and then get involved into the cyclonic rotation. The cyclonic rotation implies the convergence of currents in the BBL due the Ekman transport and, in view of continuity, the upwelling, so that the particles will remain trapped within the Bornholm Deep if the settling velocity is large enough to overcome the upwelling. Since the westerly wind conditions dominate in the climatic sense, the trapping effect may be considered as an important factor that controls dispersion of chemical warfare agents dumped in the Bornholm Deep after the World War II.

Features of summertime circulation in the Gulf of Riga (A numerical simulation)

A regional, 0.5 n.m. and 20 sigma layer grid model of the Gulf of Riga (GoR) was implemented based on the Princeton Ocean Model code with initial conditions and atmospheric forcing taken from HIROMB and HIRLAM output. A basic run that covers an ice-free period of May 1-December 31, 2012, brought the following results: - the mean summertime surface circulation in GoR displays a whole-basin anticyclonic gyre with more intense currents in the western half of the gyre; - two seasonal currents are identified: the Northern Longshore Current (NLC) in the western part of GoR and the Southern Subsurface Longshore Current in the eastern part of GoR; - in the cold season of the year (when seasonal pycnocline is absent), the anticyclonic gyre in GoR is replaced by a cyclonic gyre. The seasonal change in sign of the whole-basin surface circulation in GoR is likely controlled by the NLC, a baroclinic jet-like current formed along the western shore in summer due to the prevalence of southwesterly, upwelling-favored winds, and presence of seasonal stratification that can be approximated as a two-layer vertical structure; this baroclinic jet disappears in winter when the seasonal pycnocline is absent.