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Variability of currents over the southern slope of the Gulf of Finland

Suhhova I., Pavelson J., Lagemaa P.

Oceanologia

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2015

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No. 57 (2)

132--143

EN

In our intraseasonal variability studies of currents in the coastal sea of the Gulf of Finland northeast of Pakri Peninsula, we compared the observation data from a bottom-mounted ADCP (March–June of 2009, 50 m depth) with the simulation data from High Resolution Operational Model of the Baltic (HIROMB). The structure of the current pattern appeared strongly dependent on the stratification conditions. The flow was quasi-barotropic during the periods of weak inverse thermal stratification at the end of winter season and at transition from the inverse thermal stratification to summer type stratification when the sea was thermally unstratified, but mostly two-layered (baroclinic) when the summer type thermal stratification had developed. The alternation of strong westward (eastward) currents (up to 30 cm s-1) in the upper layer is explained in terms of coastal upwelling (downwelling) due to favourable background winds. The measured and the modelled upper layers along isobath currents showed a noticeable correlation with the correlation coefficient of 0.52 and 0.82 during the periods of winter type and summer type stratifications, respectively, and the absence of a significant correlation during the transition period. The eastward (upwind) current episodes with speeds reaching 18 cm s-1 below the seasonal thermocline are likely to reflect the specific circulation response in the elongated basin caused by the easterly wind. The long-term mean (over 3.5 months) current vector (−2.0 cm s-1, −2.9 cm s-1) was westward in the upper sea and eastward, nearly along isobaths (1.1 cm s-1, −0.3 cm s-1) in the deeper layers.

2

Flow, waves and water exchange in the Suur Strait, Gulf of Riga, in 2008

Raudsepp U., Laanemets J., Haran G., Alari V., Pavelson J., Köuts T.

Oceanologia

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2011

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No. 53 (1)

35-56

EN

Wind, flow and wave measurements were performed in November-December in 2008 in the relatively narrow and shallow Suur Strait connecting the waters of the Väinameri and the Gulf of Riga. During the measurement period wind conditions were extremely variable, including a severe storm on 23 November. The flow speed along the strait varied between ±0.2 m s-1, except for the 0.4 m s-1 that occurred after the storm as a result of the sea level gradient. The mean and maximum significant wave heights were 0.53 m and 1.6 m respectively. Because of their longer fetch, southerly winds generated higher waves in the strait than winds from the north. All wave events caused by the stronger southerly winds induced sediment resuspension, whereas the current-induced shear velocity slightly exceeded the critical value for resuspension only when the current speed was 0.4 m s-1. A triple-nested two-dimensional high resolution (100 m in the Suur Strait) circulation model and the SWAN wave model were used to simulate water exchange in 2008 and the wave-induced shear velocity field in the Suur Strait respectively. Circulation model simulations demonstrated that water exchange was highly variable, that cumulative transport followed an evident seasonal cycle, and that there was an gross annual outflow of 23 km3 from the Gulf of Riga. The horizontal distribution of wave-induced shear velocity during the strong southerly wind event indicated large shear velocities and substantial horizontal variability. The shear velocities were less than the critical value for resuspension in the deep area of the Suur Strait.

3

Laanemets J., Pavelson J., Lips U., Kononen K.

Oceanological and Hydrobiological Studies

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2005

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Vol. 34, No.2

15--36

EN

Hydrographic (towed CTD) and acoustic Doppler current profiler (ADCP) velocity surveys were conducted daily aboard the RV Aranda from July 15 to 26, 1996 at the entrance to the Gulf of Finland, Baltic Sea. Strong alongshore wind forcing that lasted two days caused an intensive downwelling event north of Hiiumaa Island with an approximate 20 m onshore descent of the thermocline. The associated eastward downwelling jet (~30 cm s-1, width 8-12 km) developed into an anticyclonic eddy with a diameter of ~20 km. A strong jet (~35 cm s-1, width 4-6 km) was observed in the periphery of the anticyclonic eddy, centered at the depth of reversal in baroclinicity. The geostrophic streamfunctions were derived from ADCP data and combined with the CTD density field to study the variations of isopycnal potential vorticity. The variation of relative vorticity from -0.95f to 1.2f and five-fold changes in the thickness of the selected isopycnal band caused up to fifty-fold variation of isopycnal potential vorticity over the survey area. The distribution of isopycnal potential vorticity as a conservative property correlated well with the isopycnal salinity distribution. The maximum upward and downward velocities, 35 and 26 m d-1, correspondingly, were estimated through the divergence of the Q-vector using the ω-equation diagnostic technique.