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On the buoyant sub-surface salinity maxima in the Gulf of Riga

Liblik T., Skudra M., Lips U.

Oceanologia

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2017

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No. 59 (2)

113--128

EN

Thermohaline structure in the Gulf of Riga (GoR) was investigated by a multi-platform measurement campaign in summer 2015. Stratification of the water column was mainly controlled by the temperature while salinity had only a minor contribution. Buoyant salinity maxima with variable strength were observed in the intermediate layer of the Gulf of Riga. The salinity maxima were likely formed by a simultaneous upwelling–downwelling event at the two opposite sides of the Irbe strait. The inflowing salty water did not reach the deeper (> 35 m) parts of the gulf and, therefore, the near-bottom layer of the gulf remained isolated throughout the summer. Thus, the lateral water exchange regime in the near bottom layer of the Gulf of Riga is more complicated than it was thought previously. We suggest that the occurrence of this type of water exchange resulting in a buoyant inflow and lack of lateral transport into the near-bottom layers might contribute to the rapid seasonal oxygen decline in the Gulf of Riga.

2

Characteristics and inter-annual changes in temperature, salinity and density distribution in the Gulf of Riga

Skudra M., Lips U.

Oceanologia

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2017

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No. 59 (1)

37--48

EN

Available CTD profiles from the Gulf of Riga (May–August, 1993–2012) were analyzed to study inter-annual and long-term changes in temperature, salinity and density in relation to river runoff and atmospheric forcing (e.g. Baltic Sea Index). To describe temporal changes in vertical stratification, the upper mixed layer (UML) and deep layer (DL) parameters were estimated. On average the UML depth increases from 8.7 m in May to 9.0, 11.5 and 13.7 m in June, July and August, respectively, and the UML temperature increases from 8.0°C to 12.5, 18.7 and 18.6°C (May, June, July and August) while the UML salinity increases from 4.90 g kg−1 to 5.14, 5.28 and 5.38 g kg−1, respectively. High correlation (r = −0.82) was found between the inter-annual changes in river runoff (spring) and mean salinity in the UML in August as well as between DL mean salinity (r = 0.88) and density (r = 0.84) in the Irbe Strait and DL mean salinity and density in the Gulf of Riga. Inter-annual changes in the UML depth as well as in DL salinity and density had a significant correlation with the changes in Baltic Sea Index. The strongest stratification (August) can be observed in the years with the highest UML temperature and the highest river run-off in spring. We suggest that the predicted increase in water temperature and changes in river run-off due to the climate change would result in faster development of the seasonal thermocline in spring and stronger vertical stratification in summer.

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.