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2 2005

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

Oceanological and Hydrobiological Studies

2005

Vol. 34, No.2

15--36

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.

Coastal up- and downwelling in the southern Baltic

Kowalewski M., Ostrowski M.

Oceanologia

2005

No. 47 (4)

453-475

A three-dimensional hydrodynamic model was used to determine 12 zones of up- and downwelling in the southern part of the Baltic Sea. On the basis of a seven-year numerical simulation, the annual frequency of up- and downwelling events in various regions was analysed, their vertical velocity evaluated and the probability of their occurrence for different wind directions calculated. Verification of the model results demonstrated their good correspondence with satellite images, on average equal to 92%. The poorest consistency was recorded for upwelling in the Bornholm region (81%). The annual average frequency of strong upwelling (velocities >10-4 m s-1) ranged from c. 5-7% off the eastern coasts of the southern Baltic to >31% along the north-eastern coast of Bornholm. Along the Polish coast (excepting the Vistula Spit) downwelling was recorded more frequently than upwelling. The frequency of strong vertical currents was highest in the area to the north of the Hel Peninsula, where high percentages of strong upwelling (27.1%) and downwelling (37.1%) were recorded.