No abstract

Safe navigation in complex archipelagos requires knowledge and understanding of oceanographic conditions in the fairways. We have studied oceanographic conditions and their relation to weather in a crossing in the Finnish archipelago, which is known to have events when strong currents affect marine traffic. Our main dataset is ADCP (Acoustic Doppler Current Profiler) current measurements, done in the cross section of five months in 2013. We found that the local currents flow mainly to two directions, either to north-northeast (NNE) or to south-southwest (SSW), which is nearly perpendicular to the deepest fairway in the area. The mean value of the currents in the surface layer was 0.087 ms - 1 , but during the high wind situations, the current speed rose over 0.4 ms - 1 . These strong currents were also shown, according to AIS (Automatic Identification System) data, to cause drift of the vessels passing the cross section, though the effect of wind and current to the ship may sometimes be hard to separate. We studied whether the strong currents could be predicted from routine observations of wind and sea level available in the area, and we found that prediction of these currents is possible to some extent. We also found that winds of over 10 ms - 1 blowing from NW (300 ∘ –350 ∘ ) and SE (135 ∘ –180 ∘ ) generated strong currents of over 0.2 ms - 1 , whereas most commonly measured winds from SW (190 ∘ –275 ∘ ) did not generate currents even with winds as high as 15 ms - 1 .

Currents in the Baltic Sea are generally weak, but during strong winds they can grow high enough to affect the surface wave propagation and evolution. To evaluate the significance of wave-current interactions in the Baltic Sea, we conducted a study using the wave model WAM, comparing a run without surface currents to one with current forcing from a NEMO hydrodynamical model simulation. The overall changes to the wave field caused by currents were quite small. Changes of over 10 cm in significant wave height (SWH) or 1 s in the peak period (Tp) occurred only in some areas and typically less than 3% of the time. Current refraction changed the SWH annual mean by up to 2 cm, but changes up to 60 cm were seen in the maximum values. Tp had occasionally large changes due to shifts in the peak energy in two-peaked swell and wind-sea spectra. Including currents typically led to a stronger changes in swell energy compared to the changes in wind sea energy. A comparison with a wave buoy in the Gulf of Finland showed that this change in the swell energy improved the accuracy of the simulation in this narrow gulf. Current-induced refraction was most prominent near the coastal areas, where current speed occasionally exceeded 0.3 m/s. In general, SWH decreased in the coastal areas with strong currents and slightly increased in adjacent open sea areas. The current effects were most frequent in the Gulf of Finland, the Western Gotland Basin and the Åland Sea.

No abstract

<p>Sea floor erosion can be induced by waves, bottom currents and ice. Although the Gulf of Bothnia in the Baltic Sea is a relatively small basin, the record value of significant wave height is 8.1 m with highest individual wave of 15 m. In the present climate the seasonal ice cover limits the wave growth during winter time, but in the future climate it is estimated that ice extent will reduce which can lead to more severe wave climate. Thus, the effect of waves on the bottom sediment erosion is expected to increase. We used wave model WAVEWATCH III to do a 30-year high-resolution hindcast for the Gulf of Bothnia. The hindcast wave parameters were validated against wave buoy and altimeter wave measurements to ensure good quality of the wave hindcast. The hindcast near-bottom orbital velocities and amplitudes were used to estimate wave-induced bottom shear stress. These calculations are based on the wave spectra, taking into account the effect of different wave heights and wave lengths. The results were used to evaluate the extent of areas that experience significant wave-induced bottom stress under the present climate. Furthermore, the results show how often and for how long periods the wave-induced stress exceeds the critical values for sediment resuspension to take place. The estimates of the critical values for resuspension are calculated utilising the seabed sediment data available for the Gulf of Bothnia. The adequacy of the results is evaluated by comparing the known erosional seafloor areas to the ones estmated based on the hindcast values. This study is part of the SmartSea project of the Strategic Research Council of the Academy of Finland (grant no. 292 985).</p>