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 (T_p) 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. T_p 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.

The Archipelago Sea (in the Baltic Sea) is characterised by thousands of islands of various sizes and steep gradients of the bottom topography. Together with the much deeper Åland Sea, the Archipelago Sea acts as a pathway to the water exchange between the neighbouring basins, Baltic proper and Bothnian Sea. We studied circulation and water transports in the Archipelago Sea using a new configuration of the NEMO 3D hydrodynamic model that covers the Åland Sea–Archipelago Sea region with a horizontal resolution of around 500 m. The results show that currents are steered by the geometry of the islands and straits and the bottom topography. Currents are highest and strongly aligned in the narrow channels in the northern part of the area, with the directions alternating between south and north. In more open areas, the currents are weaker with wider directional distribution. During our study period of 2013–2017, southward currents were more frequent in the surface layer. In the bottom layer, in areas deeper than 25 m, northward currents dominated in the southern part of the Archipelago Sea, while in the northern part southward and northward currents were more evenly represented. Due to the variation in current directions, both northward and southward transports occur. During our study period, the net transport in the upper 20 m layer was southward. Below 20 m depth, the net transport was southward at the northern edge and northward at the southern edge of the Archipelago Sea. There were seasonal and inter-annual variations in the transport volumes and directions in the upper layer. Southward transport was usually largest in spring and summer months, and northward transport was largest in autumn and winter months. The transport dynamics in the Archipelago Sea show different variabilities in the north and south. A single transect cannot describe water transport through the whole area in all cases. Further studies on the water exchange processes between the Baltic proper and the Bothnian Sea through the Archipelago Sea would benefit from using a two-way nested model set-up for the region.

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 ${}^{\circ }$ –350 ${}^{\circ }$ ) and SE (135 ${}^{\circ }$ –180 ${}^{\circ }$ ) generated strong currents of over 0.2 ms ^$-1$ , whereas most commonly measured winds from SW (190 ${}^{\circ }$ –275 ${}^{\circ }$ ) did not generate currents even with winds as high as 15 ms ^$-1$ .

Atmospheric ducting can cause a radar to detect over its standard horizon. Two years of X-band coastal surveillance radar imagery from the island of Utö in the Baltic Sea was processed and analyzed to study the statistics of visibility over the standard horizon of the radar. The results were compared to the modified refractivity profiles and weather observations measured at Utö. In total, 275 ducting events were observed with the Utö coastal surveillance radar and the median duration of an event was 4 h. Over-the-horizon observation statistics from the Utö coastal surveillance radar show that ducting occurs most frequently during spring (MAM) and summer (JJA) months, 37% and 42% of the time, respectively. A peak is observed in the evening toward the night during both seasons. Ducting was significantly less frequent during the autumn (SON) and winter (DJF) months, 1% and 2% of the time, respectively. The corresponding frequencies from the modified refractivity profiles were 31% for spring, 56% for summer, 40% for autumn, and 16% for winter months. During the winter and autumn months, a shallow surface duct was often present in the profiles. The presence of this weak duct did not always result in over-the-horizon observations with the Utö coastal surveillance radar but could affect surveillance radars that are closer to the sea surface or radar systems operating at a different frequency to that of the Utö X-band radar. Significance Statement: In our article, we explore how an X-band coastal surveillance radar can be used to study atmospheric ducting, a phenomenon where radar signals can travel farther than usual. This is important for understanding how radars work in different weather conditions and how often the conditions occur that allow radars to see farther. Our study takes place on Utö, an island at the southern edge of the Archipelago Sea in the Baltic Sea. This location allows us to study ducting over both open sea and complex island environments. To our knowledge, this study presents long-term ducting statistics for the first time based on an X-band coastal surveillance radar.

The Archipelago Sea (AS) in the Baltic Sea is a complicated fragmented sea area with numerous small islands and islets that are crossed by several deeper straits. The area functions as a major route for both transport and leisure activities on the sea as well as many other forms of the blue economy. Even with the large number of maritime activities in this sea area, knowledge of the currents along the deep channels crossing the AS has been limited due to the lack of quality-assured measurements. To enhance the general understanding of the dynamics in the AS, we have collected and analysed currents in 10 different locations across the AS using acoustic Doppler current profiler (ADCP) current measurements made during numerous short measurement campaigns over the last two decades. Currents in the AS are restricted by the geometry of the area and typically have two main flow directions, with a slightly wider directional distribution at the southern parts of the AS. In open areas of the AS, surface currents have a mean magnitude of around 8 cm s⁻¹ and can momentarily grow to significant magnitudes, as even the shortest measurement time series (of around 4 months) have measured currents exceeding at least 40 cm s⁻¹. The current magnitudes in the long and narrow straits are almost double (14 cm s⁻¹) the general current magnitudes of the area, and current magnitudes up to 115 cm s⁻¹ have been measured at the northern end of the long northeastern strait. The AS currents are mostly driven by local winds, but strong oscillations in the surrounding basins can cause significant currents (up to 80 cm s⁻¹ in narrow straits) if the local winds are weak enough (below 10 m s⁻¹) or from non-optimal directions with respect to the straits. Our analysis shows that especially strong seiche in the Gulf of Finland combined with low sea levels in the Gulf of Bothnia is a significant factor in forcing currents in the AS.

Typical statistics, such as mean or percentiles, provide an excellent baseline for studying variations and changes in physical variables that have socioeconomic relevance. Nonetheless, they lack information on how often, and for how long, a certain wave height is exceeded, which might be needed for practical applications, such as planning marine operations. Using a 29-year wave hindcast, we determined the individual events where the significant wave height exceeded warning thresholds for Baltic Sea marine traffic (2.5, 4, and 7 m). During the summer months (JJA), the significant wave height exceeded 2.5 m less than twice a month. During the winter months (DJF), a significant wave height of 2.5 m was exceeded on average once a week in the larger Baltic Proper and Bothnian Sea sub-basins. Events with wave heights of over 7 m occurred roughly once every other year in the larger sub-basins. Our case study for fish-farm-related operations compared two sites that are located 10 and 30 km from the coast in the Bothnian Sea, where we determined wave events that could affect operations at the cage – meaning a significant wave height over 1 m. During the growth period of rainbow trout (May–October) there were roughly twice as many possibly disruptive events at the location further offshore than at the location closer to the coast. Even at the less-exposed location half of the wave events lasted more than 12 h, with a few events in September and October lasting around a week.