Observed currents in the Archipelago Sea

Abstract. 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 blue economy. Even with the high 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 ensured 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 ADCP (Acoustic Doppler Current Profiler) 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. Currents in the AS 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 NE strait. The AS currents are mostly driven by local winds, but strong oscillations in the surrounding basing can cause significant current (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 direction related 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 significant factor in forcing currents in the AS.