Retrieving the signal of climate change from numerically simulated sediment transport along the eastern baltic sea coast

Abstract

We explore long-term variations in the properties of wave-driven sediment transport for 1970–2007 on the eastern Baltic Sea coast (incl. The Gulf of Riga) and temporal patterns of extreme wave-induced set-up in the vicinity of City of Tallinn on the southern coast of the Gulf of Finland for 1981–2014. The wave properties in the Baltic Proper and in the Gulf of Finland are reconstructed using the WAM model with a moderate resolution of about 3 nautical miles and a higher resolution of about 470 m, respectively. The reconstruction for the Baltic Proper is based on adjusted geostrophic winds from the Swedish Meteorological and Hydrological Institute while the hindcast for the Tallinn area is driven by high-quality open sea wind data measured at Kalbådagrund. The course of net and bulk transport in the Gulf of Riga is coherent with the similar course on the Baltic Proper coast until about 1990 but behaves substantially different since then. The simulated bulk potential sediment transport along the Curonian Spit and along the entire eastern coast of the sea (incl. the Gulf of Riga) gradually increases during the entire simulation interval but the net transport reveals a decrease from the end of the 1980s. This change is associated with an abrupt turn of the geostrophic air-flow by about 40° over the southern Baltic Sea since 1987. This turn was accompanied with substantial fluctuations in the air-flow also at the latitudes of the Gulf of Finland where the heights of waves from several directions have decreased since the beginning of the 1980s. Additionally to the obvious impact of the rotation of wind directions on the wave fields and the course of coastal processes, this turn may serve as an alternative explanation for a radical decrease in the frequency of major inflows of North Sea water into the Baltic Sea since the mid-1980s.