Changes in extreme sea-levels in the Baltic Sea

In a climate change context, changes in extreme sea-levels rather than changes in the mean are of particular interest from the coastal protection point of view. In this work, extreme sea-levels in the Baltic Sea are investigated based on daily tide gauge records for the period 1916–2005 using the annual block maxima approach. Extreme events are analysed based on the generalised extreme value distribution considering both stationary and time-varying models. The likelihood ratio test is applied to select between stationary and non-stationary models for the maxima and return values are estimated from the final model. As an independent and complementary approach, quantile regression is applied for comparison with the results from the extreme value approach. The rates of change in the uppermost quantiles are in general consistent and most pronounced for the northernmost stations

Atmospheric stilling leads to prolonged thermal stratification in a large shallow polymictic lake

To quantify the effects of recent and potential future decreases in surface wind speeds on lake thermal stratification, we apply the one-dimensional process-based model MyLake to a large, shallow, polymictic lake, Võrtsjärv. The model is validated for a 3-year period and run separately for 28 years using long-term daily atmospheric forcing data from a nearby meteorological station. Model simulations show exceptionally good agreement with observed surface and bottom water temperatures during the 3-year period. Similarly, simulated surface water temperatures for 28 years show remarkably good agreement with long-term in situ water temperatures. Sensitivity analysis demonstrates that decreasing wind speeds has resulted in substantial changes in stratification dynamics since 1982, while increasing air temperatures during the same period had a negligible effect. Atmospheric stilling is a phenomenon observed globally, and in addition to recent increases in surface air temperature, needs to be considered when evaluating the influence of climate change on lake ecosystems

Combined impact of summer heat waves and coastal upwelling in the Baltic Sea

Under warming climates, heat waves (HWs) have occurred in increasing intensity in Europe. Also, public interest towards HWs has considerably increased over the last decades. The paper discusses the manifestations of the summer 2014 HW and simultaneously occurring coastal upwelling (CU) events in the Gulf of Finland. Caused by an anticyclonic weather pattern and persisting easterly winds, CUs evolved along the southern coast of the Gulf in four episodes from June to August. Based on data from coastal weather stations, 115 days-long measurements with a Recording Doppler Current Profiler (RDCP) oceanographic complex and sea surface temperature (SST) satellite images, the partly opposing impacts of these events are analysed. Occurring on the background of a marine HW (up to 26°C), the CU-forced SST variations reached about 20 degrees. At the 10 m deep RDCP mooring location, a drop from 21.5 to 2.9°C occurred within 60 hours. Salinity varied between 3.6 and 6.2 and an alongshore coastal jet was observed; the statistically preferred westerly current frequently flowed against the wind. Locally, the cooling effect of the CUs occasionally mitigated the overheating effects by the HWs both in the sea and on the marine-land boundary. However, in the elongated channel-like Gulf of Finland, upwelling at one coast is usually paired with downwelling at the opposite coast, and simultaneously or subsequently occurring HWs and CUs effectively contribute to heat transfer from the atmosphere to the water mass. Rising extremes of HWs and rapid variations by CUs may put the ecosystems under increasing stress

Winter upwelling in the Gulf of Finland, Baltic Sea

Traditionally, upwelling-related studies in the Baltic Sea have been limited to the period from May to September. Based on wintertime in situ measurements at two nearshore locations in the Gulf of Finland, clear evidence of winter “warm” upwelling events was detected and analysed. The process was very common. At a 10 m deep location, upwelling caused water temperature (T) to switch from 0—1 to 4—5°C and salinity (S) to switch from 4.5 to 6 PSU; at 20 m depth it caused a switch in T between 1 and 2—4°C and in S between 5.5 and 6.8 PSU. Differently from summer upwelling, T and S variations were positively correlated to each other. Salinity variations remained roughly the same throughout the winter, whereas T differences were higher in winter onset, then decreased to ca. 1°C, and increased again after the process reversed to summer-type upwelling in April—May. Based on analysis of SatBaltyk (January to March) sea surface temperature and salinity product imagery, winter upwelling occurrence along the North Estonian coast was 21—28% over 2010—2021, and slightly less along the Finnish coast. Regarding S variations, winter upwelling occurred with roughly similar frequencies and impacts in the northern and southern parts of the gulf. However, the impacts on T and sea ice conditions were highly asymmetrical. Upwelling kept the Estonian coast ice-free longer and water temperatures slightly higher than at the Finnish coast. Winter upwelling as a phenomenon has long been ignored and therefore probably underestimated

Decadal variations in wave heights off Cape Kelba, Saaremaa Island, and their relationships with changes in wind climate

Based on wind data from the Vilsandi meteorological station and a 5-month calibration measurement with a bottom-mounted Recording Doppler Current Profiler (RDCP), a semi-empirical hindcast of wave parameters near the quickly developing accumulative Kelba Spit is presented for the period 1966–2006. The significant wave heights with a gross mean value of 0.56 m exhibited some quasiperiodic cycles, with the last high stage in 1980–95 and a decreasing overall trend of −0.001 m per year. At the same time, both the frequency and intensity of high wave events showed rising trends, and the mean wave heights during winter (December to February) increased as well. As the study area has the longest fetches in westerly directions, the discussed tendencies in wave conditions are sensitive to regional changes in the wind climate and can be related to a decrease in the local average wind speed on the one hand, but an intensification of westerly winds, storm events and the wintertime NAO index on the other. The roughest wave storms on record were associated with prominent W-storms on 2 November 1969 and 9 January 2005; a few other extreme wind events (e.g. in 1967, 1999, 2001), however, did not yield equally prominent waves

Currents and waves in the Northern Gulf of Riga: measurement and long-term hindcast

Based on measurements of waves and currents obtained for a period of 302 days with a bottom-mounted RDCP (Recording Doppler Current Profiler) at two differently exposed locations, a model for significant wave height was calibrated separately for those locations; in addition, the Gulf of Riga-Vainameri 2D model was validated, and the hydrodynamic conditions were studied. Using wind forcing data from the Kihnu meteorological station, a set of current, water exchange and wave hindcasts were obtained for the period 1966–2011. Current patterns in the Gulf and in the straits were wind-dependent with characteristic wind switch directions. The Matsi coast was prone to upwelling in persistent northerly wind conditions. During the hindcast period, currents increased along the Koiguste coast and in the Suur Strait, waves decreased noticeably off Koiguste but fluctuated without a clear linear trend near Matsi. The spatially contrasting results for differently exposed coasts were related to the corresponding variations in local wind conditions and to changes in atmospheric circulation patterns over northern Europe