Abstract
Wave heights in the Baltic Sea in the period 1992–2015 have mainly increased in the sea’s western parts. The linear trends in the winter wave heights exhibit a prominent meridional pattern. Using the technique of Empirical Orthogonal Functions (EOF) applied to multi-mission satellite altimetry data, we explain a large part of this increase with the Scandinavia pattern, North Atlantic Oscillation and Arctic Oscillation climatic indices. The winter trends show a statistically significant negative correlation (correlation coefficient –0.47 ± 0.19) with the Scandinavia pattern and a positive correlation with the North Atlantic Oscillation (0.31 ± 0.22) and Arctic Oscillation (0.42 ± 0.20). The meridional pattern is associated with more dominant north-westerly and westerly winds driven by the Scandinavia pattern and North Atlantic Oscillation, respectively. All three climatic indices show a statistically significant time-variable correlation with Baltic Sea wave heights during the winter season. When the Scandinavia pattern’s influence is strong, the North Atlantic and Arctic Oscillation effects are low and vice versa. The results are backed up by simulations using synthetic data that demonstrate that the percentage of variance explained using EOF analysis from the satellite-derived wave measurements is directly related to the percentage of noise in the data and that the retrieved spatial patterns are insensitive to the level of noise.
Highlights
The knowledge of both long-term changes and short-term variations in the wave climate has great importance for the safety of navigation (e.g., Barbariol et al 2019), design purposes (e.g., Hemer et al 2013), coastal protection (e.g., Weisse et al 2012), and sediment transport (e.g., Masselink et al 2016)
The first one uses the classification of the seasons into "stormy" period, which includes the months from January to March and from August to December and the "calm" period, which lasts from April to July (Soomere and Pindsoo 2016; Männikus et al 2020)
In order to interpret the appearance of various modes and parameters of the Empirical Orthogonal Functions (EOF) modes of the Baltic Sea wave climate derived from satellite altimetry, it was demonstrated that (i) the percentage of variability in the retrieved wave fields is directly related to the percentage of noise in the data and that (ii) the retrieved spatial patterns are practically not affected by the noise
Summary
The knowledge of both long-term changes and short-term variations in the wave climate has great importance for the safety of navigation (e.g., Barbariol et al 2019), design purposes (e.g., Hemer et al 2013), coastal protection (e.g., Weisse et al 2012), and sediment transport (e.g., Masselink et al 2016). In semi-sheltered seas, such as the Baltic Sea (Fig. 1), due to the limited size of the water body, small changes in the wind direction can lead to large spatiotemporal variations in the wave climate (e.g., Jönsson, Broman and Rahm 2002; Soomere and Räämet 2011; Kudryavtseva and Soomere 2017) and alongshore transport patterns (Soomere et al 2015). Semi-enclosed areas of this basin, such as the Gulf of Finland, the Gulf of Riga, and the Bay of Bothnia are characterised by a lower range of the mean wave height of 0.5–1.0 m (Tuomi, Kahma and Pettersson 2011; Nikolkina, Soomere and Räämet 2014; Kudryavtseva and Soomere 2017)
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