Radiational tides at the southeastern coast of the Baltic Sea

Abstract

The tides in the Baltic Sea are weak but unmistakably recognizable in sea-level spectra. Multiyear hourly tide gauge series enable us to examine specific tidal properties in detail and to identify some unexpected effects. Two nearby stations on the southeastern coast of the sea, namely Baltiysk, located in the strait connecting the Vistula Lagoon to the Baltic Sea, and Otkrytoe, on the coast of the Curonian Lagoon, are found to have very different character of tidal motions. Based on 13 years of simultaneous observations at these stations we constructed high-resolution spectra of sea level fluctuations that demonstrated that the tidal spectrum at Baltiysk is “classical” with sharp peaks corresponding to the major tidal constituents: M2, S2, N2, K1, O1, and P1. Unlike at Baltiysk, the main tidal peak at Otkrytoe is at the S1 frequency; the K1, P1, and S2 tidal harmonics also have significant peaks, while the other harmonics (including M2 and O1) are undistinguishable. Further analysis indicated that the tides at Baltiysk are generated by ordinary gravitational tidal forces, while at Otkrytoe they are induced by solar radiation, specifically by the breeze winds creating wind setups and setdowns in the Curonian Lagoon. Moreover, our findings demonstrate that the observed K1 and P1 peaks at Otkrytoe are related not to gravitational forces but to the seasonal modulation of the S1 radiational tidal constituent. The separate analysis of the “summer” and “winter” sea level spectra at Otkrytoe reveals prominent radiational tidal peaks (S1 and S2) in summer and the absence of these peaks in winter. The results of the analysis support the assumption that these motions are generated by the breeze winds associated with the sea/land temperature contrasts, which are substantial during the warm (ice-free) season but vague during the cold season when ice cover diminishes these contrasts.