Pycnocline variations in the Baltic Sea affect background conditions for internal waves

Significant changes in the vertical structure of the Baltic Sea water masses within the latter decades may substantially influence the dynamics of this water body through changing the properties of single internal waves, their propagation pathways, regions of breaking and associated areas of intense mixing and resuspension of bottom sediments. We evaluate the average nonlinear parameters that govern the field of long high-frequency weakly nonlinear internal waves in the framework of Gardner's equation. The calculations are performed using hydrographic data calculated by the Rossby Centre Ocean circulation model (RCO) for the entire Baltic Sea for 1961–2005 with a horizontal resolution of 2 nautical miles. The focus is on changes in the nonlinear wave regimes such as wave polarities and limiting amplitudes of solitary internal waves. The extension of the changes is demonstrated by comparison of the parameters in question in the 1960s (characterizing the situation when strong salt-water inflows were frequent) and 1990s (when inflows were weak and rare). The spatial and seasonal distributions of the listed parameters differ considerably for these years. The largest change is a shift in the possible nonlinear wave regime (e.g., (dis)appearance of probable breather generation, polarity change and/or transition from one family of soliton solutions to another) over the sea areas between Gotland and the Swedish mainland. The typical areas where the internal waves alter their appearance (through adjustment, transformation or breaking) and areas of intense breaking have also been shifted over the three decades. The overall geographical distribution of the listed parameters of internal waves is still quite stable despite the notable changes of pycnocline depth both in summer and in winter.