Complex sentence: The Baltic Sea, for example, is in a cool climatic region with low evaporation, has many rivers flowing into it and intermittent replenishment from the North Sea which creates a cold, dense under layer that hardly mixes with the surface layers.
Split into simpler sentences: The Baltic Sea, for example, is in a cool climatic region with low evaporation, and has many rivers flowing into it and intermittent replenishment from the open ocean. The occasional influx of water from the North Sea creates a cold, dense under layer that hardly mixes with the surface layers.