Geography:Why don't the Baltic Sea and the North Sea merge?
The following is supposedly an image of the Baltic Sea - North Sea boundary, and it seems they don't mix well. Why is this? On googling I found that their densities are different. Can somebody shed more light on it?
This is an urban legend. The picture shown is not the Baltic Sea and North Sea, and the border shown is not due to salinity gradient. The picture is from Alaska, showing where dirty glacial meltwater was flowing into the open ocean. It's a temporary phenomenon caused by sediment washed into the ocean -- microorganisms thrive on the nutrients. The color is not due salinity.
There is no such border between the North Sea and Baltic Sea. It's an urban legend spread by Reddit.
Waters of different salinity levels will mix quite easily when exposed to disturbances from wind, waves, and current. Salinity separation phenomena like haloclines are only stable in low-energy systems like caves or when constantly regenerated by persistent effects like surface evaporation. In either case, the halocline is a horizontal boundary (eg gravity puts the lighter fluid on top), not a vertical line like is shown in the picture.