Annually, fresh water of ca. 3,300 cubic kilometers flows into the Arctic Ocean. Once upon a time, this amount was even bigger that the Arctic Ocean was an enormous freshwater lake. It was only when the land bridge between Scotland and Greenland submerged did huge quantities of salt water from the Atlantic pour into the Arctic Ocean making it salty as it is today. This process of tectonic submersion occurred over a period of several million years.
Scientists from the Alfred Wegener Institute have explained how this process occurred, by using a climate model. They gradually submerged the land bridge 200 meters deep in their simulations. Only when it has submerged to over 50 meters deep, biggest changes in the characteristics and circulation patterns of the Arctic Ocean occurred.
This threshold depth correlates with the surface mixed layer’s depth, and determines the end of the relatively light Arctic surface water and the beginning of the below-lying layer of inflowing North Atlantic water. "Once the ocean passage between Greenland and Scotland had reached this critical depth, the saline Arctic Ocean as we know it today was created” said one of the researchers.