Mediterranean Outflow
The North Atlantic has different water masses with distinctive temperature, salinity, and densities.
The Mediterranean Outflowoccurs at the region where the Mediterranean Sea meets the Atlantic, and has a different enough density to be distinct as it's own water mass.
This figure shows some of the water masses in the Atlantic Ocean from Antarctica (60°S) to the Northern Arctic Ocean (70°N).
Image Source: http://www.atmosedu.com/Geol390/Life/OceanCirculation.html