Add to ORKGOne of the key processes of the ocean circulation and large scale air-sea interaction is water mass formation. In the north Atlantic, the upper ocean circulation brings warm waters to higher latitudes, where vigorous cooling leads, during the winter, to the formation of deep mixed layers (up to several hundred meters deep). As these newly formed water masses are advected and spread southward they are overlaid during the spring and summer by warmer water. Thus they lose contact with the atmosphere and become part of ocean circulation as intermediate water. This succession of events constitute subduction. The newly formed water mass is typically of low potential vorticity (nearly homogeneous), with characteristics imparted by the local prevai...