From Glossary of Meteorology
A convergence in the wind-driven surface current field pushes water down along constant- density surfaces. During winter this water is convectively mixed as a result of surface cooling. When spring warming sets in, the mixed water is isolated under a thin surface layer of warmer water but continues to move downward on constant-density surfaces in response to the current convergence, eventually moving out of reach of next winter's mixing. It is then injected into the ocean interior as a new water mass contribution.