Surface water traveling northward in the North Atlantic becomes cold and saltier (due to evaporation) and hence more dense. This dense water tends to sink to deep levels by the time it reaches the area around Greenland and then returns southward toward the equator at depth. This so-called meridional circulation can be interrupted by processes such as flooding the North Atlantic with freshwater (low density surface water that retards the process causing northward-moving water to become more dense). McManus et al. (2004) report that events about 17,500 years ago and 12,700 years ago shut down and slowed down, respectively, the meridional circulation and led to global glacial events. When the meridional circulation became re-established, rapid regional warming ensued.
Reference
McManus, J.F., R. Francois, J.-M. Gherardi, L.D. Keigwin, and S. Brown-Leger, 2004: Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes. Nature, 428, 834-837.