Greenland or Antarctica. There, the water sinks, and then crawls across the bottom of the ocean, miles/kilometers deep, over hundreds of years, gradually rising in the Pacific and Indian oceans. Northward surface flow transports a substantial amount of heat energy from the tropics and Southern Hemisphere toward the North Atlantic, where the heat is lost to the atmosphere due to the strong temperature gradient. Upon losing its heat, the water becomes denser and sinks. This densification links the warm, surface limb with the cold, deep return limb at regions of convection in the Nordic and Labrador Seas. The limbs are also linked in regions of upwelling, where a divergence of surface waters causes Ekman suction and an upward flux of deep water. AMOC consists of upper and lower cells. The upper cell consists of northward surface flow as well as southward return flow of North Atlantic Deep Water (NADW). The lower cell represents northward flow of dense Antarctic Bottom Water (AABW) – this bathes the abyssal ocean. AMOC exerts a major control on North Atlantic sea level, particularly along the Northeast Coast of North America. Exceptional AMOC weakening during the winter of 2009–10 has been implicated in a damaging 13 cm sea level rise along the New York coastline. There may be two stable states of the AMOC: a strong circulation (as seen over recent millennia) and a weak circulation mode, as suggested by atmosphere-ocean coupled general circulation models and Earth systems models of intermediate complexity. A number of Earth system models do not identify this bistability, however. AMOC and climate The net northward heat transport in the Atlantic is unique among global oceans, and is responsible for the relative warmth of the Northern Hemisphere. AMOC carries up to 25% of the northward global atmosphere-ocean heat transport in the northern hemisphere. This is generally thought to ameliorate the climate of Northwest Europe, although this effect is the subject of debate. As well as acting as a heat pump and high-latitude heat sink, AMOC is the largest carbon sink in the Northern Hemisphere, sequestering approximately C/year. This sequestration has significant implications for evolution of anthropogenic global warming – especially with respect to the recent and projected future decline in AMOC vigor. Thermohaline circulation and fresh water Heat is transported from the equator polewards mostly by the atmosphere but also by ocean currents, with warm water near the surface and cold water