On much longer time scales, vertical motion can also occur in the ocean through deep
convection, rather than shallow wind-driven overturning. This almost always takes place at high latitudes
during the wintertime, when the cold atmosphere extracts huge quantities
of heat
from the surface ocean. If this
process extracts enough heat, the water can become dense enough to sink to the depths of the
ocean. Once it sinks, this water spreads throughout the
global ocean. Eventually it returns to the surface.
This overturning circulation is known as the thermohaline
circulation. It typically takes about 1000 years for a chunk of water to
sink, flow through the deep ocean, and then return to the surface. This is an indication of how sluggish the
deep ocean circulation is.