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.