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.