Geoscience Reference
In-Depth Information
This pattern of sinking due to instability and the adjacent ascending motion
is called convection, and it acts to mix the water in oceans vertically. Convection
normally reaches depths from tens to hundreds of metres and occurs in both a daily
cycle driven by surface cooling during the night and in an annual cycle during
cooling in autumn. Under certain conditions it can reach thousands of metres depth.
Then it is called deep convection which occurs at only at few places in the world
ocean. It was observed in the Southern Ocean near Maud Rise where it had occurred
in the 1970s but never since. However, the impact on water mass formation was
dramatic and it is still not clear under which conditions such an event would
occur again. If it did occur again, renewal of deep waters in the Southern Ocean
would shift from the coast into the open ocean affecting turnover rates and
timescales. Since ocean water loses density by addition of fresh water, the water
column can be stabilised by the input of freshwater, e.g. by river in
ow or ice
melt, which then suppresses convection.
Seawater does not freeze at 0 C as its dissolved salts depress the freezing
point. So when sea ice is formed at oceanic salinities of 3.4 to 3.5 kgm -3 the freezing
point is -1.8 to -1.9 C. This sea ice formation extracts freshwater from the ocean,
increasing the salinity of the underlying seawater and making it denser.
The
flow of ocean water under the ice shelves
In the Southern Ocean water masses can also be cooled where they are in contact
with ice shelves
floating on the ocean surface. This occurs either at the ice shelf front
or in cavities under the ice shelf into which ocean water penetrates.
The freezing point of water decreases with pressure. So if seawater comes into
contact with the under side of the shelf ice at depth it can melt the ice shelf even if
it had already reached freezing temperature at the sea surface. It is this process that
produces the coldest water in the ocean where, for example, at a depth of 1500m water
reaches a temperature of
2.8 C yet is still in its liquid phase. Since the ice shelves
originate from precipitation over the continent, melting them adds freshwater directly
to the ocean and is comparable to river in
ow into more temperate seas. However,
due to the low temperature and the depth of the ice shelf cavities it is injected deep into
the ocean unlike the river water in
ow, which is a surface addition.
Due to the ocean circulation in the ice shelf cavity, signi
cant melting occurs
at the grounding line where the shelf rests on the underlying rock ( Figure 5.8 ). Due
to a pressure gradient the relative salty ocean water penetrates into the cavity and
follows the descending sea bottom to the grounding line. There, melting of the ice
shelf occurs, adding freshwater to reduce the local salinity and its density, which in
turn forces the water to ascend following the underside of the ice. If the fresher
water reaches shallower depths the pressure decreases and the water starts
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