Stormy and icy seas - Antarctica: Global Science from a Frozen Continent - page 150

topographical features. In addition to the transport of water masses, they are of
importance to ecosystems, in particular, through transport of plankton and control
of how long organisms spend in certain hydrographic regimes.

The role of the Southern Ocean in the global
climate system

The climate system is driven by the incoming heat from the Sun and the
loss of heat back to space. Net heat gain occurs in the low to mid-latitudes
and net heat loss in the high latitudes. Heat has to be transported from areas of heat
gain to heat loss. Poleward of 40 latitude most of the meridional heat transport
occurs in the atmosphere. In the Southern Ocean signi cant heat transport occurs

from ocean to ocean by the eastward Antarctic Circumpolar Current and by the
westward Agulhas Current south of Africa. From this circumpolar heat transport a
signi cant portion enters the South Atlantic. This surplus of heat from the other

oceans results, by ocean currents, in a cross-equatorial heat transport which finally

provides heat to the north Atlantic where it is transferred to the atmosphere giving
rise to the mild climate of mid- and northern Europe (the Gulf Stream effect).
Water can store much more heat than air. The amount of heat contained in
a 100 km high column of the atmosphere can be stored in a 3m high water column.
Because of this difference in heat capacity the ocean serves as a heat buffer for the
whole global system. Up to now, 80% of the additional heat gained by the increased
greenhouse effect in the atmosphere has been absorbed by the ocean. However,
the largest part of the ocean cannot be heated directly due to its stable strati cation.

Therefore, the deeper layers of the ocean are only warmed by sinking water masses,
i.e. by formation of dense water masses and convection cells. Due to the large
volume of the deep ocean a temperature increase there of 0.001 C requires heat
quantities which would increase the temperature of the atmosphere in the order
of 1 C. Since the sinking to the deep ocean occurs in high latitudes they control
the role of the ocean as a long-term heat buffer.
Finally, the heat budget of the Earth is controlled by the re ection or

absorption of radiation at the surface. Bright surfaces re ect much more strongly

than dark surfaces. This surface property to absorb or re ect radiation is called

albedo. The albedo of ice and snow is high, whilst that for the dark ocean is low,
i.e. the open ocean absorbs most of the incoming radiation and sea ice re ects most

of it ( Figure 5.11 ). If by warming, sea ice starts melting, becomes darker and finally

produces open water, more heat is absorbed by the dark ocean and warming is
ampli cation of

change in the context of global warming can be understood. In the Southern Ocean,

ed. This is a positive feedback, through which the polar ampli