Physical and chemical changes in the ocean over basin-wide zones and decadal or longer time-scales: perspectives on current and future conditions - Ocean Sustainability in the 21st Century - page 86

3.4 Ocean circulation
Ocean circulation is key to correcting the global heat imbalance by transporting excess heat
from the low to the high latitudes, in redressing the global hydrological cycle, and in estab-
lishing a marine ecology based on advection of abiotic and biotic properties. This section
describes ocean surface current changes, transports derived from ocean surface currents, and
features such as rings inferred from surface currents. Surface currents are obtained from in
situ and satellite (altimetry and wind) observations. The changes of heat and salt content are
linked to changes in ocean circulation. The salinity changes are consistent with an increase
in the hydrological cycle over the oceans and will drive changes in ocean advection.
In the North Atlantic the warming has extended well below 1000 m, and is particularly
pronounced under the Gulf Stream and the North Atlantic Current at 40 °N (Bindoff et al .,
2007 ; Levitus et al ., 2012 ) . This is consistent with a predominantly positive phase (increas-
ing trend) of the North Atlantic Oscillation (NAO) during past several decades.
The NAO follows the variation of the pressure difference between the Icelandic low
and the Azores high. The climates of western Europe, north-western Asia, and the north-
west coast of North America are closely linked with this pressure difference, thus reflected
in the NAO index (e.g. Voituriez, 2003 ) . When the NAO is high, strong westerlies dominate,
bringing oceanic air towards western Europe. On the other hand a low NAO implies weak
westerlies, favouring polar or continental air over north-western Europe. Observations from
aroundtheyear1850providefordirectmeasurementoftheNAO,andanalysisofthegrowth
rings of trees have made it possible to reconstruct the NAO since around 1700 (Voituriez,
2003 ) . The time series show several periods of oscillation. However, the reasons for these
oscillations have not yet been clarified .
Within the Arctic Ocean the ice coverage is decreasing faster than predicted by climate
models, with consequences to both the wind-driven and thermohaline circulations. The
change in size of the ice-snow cover areas provides an important positive feedback through
the change of reflection of the incoming energy, a change in albedo. Decreasing ice-snow