26
31 34
Practical salinity unit
Deep water formation
Surface
current
Deep current
Deep water formation
Deep water formation
Pacific
Ocean
Pacific
Ocean
Atlantic
Ocean
Indian
Ocean
36 39
(1 psu = 1 gram of salt per kilogram of water)
Thermohaline circulation
Source : NASA, 2009.
Figure 8: Thermohaline circulation
is a 3-dimensional flow involving surface and deep ocean waters, which
is driven by differences in water temperature and salinity. (
Image source: NOAA/NCDC
).
pands causing the ocean surface to rise (UNEP, 2008b). Over
time, this heat will descend to greater ocean depths, increasing
expansion and triggering further changes in sea level.
Melting of sea ice in the Arctic, inland glaciers and continen-
tal ice sheets of Greenland and Antarctica is changing the sa-
linity of sea water and in some cases also contributing to sea
level rise (UNEP, 2008b). So, melting and warming will have
further consequences on ocean circulation, as ocean currents
are driven by the interactions between water masses through a
balance with temperature and salinity, which controls the den-
sity. Changes in oceanic currents could expose local climates
to abrupt changes in temperature. Higher water temperatures
also lead to increased evaporation, making more energy avail-
able for the atmosphere. This has direct consequences on
extreme weather events, as warming sea temperatures boost
the destructive energy of hurricanes, typhoons, etc. Tropical
sea-surface temperatures have warmed by only half a degree
Celsius, while a 40% increase in the energy of hurricanes has
been observed (Saunders and Lea, 2008).
Warmer, low salinity surface waters together with the annual sea-
sonal heating are extending and strengthening the seasonal lay-
ers in the water-column (stratification), limiting the vertical move-
ment of water masses. This phenomenon together with changes
in wind regimes has implications for some of the most produc-
tive parts of earth’s oceans (Le Quéré
et al.
, 2007), where upwell-
ing of deep waters and nutrients enhances primary production,
supporting massively abundant surface ecosystems. If reduction
of upwelling occurs to any degree, marine ecosystems, fisheries
