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