京都大学21世紀COEプログラム 活地球圏の変動解明 アジア・オセアニアから世界への発信
◆「 Oceanic response to large changes in Atmospheric carbon
dioxide: The study of contrast between the glacial and warm oceans」
真鍋淑郎(プリンストン大学客員研究員、海洋開発研究機構顧問)
Using a coupled ocean-atmosphere model, this study evaluates the
equilibrium response of ocean to the doubling, quadrupling and halving of
CO2 concentration in the atmosphere. Special emphasis of the study is
placed upon the CO2-induced changes in thermohaline circulation (THC) and
the structure of water masses. The paleoclimatic implication of the result
is discussed.
When the atmospheric concentration of CO2 is halved, the climate of
the coupled model becomes substantially colder than the last glacial
maximum in the Southern Hemisphere, yielding the Atlantic THC, which is
very weak and shallow. In the Circumpolar Ocean of the Southern Hemisphere,
thick perennial sea ice extends beyond the Antarctic polar fronts, covering
the region of deepwater upwelling in summer as well as in winter. One can
speculate that, during the last glacial maximum, such a thick sea ice could
have reduced markedly the ventilation of CO2-rich water in the Circumpolar
Ocean and is responsible for maintaining low CO2 concentration in the
atmosphere.
As the thick sea ice diverges due to the Ekman drift of surface
water, it is replaced by newly formed sea ice. Because of brine rejection
resulting from the rapid freezing of sea ice, very saline surface water is
generated, thereby enhancing the formation of Antarctic Bottom Water. Thus,
a very thick layer of saline and cold bottom water develops, thereby making
the THC very shallow and weak in the Atlantic Ocean.
On the other hand, the Atlantic THC is active and has the intensity,
which is comparable among the quasi-equilibrium states with the standard,
twice the standard, and four times the standard CO2 concentrations in the
atmosphere. For these CO2 concentrations at least, there is another
distinctly different, stable mode of the THC, which is characterized by the
very weak, reverse overturning circulation with no ventilation of
subsurface water in the North Atlantic Ocean.
Reference
Stouffer, R.J., and S. Manabe, 2003, Equilibrium response of thermohaline
circulation to large changes in atmospheric CO2 concentration, Climate
Dynamics, 20, 759-773.
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