| Abstract: This study evaluates the
equilibrium response of a coupled ocean–atmosphere model to the
doubling, quadrupling, and halving of CO2 concentration in
the atmosphere. Special emphasis in the study is placed upon the
response of the thermohaline circulation in the Atlantic Ocean to the
changes in CO2 concentration of the atmosphere. The simulated
intensity of the thermohaline circulation (THC) is similar among three
quasi-equilibrium states with the standard, double the standard, and
quadruple the standard amounts of CO2 concentration in the
atmosphere. When the model atmosphere has half the standard
concentration of CO2, however, the THC is very weak and
shallow in the Atlantic Ocean. Below a depth of 3 km, the model
oceans maintain very thick layer of cold bottom water with temperature
close to –2 °C, preventing the deeper penetration of the THC in
the Atlantic Ocean. In the Circumpolar Ocean of the Southern Hemisphere,
sea ice extends beyond the Antarctic Polar front, almost entirely
covering the regions of deepwater ventilation. In addition to the active
mode of the THC, there exists another stable mode of the THC for the
standard, possibly double the standard (not yet confirmed), and
quadruple the standard concentration of atmospheric carbon dioxide. This
second mode is characterized by the weak, reverse overturning
circulation over the entire Atlantic basin, and has no ventilation of
the entire subsurface water in the North Atlantic Ocean. At one half the
standard CO2 concentration, however, the intensity of the
first mode is so weak that it is not certain whether there are two
distinct stable modes or not. The paleoceanographic implications of the
results obtained here are discussed as they relate to the signatures of
the Cenozoic changes in the oceans. |