Equilibrium response of thermohaline circulation to large changes in atmospheric CO2 concentration

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 ofthe Cenozoic changes in the oceans.