Changes in the Variability of the Thermohaline Circulation Associated With Increased Atmospheric Carbon Dioxide Concentrations in a Coupled Model

Northward heat transport in the Atlantic Ocean is an important feature of the global climate. As such, climate variability related to fluctuations in the thermohaline circulation (THC) strength may have significant implications not just for northern Europe, but also globally. We examine model results to see how such fluctuations and their relationship with global warming change as a result of increased atmospheric CO2 concentrations. Results from a 10,000 year integration using a low-resolution GFDL coupled ocean-atmosphere model show that decadal THC variability becomes larger when CO2 concentrations are increased. We look at cases where concentrations are increased two-, four- and eight-fold. The variability seen after a complete recovery of the THC correlates well with sea surface temperatures in parts of the North Atlantic. In particular, the annual mean sea surface temperatures of the Labrador and Nordic Seas, where downwelling occurs, appear to be in and out of phase with the THC intensity, respectively. The variance of the THC intensity also appears to increase with CO2 concentration, as do those of the sea surface temperature and salinity in the Labrador and Nordic Seas. In the case where CO2 concentrations are raised to 8 times the normal level, a strong centennial variability appears after the transient period but before a complete recovery in the THC. During this stage, which lasts a few thousand years, the THC intensity remains at approximately half of the final value. Sea surface temperatures in the North Atlantic, especially those of the Labrador Sea and south of Greenland lag the THC intensity by about 7 years. However, the sea surface salinity and density exhibit large variances and regression coefficients with the THC intensity in the Labrador Sea and south of Newfoundland only.