Atlantic Thermohaline Circulation in a Coupled General Circulation Model Unforced Variations versus Forced Changes

• Published in: Journal of climate Vol. 18; no. 16; pp. 3270 - 3293
• Main Authors: Dai, Aiguo, Hu, A., Meehl, G. A., Washington, W. M., Strand, W. G.
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 15.08.2005
• Subjects: Atmospheric circulation; Atmospherics; Climate change; Climate models; Earth, ocean, space; Exact sciences and technology; External geophysics; General circulation models; Marine; Meteorology; Ocean circulation; Ocean temperature; Oceanic climates; Oceans; Physics of the oceans; Salinity; Sea ice; Sea transportation; Sea-air exchange processes; Seas; Thermohaline structure and circulation. Turbulence and diffusion; Atlantic Ocean; AN, North Atlantic; AN, North Atlantic, Gulf Stream; ANE, North Atlantic, Subpolar Gyre; ANW, Labrador Sea; AN, North Atlantic, North Atlantic Oscillation; AN, North Atlantic, North Atlantic Current; A, Atlantic; ocean currents; Flux adjustment; General circulation models; Thermohaline convection; Atlantic Ocean; Atmosphere cryosphere interaction; Climate models; digital simulation; sulfates; ocean-atmosphere interaction; greenhouse gas; Ocean-atmosphere model; North Atlantic oscillation; climate modification; sea ice; Forcing; ocean circulation
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/JCLI3481.1

A 1200-yr unforced control run and future climate change simulations using the Parallel Climate Model (PCM), a coupled atmosphere–ocean–land–sea ice global model with no flux adjustments and relatively high resolution (∼2.8° for the atmosphere and 2/3° for the oceans) are analyzed for changes in Atlantic Ocean circulations. For the forced simulations, historical greenhouse gas and sulfate forcing of the twentieth century and projected forcing for the next two centuries are used. The Atlantic thermohaline circulation (THC) shows large multidecadal (15–40 yr) variations with mean-peak amplitudes of 1.5–3.0 Sv (1 Sv ≡ 10⁶ m³ s−1) and a sharp peak of power around a 24-yr period in the control run. Associated with the THC oscillations, there are large variations in North Atlantic Ocean heat transport, sea surface temperature (SST) and salinity (SSS), sea ice fraction, and net surface water and energy fluxes, which all lag the variations in THC strength by 2–3 yr. However, the net effect of the SST and SSS variations on upper-ocean density in the midlatitude North Atlantic leads the THC variations by about 6 yr, which results in the 24-yr period. The simulated SST and sea ice spatial patterns associated with the THC oscillations resemble those in observed SST and sea ice concentrations that are associated with the North Atlantic Oscillation (NAO). The results suggest a dominant role of the advective mechanism and strong coupling between the THC and the NAO, whose index also shows a sharp peak around the 24-yr time scale in the control run. In the forced simulations, the THC weakens by ∼12% in the twenty-first century and continues to weaken by an additional ∼10% in the twenty-second century if CO₂ keeps rising, but the THC stabilizes if CO₂ levels off. The THC weakening results from stabilizing temperature increases that are larger in the upper and northern Atlantic Ocean than in the deep and southern parts of the basin. In both the control and forced simulations, as the THC gains (loses) strength and depth, the separated Gulf Stream (GS) moves southward (northward) while the subpolar gyre centered at the Labrador Sea contracts from (expands to) the east with the North Atlantic Current (NAC) being shifted westward (eastward). These horizontal circulation changes, which are dynamically linked to the THC changes, induce large temperature and salinity variations around the GS and NAC paths.