Response of the Atlantic thermohaline circulation to increased atmospheric CO2 in a coupled model

• Main Authors: Aixue Hu, Gerald A. Meehl, Warren M. Washington, Aiguo Dai
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2004
• Subjects: Arctic; Greenland; Denmark Strait; Iceland; Labrador Sea; North Atlantic; Sea ice
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.595.9383; http://www.cgd.ucar.edu/ccr/ahu/papers/ams_annual.pdf

Changes in the thermohaline circulation (THC) due to increased CO2 are important in future climate regimes. Using a coupled climate system model – Parallel Climate Model (PCM), regional responses of the THC in the North Atlantic to increased CO2 and the underlying physical processes are studied here. The Atlantic THC shows a 20-year cycle in the control run, qualitatively consistent with observations. Compared with the control run, the simulated maximum of the Atlantic THC weakens by about 5 Sv or 14 % in an ensemble of transient experiments with 1 % CO2 increase per year at the time of CO2 doubling. The weakening of the THC is accompanied by reduced poleward heat transport in mid-latitude North Atlantic. Analyses show that oceanic deep convective activity strengthens significantly in the Greenland-Iceland-Norwegian (GIN) Seas, but weakens in the Labrador Sea and the south of Denmark Strait Region (SDSR). The strengthening of deep convective activity in the GIN Seas is mainly caused by an increased salty North Atlantic inflow, reduced sea ice volume fluxes from the Arctic into this region, all of which lead to a saltier (denser) upper ocean. The weakening of deep convective activity in the SDSR is induced by a reduced sea ice flux into this region and a reduced heat loss to the atmosphere which leads to a warmer (lighter) upper ocean. On the other hand, the weakening in the Labrador Sea is mainly attributed to increased precipitation that freshens the surface ocean. These regional changes produce the overall weakening of the THC in the Labrador Sea and SDSR, and more vigorous ocean overturning in the GIN Seas. The northward heat transport south of 60◦N is reduced with increased CO2, but increased north of 60◦N due to the increased North Atlantic water across this latitude. 2 1.