Reconstructing changes in Atlantic thermohaline circulation during the 20th century under two possible scenarios

Atlantic thermohaline circulation (THC) is a key component of the Earth Climate System and identification of its changes during the 20th Century is critical to the understanding of its variation characteristics and the corresponding climatic impacts. Previous researches have been inconclusive, with the results varying depending on the approach used to measure THC. The results for the two established approaches for measurement of the phenomenon (direct observation and indirect reconstruction) are contradictive (weakening and non-weakening), and their credibility needs improving. Based on the tight relationship between THC anomaly and "see-saw" intensities of Sea Surface Temperature (SST) and Surface Air Temperature (SAT), we first diagnose their quantitative relationship in the model experiments, which is corresponding to its two possible scenarios, and then reconstruct the changes of THC during the 20th Century respectively with multiple observed datasets of SST and SAT. Model results show that THC anomaly and SST/SAT "see-saw" intensities are well correlated in timescales longer than 10/40 years under scenarios of weakening/non-weakening respectively. Two kinds of reconstructions here are consistent with each other, and we propose that THC has undergone a 2-cycle oscillation with inter-decadal scale since the Industrial Revolution with a magnitude of about 1 Sv. The transformation times of decadal trend are around the mid-1910s, the 1940s, and the mid-1970s. This research further validates the main results of previous reconstructions, and points out that THC does not have a long-term weakening during the 20th Century.