The Application of Causality Analysis Based on the Theory of Information Flow in Distinguishing the Atlantic Multi-decadal Oscillation Driving Mechanism

Atlantic Multi-decadal Oscillation (AMO) is the average sea surface temperature (SST) over the entire North Atlantic after the long-term warming trend is removed. In the reconstruction of North Atlantic SST from the mid-1800s to present, the basin-wide mean time series appears to oscillate with a period of 50-80 years. One of the most accepted theories is that the AMO is primarily driven by variation in the strength of Atlantic meridional overturning circulation (AMOC). Recently, the atmosphere-forced thermodynamics for the AMO has been proposed. There is a clear need to figure out the causality between AMO and North Atlantic surface heat flux and it may provide a key feature to distinguish the AMO mechanisms. We use a newly developed technique that is based on the information flow concept to investigate the causal structure between the North Atlantic surface heat flux and the observed AMO from the mid-1800s. Our study shows one-way causality between North Atlantic surface heat flux and AMO. It is confirmed that the AMO is the main causal driver of the North Atlantic surface heat flux, which means ocean dynamics mainly dominates heat transport between ocean and atmosphere. The stochastic atmosphere-forced thermodynamics mechanism cannot explain the results. The results between the AMO and land surface air temperature further prove that the AMO dominates the North Atlantic surface heat flux transport. We conduct a SST forced ATL_VAR_AGCM experiment, and the results of the five ensemble members from the experiment verified the direct impact of the Atlantic SST forcing on land surface air temperature. The results of this study provide a new clue to distinguish the AMO mechanisms and further show that, instead of atmosphere-forced thermodynamics mechanism, ocean circulation may mainly drive the AMO.