Evolution characteristics of the Atlantic Meridional Overturning Circulation and its thermodynamic and dynamic effects on surface air temperature in the Northern Hemisphere

Based on modern observations, historical proxy data, and climate model simulations, this paper provides a comprehensive overview of the past, present and future evolution characteristics of the Atlantic Meridional Overturning Circulation (AMOC), as well as its impact on the surface air temperature (SAT) at regional and hemispherical scales. The reconstruction results based on the proxy data indicate that the AMOC has weakened since the late 19th century and experienced overall weakening throughout the 20th century with low confidence. Direct observations show that the AMOC weakened during 2004–2016, but it is not possible to distinguish between its decadal variability and long-term trend. Climate models predict that if greenhouse gas emissions continue to increase, AMOC will weaken in the future, but there will not be a sudden collapse before 2100. For the thermodynamic effects of AMOC, the increased surface heat flux release and meridional heat transport (MHT) over the North Atlantic associated with the strong AMOC cause an increase in the hemispherical SAT. At the millennial scale, climate cooling (warming) periods correspond to a weakened (strengthened) AMOC. The enhanced MHT of a strong AMOC can affect Arctic warming and thus influence regional SAT anomalies and SAT extremes through mutual feedback between Arctic sea ice and AMOC. In terms of dynamic effects, a strong AMOC modulates the Rossby wave trains originating from the North Atlantic and spreading across mid-to-high latitudes in the Northern Hemisphere and causes an increase in the variabilities in the circulation anomalies over the Ural and Siberian regions. Ultimately, a strong AMOC significantly affects the frequencies of extreme cold and warm events in the mid-to-high latitude regions over Eurasia. In addition, AMOC can also influence regional and global SAT anomalies through its dynamic adjustment of planetary-scale circulation. Decadal variation in AMOC is closely related to the Atlantic Multidecadal Oscillation (AMO). During positive phases of AMO and AMOC, enhanced surface heat fluxes over the North Atlantic lead to abnormal warming in the Northern Hemisphere, while during negative phases, the reverse case occurs. Under high emission scenarios in the future, the possibility of AMOC collapse increases due to freshwater forcing. However, most advanced climate models underestimate the strength of the AMOC and its impact on the AMO and relevant climate change, which presents a major challenge for future understanding and prediction of the AMOC and its climate effects.