Disentangling Internal and External Contributions to Atlantic Multidecadal Variability Over the Past Millennium

• Published in: Geophysical research letters Vol. 48; no. 23
• Main Authors: Fang, Shih‐Wei, Khodri, Myriam, Timmreck, Claudia, Zanchettin, Davide, Jungclaus, Johann
• Format: Journal Article
• Language: English
• Published: American Geophysical Union 16.12.2021
• Subjects: Atlantic multidecadal variability; Earth Sciences; Geophysics; internal variability; paleo‐climate; past1000; Physics; PMIP4; Sciences of the Universe; volcano
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL095990

The Atlantic multidecadal variability (AMV) modulates the North Atlantic surface ocean variability and affects decadal climates over the globe; its underlying mechanisms remain, however, under debate. In this study, we use a multi‐model ensemble of transient past‐millennium (850–1849) and unperturbed preindustrial control simulations contributing to the paleoclimate modeling intercomparison project—phase 4 (PMIP4) to decompose the AMV signal into the internal AMV and the external signal. The internal component of AMV exhibits no robust behavior across simulations during periods of major forcing such as strong volcanic eruptions, whereas the external forced temperature responds to volcanic eruptions with an immediate radiative cooling followed, in some simulations, by a sequence of damped multidecadal oscillations. The internal component tightly relates with the Atlantic meridional overturning circulation (AMOC) and dominates the fluctuations of AMV; whereas the external signal has limited impacts on AMOC and explains ∼25% of the AMV variance over the past millennium. Plain Language Summary The decadal‐scale changes of the sea surface temperature over the North Atlantic is referred to as the Atlantic multidecadal variability (AMV), which modulates the North Atlantic surface ocean variability and affects decadal climates over the globe; however, how different forcing processes can affect AMV remains debated. In this study, a multi‐model ensemble participating in the paleoclimate modeling intercomparison project—phase 4 (PMIP4) of preindustrial millennium simulation (850–1849 CE) is used to decompose the AMV signal into the internal AMV and the external signal and to compare it with the preindustrial control simulation not including external forcing (such as volcanic eruptions, solar irradiance, and anthropogenic greenhouse gas). The internal component of AMV exhibits no robust signature across simulations during periods of major forcing such as strong volcanic eruptions, whereas the external forced temperature responds to volcanic eruptions with an immediate radiative cooling followed, in some simulations, by a sequence of damped multidecadal oscillations. The internal component of AMV tightly relates with the Atlantic subsurface ocean circulation called Atlantic meridional overturning circulation (AMOC) and dominates the variations of AMV, while the external signal has limited impacts on AMOC and explains ∼25% of the AMV variance over past millennium. Key Points The internal and external contributions to Atlantic multidecadal variability (AMV) over the past millennium can be separated by a pattern‐based calculation of AMV The internal AMV has a robust tripole pattern and dominates the relationship with meridional overturning circulation The North Atlantic response to post‐eruption global cooling has a damped behavior and is identified as a more uniform spatial pattern