Regional Signatures of Forced North Atlantic SST Variability: A Limited Role for Aerosols and Greenhouse Gases. Issue 8 (16th April 2022)

• Record Type: Journal Article
• Authors: Baek, Seung H.; Kushnir, Yochanan; Ting, Mingfang; Smerdon, Jason E.; Lora, Juan M.
• Is Part Of: Geophysical research letters. Volume 49:Issue 8(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-04-16
• Subjects: Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL097794
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Prior studies that argue external forcings drive Atlantic Multidecadal Variability (AMV) use linear detrending to remove the global warming trend from North Atlantic sea surface temperatures (SSTs). The linear detrending method, however, aliases residual nonlinear, global‐scale warming, affecting the interpretation of results. Here we revisit the role of external forcing in AMV using large climate model ensembles, examining the influences of greenhouse gases (GHGs) and industrial aerosols—the two dominant 20th‐century external forcing agents—on North Atlantic SSTs separately from their respective global response. Our approach shows that GHGs have little to no influence unique to the North Atlantic, while industrial aerosols exert only modest influences as part of a broader loading over the Northern Hemisphere. We demonstrate that a prominent role for external forcings on AMV is an artifact of the linear detrending method that disappears when their global responses common to the World Ocean are correctly accounted for. Plain Language Summary: The origin of Atlantic Multidecadal Variability (AMV) is widely debated, with discussions focused on whether it is an internal mode of the coupled climate system or a response to external radiative forcings. Most studies that identify a dominant role for external forcings in driving AMV apply linear detrending to remove the global warming trend from North Atlantic SSTs. This is problematic because the linear detrending method does not sufficiently remove the global warming signal. In this study, we re‐examine the role of external forcing in 20th‐century AMV, correctly accounting for global forcing influences common to the entire World Ocean. Separate analysis of the roles of greenhouse gases (GHGs) and industrial aerosols on AMV, independent of their respective planetary influence, shows that GHGs have little to no influence unique to the North Atlantic, while industrial aerosols exert only modest influences as part of a broader impact on the Northern Hemisphere oceans. Our results suggest that linear signal separation methods that do not fully isolate the influences of global forcing on North Atlantic temperatures incorrectly inflate the role played by external forcing. When these approaches are avoided, our results indicate that external forcing has little influence on AMV compared to variability internal to the climate system. Key Points: Linearly detrending North Atlantic sea surface temperatures conflate variability unique to the Atlantic with aliased residual, nonlinear, global warming The role of external forcing on Atlantic Multidecadal Variability (AMV) is greatly diminished when global forcing signals are sufficiently removed Roles for historical forcing on AMV previously identified with the linear detrending method are likely an unphysical artifact