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Quantifying the External vs. Internal Mechanisms Behind the Atlantic Multidecadal Variability


The Atlantic Multidecadal Variability (AMV) describes the large-scale, quasi-oscillation in North Atlantic sea surface temperatures which occur on a 60-80-year cycle. The AMV impacts important climatic behavior such as Atlantic hurricane activity, Arctic sea ice extent, and global-mean temperature. A major scientific question surrounding the AMV is: have external forcings, such as volcanic or human activity, played a major role in the multidecadal variations of North Atlantic sea surface temperatures over the last 150 years? Some recent studies suggest that, yes, external forcings have contributed significantly to recent AMV, challenging the conventional view that AMV is an internally-generated mode of variability. This study, published in Geophysical Research Letters, relies on observations and numerous climate model simulations to quantify the contributions from both internal variability and external forcing to the AMV since 1870. The researchers, supported in part by CPO’s Climate Observations and Monitoring (COM) program, have found that while internal variability has played a dominant role in AMV since 1870, decadal variations in volcanic and anthropogenic aerosols have amplified AMV cycles induced by internal variability. Specifically, volcanic and anthropogenic aerosols were shown to have enhanced internal variability since the late 1920s and greenhouse gases since the 1970s. These findings reconcile the internal vs. external debate and provide new evidence for the recent changes observed in the AMV. 

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