Abstract
The Atlantic multidecadal oscillation (AMO) is a 60–70 year pattern of sea-surface temperature (SST) variability in the North Atlantic commonly ascribed to internal ocean dynamics and changes in northward heat transport. Recent modeling studies, however, suggest that SSTs fluctuate primarily in response to major volcanic eruptions and changes in atmospheric circulation. Here, we utilize historical SST, atmospheric reanalysis, and stratospheric aerosol optical depth data to examine the basic evidence supporting a volcanic link. We find that cool intervals across the North Atlantic coincide with two distinct episodes of explosive volcanic activity (1880s–1920s and 1960s–1990s), where key eruptions include 1883 Krakatau, 1902 Santa María, 1912 Novarupta, 1963 Agung, 1982 El Chichón, and 1991 Pinatubo. Cool SST patterns develop in association with an increased prevalence of North Atlantic Oscillation (NAO)+ atmospheric patterns caused by stratospheric aerosol loading and a steepened poleward temperature gradient. NAO+ patterns promote wind-driven advection, evaporative cooling, and increased albedo from enhanced Saharan dust transport and anthropogenic aerosols. SSTs across the subpolar gyre are regulated by strength of low pressure near Iceland and the associated wind-driven advection of cold surface water from the Labrador Sea. This is contrary to an interpretation that subpolar SSTs are driven by changes in ocean overturning circulation. We also find that North Pacific and global mean SST declines can be readily associated with the same volcanic triggers that affect the North Atlantic. Thus, external forcing from volcanic aerosols appears to underpin multi-decade SST variability observed in the historical record.
Highlights
Historical measurements of sea-surface temperature (SST) in the North Atlantic reveal a 60–70 year pattern of variability often described as the Atlantic multidecadal oscillation (AMO)[1,2] (Fig. 1)
We find that historical cool intervals across the North Atlantic coincide with two episodes of major volcanic activity (1880s–1920s and 1960s–1990s), where SST declines can be explained from the prevalence of North Atlantic oscillation (NAO)+ atmospheric patterns that promote winddriven advection, evaporative cooling, and increased albedo from enhanced Saharan dust transport
Cool intervals coincide with significant episodic volcanic aerosol loading in the stratosphere, whereas warm intervals coincide with reduced aerosol loading, or an optically clean stratosphere
Summary
Historical measurements of sea-surface temperature (SST) in the North Atlantic reveal a 60–70 year pattern of variability often described as the Atlantic multidecadal oscillation (AMO)[1,2] (Fig. 1). Cool and warm AMO modes are identified based on the assumption that SST changes are representative of oscillatory behavior, even though the record of direct observation beginning 1851 contains only two pattern wavelengths. These AMO modes occur in conjunction with a prevalence of atmospheric patterns resembling phases of the North Atlantic oscillation (NAO)[3] (Fig. 1b). This coupled atmosphere-ocean variability has important teleconnection to several systems, including rainfall over northeast Brazil and the African Sahel,[4,5] and summer climate over North America and Europe.[2,6,7] North Atlantic variability may have strong atmospheric teleconnection to SST and sea-level pressure in the North Pacific.[8,9]
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