For at least the past 1,000 years, the surface waters of the North Atlantic Ocean have undergone a series of warmer and cooler phases, each lasting about 20 to 40 years and differing by a maximum of about 0.5°C. Known as the Atlantic Multidecadal Oscillation (AMO), this pattern influences Atlantic hurricanes, Arctic sea ice, and European summer climate, as well as rainfall and droughts worldwide. What’s more, it can obscure or amplify the effects of global climate change.
Although the AMO is well documented, the underlying mechanism that drives it is unknown and remains up for debate. In a new study, Zhang presents compelling findings in support of the idea that ocean dynamics play a central role in the AMO.
Past studies, including some copublished by the author, had already indicated an important role for the ocean in powering the AMO. These studies propose that large-scale ocean circulation underpins the AMO. However, ongoing debate and recent studies suggest that stochastic atmospheric white noise is the main driver of the pattern.
In an effort to settle the debate, the author investigated several decades’ worth of monthly temperature and salinity observations for the sea surface and subsurface of the subpolar North Atlantic, the region with the most extreme temperature anomalies seen in the AMO. The author also examined the simulations using a fully coupled climate model known as Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1.
The author’s analysis revealed key statistical features of the AMO that are not adequately explained by atmospheric mechanisms. Instead, she found that the key AMO features she identified are linked with the Atlantic Meridional Overturning Circulation, a major current in which warm, salty water flows northward in the upper Atlantic while colder water flows southward at greater depths. These findings lend support to the ocean dynamics mechanism.
The debate over what drives the AMO is not yet resolved. Nonetheless, the study provides compelling evidence for the important role of ocean circulation and contributes new insights into the features that characterize the AMO. (Geophysical Research Letters, https://doi.org/10.1002/2017GL074342, 2017)
—Sarah Stanley, Freelance Writer