Biogeosciences Research Spotlight

What Causes Long-Term North Atlantic Surface Temperature Cycles?

New evidence strengthens a likely link between 20- to 40-year sea surface temperature fluctuations and varying ocean circulation patterns.

Source: Geophysical Research Letters

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For at least 1000 years, the surface temperature of the North Atlantic Ocean has fluctuated between warmer and cooler periods, each lasting between roughly 20 and 40 years. Known as the Atlantic Multidecadal Oscillation (AMO), this pattern is characterized by a 1°F difference between extremes, and it influences climate and weather in Europe and North America.

Despite its impact, scientists aren’t entirely sure what drives AMO. Now new research by O’Reilly et al. supports a previously proposed connection between AMO and variations in the Atlantic Meridional Overturning Circulation (AMOC), a major current that carries shallow, warmer water northward and deep, cooler water southward.

After a recent computer modeling study suggested that atmospheric processes—and not variations in ocean circulation—drive AMO, the authors set out to compare observations with predictions from two sets of climate models: one that incorporates ocean circulation variability and one that does not.

Specifically, the researchers predicted expected AMO patterns for the past 127 years using the third and fifth phases of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). Unlike CMIP5 simulations—which fully coupled atmospheric and ocean dynamics—CMIP3 simulations used a “slab” ocean model that did not incorporate ocean circulation variability.

The scientists compared simulation outputs with real-world data. Both types of simulations correctly predicted the observed sea surface temperature fluctuations of AMO. However, only the fully coupled simulations correctly predicted the long-term relationship between surface temperature and heat flux between the atmosphere and ocean.

The results suggest that atmospheric processes—changes in air temperature and winds—do not directly drive AMO. Instead, the scientists conclude, the earlier explanation is more plausible: Ocean circulation variability drives AMO.

Since the mid-1990s, AMO has been in a warm phase, leading to more severe hurricanes and other weather patterns. Because AMO can mask or amplify the effects of climate change, a better understanding of its drivers and impacts could improve predictions of future weather patterns. (Geophysical Research Letters, doi:10.1002/2016GL067925, 2016)

—Sarah Stanley, Freelance Writer

Citation: Stanley, S. (2016), What causes long-term North Atlantic surface temperature cycles?, Eos, 97, doi:10.1029/2016EO050997. Published on 25 April 2016.

© 2016. The authors. CC BY-NC-ND 3.0