GFDL - Geophysical Fluid Dynamics Laboratory

The North Atlantic Oscillation as a driving force for observed rapid Arctic sea ice change, hemispheric warming, and Atlantic tropical cyclone variability

June 20th, 2016

Thomas L. Delworth, Fanrong Zeng, Gabriel A. Vecchi, Xiaosong Yang, Liping Zhang, and Rong Zhang. Nature Geoscience. DOI: 10.1038/ngeo2738

Summary

In order to better understand the factors governing observed climate variability and change, it is critical to better understand the mechanisms contributing to natural climate variability, particularly on decadal and longer time scales. The ocean is thought to play a critical role in such variability. This study examined factors that influence decadal and longer time-scale variability of the Atlantic Ocean, and its subsequent influence on the overall climate system.

Model simulations were used to assess the role of the North Atlantic Oscillation (NAO) in driving observed rapid changes from the 1970s to the 2000s in Northern Hemisphere temperature, Arctic sea ice, and Atlantic tropical storms. The authors employed suites of simulations with several GFDL climate models to study how the NAO influences Atlantic multidecadal variability, through its influence on the Atlantic Meridional Overturning Circulation (AMOC). The authors found that observed variations of the NAO over the last 60 years have modulated the strength of the AMOC, and this in turn influences Northern Hemisphere climate by modulating ocean heat transport into the Atlantic and Arctic.

NAO is related to the strength of the westerly winds in the subpolar North Atlantic. An increase in the NAO from the 1970s to the 1990s leads to enhanced poleward oceanic heat transport in the North Atlantic, extending into the Arctic. This contributes to a rapid reduction of Arctic sea ice, especially in winter, and an increase in both hemispheric surface air temperature and Atlantic tropical cyclone activity. These changes, driven by the NAO, are in addition to the very substantial influence of anthropogenic forcing, illustrating the roles of natural variability and anthropogenic forcing in explaining observed climate change.

More details about this study can be found in a related Q&A with the lead author, Tom Delworth.

Figure Schematic of some of the impacts of the North Atlantic Oscillation (NAO) on the ocean. A positive phase of the NAO has stronger westerly winds over the subpolar gyre, thereby extracting more heat from the Labrador Sea and subpolar gyre. This leads to increasing density of water in the Northwest Atlantic, thereby increasing the west-east density gradient across the North Atlantic. In turn, this altered density contrast leads to a strengthening of the Atlantic Meridional Overturning Circulation (AMOC), thereby strengthening poleward heat transport in the North Atlantic extending into the Barents Sea and Arctic Ocean, and reducing Arctic sea ice and warming the Northern Hemisphere. Associated atmospheric circulation changes also increase the number of tropical storms in the Atlantic.
Figure Schematic of some of the impacts of the North Atlantic Oscillation (NAO) on the ocean. A positive phase of the NAO has stronger westerly winds over the subpolar gyre, thereby extracting more heat from the Labrador Sea and subpolar gyre. This leads to increasing density of water in the Northwest Atlantic, thereby increasing the west-east density gradient across the North Atlantic. In turn, this altered density contrast leads to a strengthening of the Atlantic Meridional Overturning Circulation (AMOC), thereby strengthening poleward heat transport in the North Atlantic extending into the Barents Sea and Arctic Ocean, and reducing Arctic sea ice and warming the Northern Hemisphere. Associated atmospheric circulation changes also increase the number of tropical storms in the Atlantic.