Importance of a vertically tilting structure for energizing the North Atlantic Oscillation

Patrick Martineau,Hisashi Nakamura,Ayako Yamamoto,Yu Kosaka

doi:10.1038/s41598-020-69551-5

Patrick Martineau, Hisashi Nakamura + Show 2 more

Open Access

https://doi.org/10.1038/s41598-020-69551-5

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Abstract

The North Atlantic Oscillation (NAO) is a prominent mode of atmospheric variability that influences weather and climate, including the occurrence of extreme events, over a large part of Europe and Northeastern America. The NAO has been considered to be maintained primarily by migratory weather disturbances and to have a deep structure with no vertical tilt. A careful inspection nonetheless reveals that the associated anomalies do exhibit a subtle vertical tilt, but its dynamical implications are still unknown. Here we show that this vertical tilt is of vital dynamical significance for the wintertime NAO. We find, using atmospheric reanalysis data, that the tilted anomalies transport heat across the pronounced thermal gradient associated with a background westerly jetstream, advecting air from the cooler North America and Greenland to the warmer Atlantic, thereby acting to reinforce NAO’s thermal anomalies. The resultant conversion of potential energy from the background state is a larger energy source for maintaining the NAO than the feedback from migratory disturbances. Our findings thus uncover a fundamental mechanism of the NAO dynamics, with implications for the improvement of seasonal predictions for the Euro-Atlantic climate and the representation of the NAO variability in climate models.

Highlights

The North Atlantic Oscillation (NAO) is one of the most dominant modes of variability of the atmospheric circulation in the extratropical Northern H emisphere[1,2,3]
The energetics shown above elucidate why the NAO is such a prominent mode of variability with persistent thermal and circulation anomalies
The wintertime enhancement of the vertical tilt of the NAO anomalies, which allows an effective conversion of potential energy from the mean flow, is likely a contributing factor to seasonal changes in the nature of the NAOassociated anomalies, such as their amplitude and persistence

Summary

Introduction

The North Atlantic Oscillation (NAO) is one of the most dominant modes of variability of the atmospheric circulation in the extratropical Northern H emisphere[1,2,3]. The eddy available potential energy (EAPE) associated with the NAO, which is proportional to the squared temperature anomalies shown, is maximized around the southern tip of Greenland, while its near-surface maximum is shifted westward over the Labrador Sea (Fig. 4, 1st column).

Results

Conclusion