North Atlantic and Arctic Oscillations

Seesaws in atmospheric pressure that steer the mid-latitude jet stream and storm tracks, governing winter weather across the North Atlantic and the wider Northern Hemisphere.

Definition

The North Atlantic Oscillation is a swing in the pressure contrast between the subpolar and subtropical North Atlantic, and the Arctic Oscillation is the broader hemispheric pattern of opposing pressure anomalies between the Arctic and mid-latitudes, both of which control the strength and position of the westerly winds.

Scope

This topic covers the leading modes of extratropical Northern Hemisphere variability: the North Atlantic Oscillation, a fluctuation in the pressure difference between the Icelandic Low and the Azores High, and the closely related Arctic Oscillation or Northern Annular Mode, a hemispheric seesaw between polar and mid-latitude pressure. It treats how these modes shift the jet stream and storm tracks, their impacts on European and North American winters, and their links to the stratosphere and ocean.

Core questions

• What pressure patterns define these oscillations?
• How do they steer the jet stream and winter storm tracks?
• What are their impacts on European and North American climate?
• How are they linked to the stratosphere and to predictability?

Key theories

Annular mode dynamics

The Arctic Oscillation reflects a hemispheric exchange of mass between polar and mid-latitudes that strengthens or weakens the westerly jet, with the North Atlantic Oscillation as its regional Atlantic expression.

Stratosphere-troposphere coupling

Variations in the strength of the stratospheric polar vortex can propagate downward to influence the surface annular mode, offering a source of extended-range predictability.

Mechanisms

When the subpolar low and subtropical high are both strong, the pressure contrast is large, the westerlies and storm track strengthen and shift north, bringing mild wet winters to northern Europe and dry conditions to the Mediterranean; the opposite phase weakens and shifts the winds south. The Arctic Oscillation expresses the same exchange of atmospheric mass on a hemispheric scale, and a strong or weak stratospheric polar vortex can nudge these surface patterns weeks in advance.

Clinical relevance

Because these modes dominate winter temperature and precipitation variability over Europe and North America, they are key to seasonal forecasting of cold spells, storms, and energy demand in heavily populated regions.

History

The North Atlantic pressure seesaw was noted by Walker and others early in the twentieth century, and Hurrell's 1990s work quantified its strong influence on European winters; Thompson and Wallace introduced the Arctic Oscillation framing in 1998, linking the regional pattern to a hemispheric annular mode and to the stratosphere.

Debates

Distinct mode or statistical artifact

Whether the Arctic Oscillation is a single physical mode or largely a statistical summary of regional patterns such as the North Atlantic Oscillation has been debated since its introduction.

Key figures

• James Hurrell
• David Thompson
• John Wallace
• Martin Visbeck

Seminal works

• hurrell2003
• thompsonwallace1998

Frequently asked questions

How does the North Atlantic Oscillation affect European weather?

In its positive phase it strengthens the westerlies, bringing mild, wet, stormy winters to northern Europe and drier conditions to the Mediterranean; the negative phase does the reverse.

Are the North Atlantic and Arctic Oscillations the same thing?

They are closely related; the North Atlantic Oscillation is the Atlantic regional expression of the hemispheric Arctic Oscillation, and their indices are strongly correlated.