North Atlantic Oscillation

The North Atlantic Oscillation (abbreviated NAO) is the dominant mode of wintertime atmospheric circulation variability over the North Atlantic Ocean and surrounding continents, accounting for more than one-third of the total variance in winter sea-level pressure (SLP) across the extratropical North Atlantic. First named and quantified by British meteorologist Sir Gilbert Walker and E. W. Bliss in 1924 and 1932 (though its opposing winter temperature effects between Greenland and Denmark had been documented by missionary Hans Egede Saabye in the 1770s), the NAO consists of a large-scale meridional redistribution of atmospheric mass between the semi-permanent subtropical high-pressure belt centered near the Azores, Lisbon, or Gibraltar (Azores High, ~35°N–40°N) and the subpolar low-pressure trough centered near Reykjavik, Iceland (Icelandic Low, ~60°N–65°N). Through thermal-wind balance and transient baroclinic eddy momentum fluxes, fluctuations in the NAO control the strength, latitude, and orientation of the North Atlantic eddy-driven westerly jet stream and storm track, governing winter temperature, precipitation, and ocean deep-water convection across North America, Europe, and North Africa.

Meteorologists quantify the state of the NAO using two complementary methods:

1. Station-Based Hurrell NAO Index: Formulated by James W. Hurrell (1995), the station-based index is defined as the difference between normalized winter (December–March, DJFM) sea-level pressure anomalies measured at a southern subtropical station—either Lisbon, Portugal, Ponta Delgada, Azores, or Gibraltar—and a northern subpolar station at Stykkishólmur / Reykjavik, Iceland:

NAO Index = ΔP_norm(Lisbon or Azores) − ΔP_norm(Iceland)

Station-based instrumental records extend continuously back to the 1820s–1860s, while tree-ring, Greenland ice-core, and speleothem proxy reconstructions extend NAO histories across the last millennium.

1. Principal Component (EOF-1) Index: Computed as the leading Empirical Orthogonal Function (EOF 1) of sea-level pressure or 500-hPa geopotential height anomalies across the North Atlantic sector (20°N–80°N, 90°W–40°E). Because the atmospheric centers of action migrate seasonally, the EOF-based index captures the full spatial dipole year-round and closely parallels the hemispheric Northern Annular Mode (NAM, or Arctic Oscillation, AO) defined by David J. Thompson and John M. Wallace (1998).

Both the Azores High and the Icelandic Low are present year-round, meaning that the mean geostrophic wind across the mid-latitude North Atlantic is always westerly. However, variations in the meridional pressure gradient between these two centers shift the North Atlantic jet stream by 10° to 15° of latitude and reorganize regional hydroclimate into two contrasting regimes:

Unlike the tropical El Niño–Southern Oscillation—which is an intrinsic coupled ocean–atmosphere oscillation with a well-defined 2-to-7-year quasi-period—the NAO is primarily an internal, eddying atmospheric mode generated by nonlinear wave–mean flow feedbacks inside the troposphere, modulated on subseasonal to multidecadal timescales by boundary forcing:

1. Baroclinic Eddy Momentum Flux Feedback: Synoptic-scale baroclinic waves (developing extratropical cyclones and anticyclones with timescales of 2 to 6 days) grow along the sharp meridional temperature gradient of the Gulf Stream / North Atlantic Current front. As these transient eddies mature and break anticyclonically on the equatorward side and cyclonically on the poleward side of the jet stream, their Reynolds stress convergence (−∂(u'v')/∂y) deposits westerly momentum directly back into the core of the eddy-driven jet, extending the persistence timescale of NAO anomalies from a few days to 10–14 days.
2. Stratosphere–Troposphere Downward Coupling: During boreal winter, the tropospheric NAO is strongly coupled to the strength of the stratospheric polar vortex. When the stratospheric polar night jet is unusually strong and cold, positive zonal wind anomalies propagate downward through the tropopause over 2 to 6 weeks, locking the troposphere into a persistent positive NAO+ phase. Conversely, following a major Sudden Stratospheric Warming (SSW) event—in which breaking planetary Rossby waves reverse the stratospheric westerlies to easterlies—negative wind anomalies descend into the troposphere and increase the probability of a persistent negative NAO− / Greenland blocking regime for the ensuing 30 to 60 days.
3. North Atlantic Sea Surface Temperature Tripole: A persistent winter NAO anomaly imprints a characteristic three-banded SST tripole onto the North Atlantic mixed layer via surface turbulent sensible and latent heat fluxes and wind-driven Ekman transport: during NAO+, enhanced cold-air winds cool the subpolar gyre (Labrador/Irminger Seas) and the tropical North Atlantic trade-wind belt (10°N–20°N), while weaker winds and northward Ekman drift warm the mid-latitude band off the US East Coast (30°N–45°N). Through thermal re-emergence of deep winter mixed-layer anomalies in subsequent autumns, this oceanic tripole imparts modest multidecadal red-noise persistence to the NAO.

Through its control of surface buoyancy loss over the subpolar North Atlantic, the NAO acts as a primary atmospheric pacemaker of deep-water formation and the Atlantic Meridional Overturning Circulation (AMOC):

• Labrador and Irminger Sea Deep Convection: During a strong positive NAO+ winter, intense, frigid northwesterly winds blow off the Canadian Arctic archipelago across the Labrador and Irminger Seas. Turbulent sensible and latent heat fluxes extract 300 to 600 W m⁻² of heat from the ocean surface, cooling the salty surface waters until they become denser than the underlying water column (σ_θ > 27.74 kg m⁻³) and convectively mix to depths of 1,500 to 2,200 meters, forming Labrador Sea Water (LSW)—the upper limb of North Atlantic Deep Water (NADW).
• Multidecadal AMOC Spin-Up: Multi-year sequences of positive NAO+ winters (such as the prolonged NAO+ upward trend observed between the 1960s and mid-1990s) build a thick reservoir of dense Labrador Sea Water, steepening the meridional steric density gradient and spinning up the northward transport of the AMOC by 1 to 3 Sverdrups (1 Sv = 10⁶ m³ s⁻¹) after a 3-to-5-year advective adjustment lag, whereas prolonged NAO− decades (such as the 1960s or the sharp NAO− winter of 2009–2010) suppress Labrador Sea convection and temporarily weaken the overturning circulation.

Variations in the NAO ripple through marine food webs, terrestrial wildlife populations, and European energy infrastructure:

• North Atlantic Plankton and Cod Recruitment: By altering winter wind mixing depth, surface temperature, and the inflow of warm Atlantic water into the North Sea and Barents Sea, the NAO shifts the biogeographic boundary between the cold-water subarctic copepod Calanus finmarchicus and the warm-water temperate congener Calanus helgolandicus. Because larval Atlantic cod (Gadus morhua) depend on synchrony with the lipid-rich spring bloom of C. finmarchicus (Cushing's match–mismatch hypothesis), positive NAO+ regimes enhance cod recruitment in the cold Barents Sea while depressing recruitment near the warm southern limit of cod in the North Sea and Irish Sea.
• Renewable Energy Balancing: During NAO+ winters, northern Europe experiences simultaneous high wind-power generation, high Norwegian hydro-reservoir inflows, and reduced heating demand due to mild temperatures; conversely, during NAO− blocking episodes (Dunkelflaute conditions), wind speeds drop across northwestern Europe precisely when sub-freezing temperatures drive peak residential electrical and natural-gas heating demand.

In 2020, a multi-model grand ensemble analysis of initialized decadal climate hindcasts demonstrated a pronounced signal-to-noise paradox over the North Atlantic, showing that coupled climate models predict multi-year fluctuations of the winter North Atlantic Oscillation with substantially higher real-world skill than their weak internal ensemble signal-to-noise ratio implies (Smith et al., 2020).

In 2020, coordinated climate model perturbation experiments and observational energy-budget diagnostics established that mid-latitude North Atlantic Oscillation wind anomalies drive winter Arctic sea-ice variability and warm-air intrusions rather than passive Barents–Kara sea-ice loss forcing negative NAO states (Blackport & Screen, 2020).

In 2022, reanalysis and atmospheric dynamic diagnostics linked summertime negative North Atlantic Oscillation blocking patterns and persistent double-jet states over Eurasia to an accelerated trend in western European heatwaves over the satellite era (Rousi et al., 2022).

1. Hurrell, J. W. (1995). Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation. Science, 269(5224), 676–679.
2. Hurrell, J. W., Kushnir, Y., Ottersen, G., & Visbeck, M. (Eds.). (2003). The North Atlantic Oscillation: Climatic Significance and Environmental Impact. Geophysical Monograph Series (Vol. 134). American Geophysical Union.
3. Thompson, D. W. J., & Wallace, J. M. (1998). The Arctic Oscillation signature in the wintertime geopotential height and temperature fields. Geophysical Research Letters, 25(9), 1297–1300.
4. Scaife, A. A., Arribas, A., Blockley, E., Brookshaw, A., Clark, R. T., Dunstone, N., et al. (2014). Skillful long-range prediction of European and North American winters. Geophysical Research Letters, 41(7), 2514–2519.
5. Smith, D. M., Scaife, A. A., Eade, R., Athanasiadis, P., Bellucci, A., Bethke, I., et al. (2020). North Atlantic climate far more predictable than models imply. Nature, 583(7818), 796–800.
6. Blackport, R., & Screen, J. A. (2020). Insignificant effect of Arctic amplification on the amplitude of midlatitude atmospheric waves. Science Advances, 6(8), eaay2880.
7. Rousi, E., Kornhuber, K., Beobide-Arsuaga, G., Luo, F., & Coumou, D. (2022). Accelerated western European heatwave trends linked to more-persistent double jets over Eurasia. Nature Communications, 13(1), 3851.