The two states of the North Atlantic Oscillation

The two cartoons attempt to visualise the dynamical structure of the two (positive and negative) states of the NAO with its major components of the ocean - atmosphere - sea-ice system. It was constructed based on an extended review of the available literature and on discussions with different colleagues. Note that the two pictures are schematic in many of the represented structures. In addition, it suggests that the two states of the NAO mode are static, quasi-stable, and based on a bimodal frequency distribution. This is a strong simplification and partly wrong because the NAO is an intermittent phenomenon and does not reside in two quasi-stable states.

The following components can be distinguished as follows:

- Two pools with anomalous ocean temperatures are situated east/southeast of Greenland and west of North Africa (both with negative anomalies in the NAO+ state, and positive anomalies in the NAO- state). Two other pools with contrasting temperature anomalies exist over the North American Basin and around the British Channel.

- Beside the interactions with the atmosphere the four pools interact with (or are the result of) the circulation in the ocean, namely the subtropical and the subpolar gyre, the Gulf Stream, the Mid-Atlantic and West Norwegian Current .

- The subtropical and mid-latitude ocean circulation system is also connected to the polar basin - namely through the Labrador Current and the Transpolar Drift Stream - and the formation and distribution of sea-ice. During the NAO+ state, the Labrador Sea is covered by sea-ice (sustaining a strong deep convection, the Labrador Current extends far south and southward water and sea-ice transport along the Transpolar Drift Stream east of Greenland is decreasing. During the NAO- state, the Labrador Sea is open (with a decreased deep convection) and the sea-ice and water transport through the Fram Strait becomes stronger with an increasing NAO.

- The sea-air exchange and the ocean surface currents at the border between the mid-latitudes and the polar basin are influenced by the air pressure configuration over Greenland. In the NAO+ state, the temperature gradient between Greenland and the cool ocean surface to the south is minor. Therefore, the flow divergence over Greenland is much smaller than in the negative mode and the air pressure tends to be low which again influences the Labrador Current and the Transpolar Drift Stream.

- Energy and mass fluxes to and from the sea and ice surfaces influence the air pressure and the wind systems from the lower to the higher atmospheric levels. In the NAO+ state, the Icelandic Low and the Azores High are well developed, with anomalous low and high pressure cores, respectively. This pressure configuration leads to strong pressure gradients and strengthens the westerlies and the trade winds. The axis of the westerlies orients southwest-northeast and shifts to the north. In the negative phase, the opposite effects are observed, with the axis of the westerlies clearly zonal and lying further south.

- The described configuration of the pressure systems influences the storm tracks and, therefore, the transport of heat and moisture to the European continent. In the NAO+ state with a more northern jet axis, wet and warm winter climate is observed in Scandinavia, while cool and dry conditions predominate in southern Europe and northern North Africa. Almost the opposite conditions prevail during the NAO- state.

- The seesaw between the two NAO states influences the northern Eurasian and American climate in a different manner: In the NAO+ state, northern Siberia is wetter and northern Canada is drier. This influences the runoff from the big rivers in both areas and modifies the freshwater balance of the polar basin, and thus the water exchange with the mid-latitudes.

- Salinities along the two inflow branches (Barents Sea throughflow and West Spitzbergen Current) appear to have declined as the NAO evolved from its negative values in the 1960s to its highly positive ones in the 1990s, a fact which is consistent with the increasing freshwater accession, the increasing volume flux of sea-ice from the Arctic and the reduction in total sea-ice over this period.

Finally, it has to be mentioned that a possible stratospheric coupling with the NAO (e.g., after strong tropical volcanic eruptions) is not represented in the two cartoons.